17 Templates [temp]

A template-argument for a template-parameter which is a type shall be a type-id.

[ Example:

template <class T> class X { };
template <class T> void f(T t) { }
struct { } unnamed_obj;

void f() {
  struct A { };
  enum { e1 };
  typedef struct { } B;
  B b;
  X<A> x1;          // OK
  X<A*> x2;         // OK
  X<B> x3;          // OK
  f(e1);            // OK
  f(unnamed_obj);   // OK
  f(b);             // OK
}

 — end example ] [ Note: A template type argument may be an incomplete type ([basic.types]).  — end note ]

If the type of a template-parameter contains a placeholder type, the deduced parameter type is determined from the type of the template-argument by placeholder type deduction. If a deduced parameter type is not permitted for a template-parameter declaration ([temp.param]), the program is ill-formed.

A template-argument for a non-type template-parameter shall be a converted constant expression of the type of the template-parameter. For a non-type template-parameter of reference or pointer type, the value of the constant expression shall not refer to (or for a pointer type, shall not be the address of):

• (2.1)

  a subobject,

• (2.2)

  a temporary object,

• (2.3)

  a string literal,

• (2.4)

  the result of a typeid expression, or

• (2.5)

  a predefined _­_­func_­_­ variable.

[ Note: If the template-argument represents a set of overloaded functions (or a pointer or member pointer to such), the matching function is selected from the set ([over.over]).  — end note ]

[ Example:

template<const int* pci> struct X { /* ... */ };
int ai[10];
X<ai> xi;                       // array to pointer and qualification conversions

struct Y { /* ... */ };
template<const Y& b> struct Z { /* ... */ };
Y y;
Z<y> z;                         // no conversion, but note extra cv-qualification

template<int (&pa)[5]> struct W { /* ... */ };
int b[5];
W<b> w;                         // no conversion

void f(char);
void f(int);

template<void (*pf)(int)> struct A { /* ... */ };

A<&f> a;                        // selects f(int)

template<auto n> struct B { /* ... */ };
B<5> b1;                        // OK: template parameter type is int
B<'a'> b2;                      // OK: template parameter type is char
B<2.5> b3;                      // error: template parameter type cannot be double

 — end example ]

[ Note: A string literal is not an acceptable template-argument. [ Example:

template<class T, const char* p> class X {
  /* ... */
};

X<int, "Studebaker"> x1;        // error: string literal as template-argument

const char p[] = "Vivisectionist";
X<int,p> x2;                    // OK

 — end example ]  — end note ]

[ Note: The address of an array element or non-static data member is not an acceptable template-argument. [ Example:

template<int* p> class X { };

int a[10];
struct S { int m; static int s; } s;

X<&a[2]> x3;                    // error: address of array element
X<&s.m> x4;                     // error: address of non-static member
X<&s.s> x5;                     // OK: address of static member
X<&S::s> x6;                    // OK: address of static member

 — end example ]  — end note ]

[ Note: A temporary object is not an acceptable template-argument when the corresponding template-parameter has reference type. [ Example:

template<const int& CRI> struct B { /* ... */ };

B<1> b2;                        // error: temporary would be required for template argument

int c = 1;
B<c> b1;                        // OK

 — end example ]  — end note ]

A template-argument for a template template-parameter shall be the name of a class template or an alias template, expressed as id-expression. When the template-argument names a class template, only primary class templates are considered when matching the template template argument with the corresponding parameter; partial specializations are not considered even if their parameter lists match that of the template template parameter.

Any partial specializations associated with the primary class template or primary variable template are considered when a specialization based on the template template-parameter is instantiated. If a specialization is not visible at the point of instantiation, and it would have been selected had it been visible, the program is ill-formed, no diagnostic required. [ Example:

template<class T> class A {     // primary template
  int x;
};
template<class T> class A<T*> { // partial specialization
  long x;
};
template<template<class U> class V> class C {
  V<int>  y;
  V<int*> z;
};
C<A> c;             // V<int> within C<A> uses the primary template, so c.y.x has type int
                    // V<int*> within C<A> uses the partial specialization, so c.z.x has type long

 — end example ]

A template-argument matches a template template-parameter P when P is at least as specialized as the template-argument A. If P contains a parameter pack, then A also matches P if each of A's template parameters matches the corresponding template parameter in the template-parameter-list of P. Two template parameters match if they are of the same kind (type, non-type, template), for non-type template-parameters, their types are equivalent ([temp.over.link]), and for template template-parameters, each of their corresponding template-parameters matches, recursively. When P's template-parameter-list contains a template parameter pack, the template parameter pack will match zero or more template parameters or template parameter packs in the template-parameter-list of A with the same type and form as the template parameter pack in P (ignoring whether those template parameters are template parameter packs).

[ Example:

template<class T> class A { /* ... */ };
template<class T, class U = T> class B { /* ... */ };
template<class ... Types> class C { /* ... */ };
template<auto n> class D { /* ... */ };
template<template<class> class P> class X { /* ... */ };
template<template<class ...> class Q> class Y { /* ... */ };
template<template<int> class R> class Z { /* ... */ };

X<A> xa;            // OK
X<B> xb;            // OK
X<C> xc;            // OK
Y<A> ya;            // OK
Y<B> yb;            // OK
Y<C> yc;            // OK
Z<D> zd;            // OK

 — end example ]

[ Example:

template <class T> struct eval;

template <template <class, class...> class TT, class T1, class... Rest>
struct eval<TT<T1, Rest...>> { };

template <class T1> struct A;
template <class T1, class T2> struct B;
template <int N> struct C;
template <class T1, int N> struct D;
template <class T1, class T2, int N = 17> struct E;

eval<A<int>> eA;            // OK: matches partial specialization of eval
eval<B<int, float>> eB;     // OK: matches partial specialization of eval
eval<C<17>> eC;             // error: C does not match TT in partial specialization
eval<D<int, 17>> eD;        // error: D does not match TT in partial specialization
eval<E<int, float>> eE;     // error: E does not match TT in partial specialization

 — end example ]

A template template-parameter P is at least as specialized as a template template-argument A if, given the following rewrite to two function templates, the function template corresponding to P is at least as specialized as the function template corresponding to A according to the partial ordering rules for function templates. Given an invented class template X with the template parameter list of A (including default arguments):

• (4.1)

  Each of the two function templates has the same template parameters, respectively, as P or A.

• (4.2)

  Each function template has a single function parameter whose type is a specialization of X with template arguments corresponding to the template parameters from the respective function template where, for each template parameter PP in the template parameter list of the function template, a corresponding template argument AA is formed. If PP declares a parameter pack, then AA is the pack expansion PP... ([temp.variadic]); otherwise, AA is the id-expression PP.

If the rewrite produces an invalid type, then P is not at least as specialized as A.

A class template defines the layout and operations for an unbounded set of related types.

[ Example: A single class template List might provide an unbounded set of class definitions: one class List<T> for every type T, each describing a linked list of elements of type T. Similarly, a class template Array describing a contiguous, dynamic array might be defined like this:

template<class T> class Array {
  T* v;
  int sz;
public:
  explicit Array(int);
  T& operator[](int);
  T& elem(int i) { return v[i]; }
};

The prefix template<class T> specifies that a template is being declared and that a type-name T may be used in the declaration. In other words, Array is a parameterized type with T as its parameter.  — end example ]

When a member function, a member class, a member enumeration, a static data member or a member template of a class template is defined outside of the class template definition, the member definition is defined as a template definition in which the template-parameters are those of the class template. The names of the template parameters used in the definition of the member may be different from the template parameter names used in the class template definition. The template argument list following the class template name in the member definition shall name the parameters in the same order as the one used in the template parameter list of the member. Each template parameter pack shall be expanded with an ellipsis in the template argument list. [ Example:

template<class T1, class T2> struct A {
  void f1();
  void f2();
};

template<class T2, class T1> void A<T2,T1>::f1() { }    // OK
template<class T2, class T1> void A<T1,T2>::f2() { }    // error

template<class ... Types> struct B {
  void f3();
  void f4();
};

template<class ... Types> void B<Types ...>::f3() { }    // OK
template<class ... Types> void B<Types>::f4() { }        // error

 — end example ]

In a redeclaration, partial specialization, explicit specialization or explicit instantiation of a class template, the class-key shall agree in kind with the original class template declaration ([dcl.type.elab]).

A template can be declared within a class or class template; such a template is called a member template. A member template can be defined within or outside its class definition or class template definition. A member template of a class template that is defined outside of its class template definition shall be specified with the template-parameters of the class template followed by the template-parameters of the member template. [ Example:

template<class T> struct string {
  template<class T2> int compare(const T2&);
  template<class T2> string(const string<T2>& s) { /* ... */ }
};

template<class T> template<class T2> int string<T>::compare(const T2& s) {
}

 — end example ]

A local class of non-closure type shall not have member templates. Access control rules apply to member template names. A destructor shall not be a member template. A non-template member function ([dcl.fct]) with a given name and type and a member function template of the same name, which could be used to generate a specialization of the same type, can both be declared in a class. When both exist, a use of that name and type refers to the non-template member unless an explicit template argument list is supplied. [ Example:

template <class T> struct A {
  void f(int);
  template <class T2> void f(T2);
};

template <> void A<int>::f(int) { }                 // non-template member function
template <> template <> void A<int>::f<>(int) { }   // member function template specialization

int main() {
  A<char> ac;
  ac.f(1);          // non-template
  ac.f('c');        // template
  ac.f<>(1);        // template
}

 — end example ]

A member function template shall not be virtual. [ Example:

template <class T> struct AA {
  template <class C> virtual void g(C);     // error
  virtual void f();                         // OK
};

 — end example ]

A specialization of a member function template does not override a virtual function from a base class. [ Example:

class B {
  virtual void f(int);
};

class D : public B {
  template <class T> void f(T); // does not override B​::​f(int)
  void f(int i) { f<>(i); }     // overriding function that calls the template instantiation
};

 — end example ]

A specialization of a conversion function template is referenced in the same way as a non-template conversion function that converts to the same type. [ Example:

struct A {
  template <class T> operator T*();
};
template <class T> A::operator T*(){ return 0; }
template <> A::operator char*(){ return 0; }    // specialization
template A::operator void*();                   // explicit instantiation

int main() {
  A a;
  int* ip;
  ip = a.operator int*();       // explicit call to template operator A​::​operator int*()
}

 — end example ] [ Note: Because the explicit template argument list follows the function template name, and because conversion member function templates and constructor member function templates are called without using a function name, there is no way to provide an explicit template argument list for these function templates.  — end note ]

A specialization of a conversion function template is not found by name lookup. Instead, any conversion function templates visible in the context of the use are considered. For each such operator, if argument deduction succeeds ([temp.deduct.conv]), the resulting specialization is used as if found by name lookup.

A using-declaration in a derived class cannot refer to a specialization of a conversion function template in a base class.

Overload resolution and partial ordering are used to select the best conversion function among multiple specializations of conversion function templates and/or non-template conversion functions.

A template parameter pack is a template parameter that accepts zero or more template arguments. [ Example:

template<class ... Types> struct Tuple { };

Tuple<> t0;                     // Types contains no arguments
Tuple<int> t1;                  // Types contains one argument: int
Tuple<int, float> t2;           // Types contains two arguments: int and float
Tuple<0> error;                 // error: 0 is not a type

 — end example ]

A function parameter pack is a function parameter that accepts zero or more function arguments. [ Example:

template<class ... Types> void f(Types ... args);

f();                            // OK: args contains no arguments
f(1);                           // OK: args contains one argument: int
f(2, 1.0);                      // OK: args contains two arguments: int and double

 — end example ]

A parameter pack is either a template parameter pack or a function parameter pack.

A pack expansion consists of a pattern and an ellipsis, the instantiation of which produces zero or more instantiations of the pattern in a list (described below). The form of the pattern depends on the context in which the expansion occurs. Pack expansions can occur in the following contexts:

• (4.1)

  In a function parameter pack ([dcl.fct]); the pattern is the parameter-declaration without the ellipsis.

• (4.2)

  In a using-declaration; the pattern is a using-declarator.

• (4.3)

  In a template parameter pack that is a pack expansion ([temp.param]):

  • (4.3.1)

    if the template parameter pack is a parameter-declaration; the pattern is the parameter-declaration without the ellipsis;

  • (4.3.2)

    if the template parameter pack is a type-parameter with a template-parameter-list; the pattern is the corresponding type-parameter without the ellipsis.

• (4.4)

  In an initializer-list; the pattern is an initializer-clause.

• (4.5)

  In a base-specifier-list; the pattern is a base-specifier.

• (4.6)

  In a mem-initializer-list for a mem-initializer whose mem-initializer-id denotes a base class; the pattern is the mem-initializer.

• (4.7)

  In a template-argument-list ([temp.arg]); the pattern is a template-argument.

• (4.8)

  In an attribute-list; the pattern is an attribute.

• (4.9)

  In an alignment-specifier ([dcl.align]); the pattern is the alignment-specifier without the ellipsis.

• (4.10)

  In a capture-list ([expr.prim.lambda]); the pattern is a capture.

• (4.11)

  In a sizeof... expression; the pattern is an identifier.

• (4.12)

  In a fold-expression; the pattern is the cast-expression that contains an unexpanded parameter pack.

[ Example:

template<class ... Types> void f(Types ... rest);
template<class ... Types> void g(Types ... rest) {
  f(&rest ...);     // “&rest ...” is a pack expansion; “&rest” is its pattern
}

 — end example ]

For the purpose of determining whether a parameter pack satisfies a rule regarding entities other than parameter packs, the parameter pack is considered to be the entity that would result from an instantiation of the pattern in which it appears.

A parameter pack whose name appears within the pattern of a pack expansion is expanded by that pack expansion. An appearance of the name of a parameter pack is only expanded by the innermost enclosing pack expansion. The pattern of a pack expansion shall name one or more parameter packs that are not expanded by a nested pack expansion; such parameter packs are called unexpanded parameter packs in the pattern. All of the parameter packs expanded by a pack expansion shall have the same number of arguments specified. An appearance of a name of a parameter pack that is not expanded is ill-formed. [ Example:

template<typename...> struct Tuple {};
template<typename T1, typename T2> struct Pair {};

template<class ... Args1> struct zip {
  template<class ... Args2> struct with {
    typedef Tuple<Pair<Args1, Args2> ... > type;
  };
};

typedef zip<short, int>::with<unsigned short, unsigned>::type T1;
    // T1 is Tuple<Pair<short, unsigned short>, Pair<int, unsigned>>
typedef zip<short>::with<unsigned short, unsigned>::type T2;
    // error: different number of arguments specified for Args1 and Args2

template<class ... Args>
  void g(Args ... args) {                   // OK: Args is expanded by the function parameter pack args
    f(const_cast<const Args*>(&args)...);   // OK: “Args” and “args” are expanded
    f(5 ...);                               // error: pattern does not contain any parameter packs
    f(args);                                // error: parameter pack “args” is not expanded
    f(h(args ...) + args ...);              // OK: first “args” expanded within h,
                                            // second “args” expanded within f
  }

 — end example ]

The instantiation of a pack expansion that is neither a sizeof... expression nor a fold-expression produces a list $E_1,E_2,\dots ,E_N$, where N is the number of elements in the pack expansion parameters. Each $E_i$ is generated by instantiating the pattern and replacing each pack expansion parameter with its ith element. Such an element, in the context of the instantiation, is interpreted as follows:

• (7.1)

  if the pack is a template parameter pack, the element is a template parameter of the corresponding kind (type or non-type) designating the type or value from the template argument; otherwise,

• (7.2)

  if the pack is a function parameter pack, the element is an id-expression designating the function parameter that resulted from the instantiation of the pattern where the pack is declared.

All of the $E_i$ become elements in the enclosing list. [ Note: The variety of list varies with the context: expression-list, base-specifier-list, template-argument-list, etc. — end note ] When N is zero, the instantiation of the expansion produces an empty list. Such an instantiation does not alter the syntactic interpretation of the enclosing construct, even in cases where omitting the list entirely would otherwise be ill-formed or would result in an ambiguity in the grammar. [ Example:

template<class... T> struct X : T... { };
template<class... T> void f(T... values) {
  X<T...> x(values...);
}

template void f<>();    // OK: X<> has no base classes
                        // x is a variable of type X<> that is value-initialized

 — end example ]

The instantiation of a sizeof... expression produces an integral constant containing the number of elements in the parameter pack it expands.

The instantiation of a fold-expression produces:

• (9.1)

  (($E_1$ op $E_2$) op ⋯) op $E_N$ for a unary left fold,

• (9.2)

  $E_1$ op (⋯ op ($E_{N-1}$ op $E_N$)) for a unary right fold,

• (9.3)

  (((E op $E_1$) op $E_2$) op ⋯) op $E_N$ for a binary left fold, and

• (9.4)

  $E_1$ op (⋯ op ($E_{N-1}$ op ($E_N$ op E))) for a binary right fold.

In each case, op is the fold-operator, N is the number of elements in the pack expansion parameters, and each $E_i$ is generated by instantiating the pattern and replacing each pack expansion parameter with its ith element. For a binary fold-expression, E is generated by instantiating the cast-expression that did not contain an unexpanded parameter pack. [ Example:

template<typename ...Args>
  bool all(Args ...args) { return (... && args); }

bool b = all(true, true, true, false);

Within the instantiation of all, the returned expression expands to ((true && true) && true) && false, which evaluates to false.  — end example ] If N is zero for a unary fold-expression, the value of the expression is shown in Table 14; if the operator is not listed in Table 14, the instantiation is ill-formed.

A friend of a class or class template can be a function template or class template, a specialization of a function template or class template, or a non-template function or class. For a friend function declaration that is not a template declaration:

• (1.1)

  if the name of the friend is a qualified or unqualified template-id, the friend declaration refers to a specialization of a function template, otherwise,

• (1.2)

  if the name of the friend is a qualified-id and a matching non-template function is found in the specified class or namespace, the friend declaration refers to that function, otherwise,

• (1.3)

  if the name of the friend is a qualified-id and a matching function template is found in the specified class or namespace, the friend declaration refers to the deduced specialization of that function template ([temp.deduct.decl]), otherwise,

• (1.4)

  the name shall be an unqualified-id that declares (or redeclares) a non-template function.

[ Example:

template<class T> class task;
template<class T> task<T>* preempt(task<T>*);

template<class T> class task {
  friend void next_time();
  friend void process(task<T>*);
  friend task<T>* preempt<T>(task<T>*);
  template<class C> friend int func(C);

  friend class task<int>;
  template<class P> friend class frd;
};

Here, each specialization of the task class template has the function next_­time as a friend; because process does not have explicit template-arguments, each specialization of the task class template has an appropriately typed function process as a friend, and this friend is not a function template specialization; because the friend preempt has an explicit template-argument T, each specialization of the task class template has the appropriate specialization of the function template preempt as a friend; and each specialization of the task class template has all specializations of the function template func as friends. Similarly, each specialization of the task class template has the class template specialization task<int> as a friend, and has all specializations of the class template frd as friends.  — end example ]

A friend template may be declared within a class or class template. A friend function template may be defined within a class or class template, but a friend class template may not be defined in a class or class template. In these cases, all specializations of the friend class or friend function template are friends of the class or class template granting friendship. [ Example:

class A {
  template<class T> friend class B;                 // OK
  template<class T> friend void f(T){ /* ... */ }   // OK
};

 — end example ]

A template friend declaration specifies that all specializations of that template, whether they are implicitly instantiated, partially specialized or explicitly specialized, are friends of the class containing the template friend declaration. [ Example:

class X {
  template<class T> friend struct A;
  class Y { };
};

template<class T> struct A { X::Y ab; };            // OK
template<class T> struct A<T*> { X::Y ab; };        // OK

 — end example ]

A member of a class template may be declared to be a friend of a non-template class. In this case, the corresponding member of every specialization of the primary class template and class template partial specializations thereof is a friend of the class granting friendship. For explicit specializations and specializations of partial specializations, the corresponding member is the member (if any) that has the same name, kind (type, function, class template, or function template), template parameters, and signature as the member of the class template instantiation that would otherwise have been generated. [ Example:

template<class T> struct A {
  struct B { };
  void f();
  struct D {
    void g();
  };
};
template<> struct A<int> {
  struct B { };
  int f();
  struct D {
    void g();
  };
};

class C {
  template<class T> friend struct A<T>::B;      // grants friendship to A<int>​::​B even though
                                                // it is not a specialization of A<T>​::​B
  template<class T> friend void A<T>::f();      // does not grant friendship to A<int>​::​f()
                                                // because its return type does not match
  template<class T> friend void A<T>::D::g();   // does not grant friendship to A<int>​::​D​::​g()
                                                // because A<int>​::​D is not a specialization of A<T>​::​D
};

 — end example ]

[ Note: A friend declaration may first declare a member of an enclosing namespace scope ([temp.inject]).  — end note ]

A friend template shall not be declared in a local class.

Friend declarations shall not declare partial specializations. [ Example:

template<class T> class A { };
class X {
  template<class T> friend class A<T*>;         // error
};

 — end example ]

When a friend declaration refers to a specialization of a function template, the function parameter declarations shall not include default arguments, nor shall the inline specifier be used in such a declaration.

A primary class template declaration is one in which the class template name is an identifier. A template declaration in which the class template name is a simple-template-id is a partial specialization of the class template named in the simple-template-id. A partial specialization of a class template provides an alternative definition of the template that is used instead of the primary definition when the arguments in a specialization match those given in the partial specialization ([temp.class.spec.match]). The primary template shall be declared before any specializations of that template. A partial specialization shall be declared before the first use of a class template specialization that would make use of the partial specialization as the result of an implicit or explicit instantiation in every translation unit in which such a use occurs; no diagnostic is required.

Each class template partial specialization is a distinct template and definitions shall be provided for the members of a template partial specialization ([temp.class.spec.mfunc]).

[ Example:

template<class T1, class T2, int I> class A             { };
template<class T, int I>            class A<T, T*, I>   { };
template<class T1, class T2, int I> class A<T1*, T2, I> { };
template<class T>                   class A<int, T*, 5> { };
template<class T1, class T2, int I> class A<T1, T2*, I> { };

The first declaration declares the primary (unspecialized) class template. The second and subsequent declarations declare partial specializations of the primary template.  — end example ]

The template parameters are specified in the angle bracket enclosed list that immediately follows the keyword template. For partial specializations, the template argument list is explicitly written immediately following the class template name. For primary templates, this list is implicitly described by the template parameter list. Specifically, the order of the template arguments is the sequence in which they appear in the template parameter list. [ Example: The template argument list for the primary template in the example above is <T1, T2, I>.  — end example ] [ Note: The template argument list shall not be specified in the primary template declaration. For example,

template<class T1, class T2, int I>
class A<T1, T2, I> { };                         // error

 — end note ]

A class template partial specialization may be declared in any scope in which the corresponding primary template may be defined ([namespace.memdef], [class.mem], [temp.mem]). [ Example:

template<class T> struct A {
  struct C {
    template<class T2> struct B { };
    template<class T2> struct B<T2**> { };      // partial specialization #1
  };
};

// partial specialization of A<T>​::​C​::​B<T2>
template<class T> template<class T2>
  struct A<T>::C::B<T2*> { };                   // #2

A<short>::C::B<int*> absip;                     // uses partial specialization #2

 — end example ]

Partial specialization declarations themselves are not found by name lookup. Rather, when the primary template name is used, any previously-declared partial specializations of the primary template are also considered. One consequence is that a using-declaration which refers to a class template does not restrict the set of partial specializations which may be found through the using-declaration. [ Example:

namespace N {
  template<class T1, class T2> class A { };     // primary template
}

using N::A;                                     // refers to the primary template

namespace N {
  template<class T> class A<T, T*> { };         // partial specialization
}

A<int,int*> a;      // uses the partial specialization, which is found through the using-declaration
                    // which refers to the primary template

 — end example ]

A non-type argument is non-specialized if it is the name of a non-type parameter. All other non-type arguments are specialized.

Within the argument list of a class template partial specialization, the following restrictions apply:

• (8.1)

  The type of a template parameter corresponding to a specialized non-type argument shall not be dependent on a parameter of the specialization. [ Example:

  template <class T, T t> struct C {};
  template <class T> struct C<T, 1>;              // error
  
  template< int X, int (*array_ptr)[X] > class A {};
  int array[5];
  template< int X > class A<X,&array> { };        // error
  

   — end example ]

• (8.2)

  The specialization shall be more specialized than the primary template.

• (8.3)

  The template parameter list of a specialization shall not contain default template argument values.138

• (8.4)

  An argument shall not contain an unexpanded parameter pack. If an argument is a pack expansion, it shall be the last argument in the template argument list.

A function template defines an unbounded set of related functions. [ Example: A family of sort functions might be declared like this:

template<class T> class Array { };
template<class T> void sort(Array<T>&);

 — end example ]

A function template can be overloaded with other function templates and with non-template functions ([dcl.fct]). A non-template function is not related to a function template (i.e., it is never considered to be a specialization), even if it has the same name and type as a potentially generated function template specialization.139

A template-declaration in which the declaration is an alias-declaration declares the identifier to be an alias template. An alias template is a name for a family of types. The name of the alias template is a template-name.

When a template-id refers to the specialization of an alias template, it is equivalent to the associated type obtained by substitution of its template-arguments for the template-parameters in the type-id of the alias template. [ Note: An alias template name is never deduced. — end note ] [ Example:

template<class T> struct Alloc { /* ... */ };
template<class T> using Vec = vector<T, Alloc<T>>;
Vec<int> v;         // same as vector<int, Alloc<int>> v;

template<class T>
  void process(Vec<T>& v)
  { /* ... */ }

template<class T>
  void process(vector<T, Alloc<T>>& w)
  { /* ... */ }     // error: redefinition

template<template<class> class TT>
  void f(TT<int>);

f(v);               // error: Vec not deduced

template<template<class,class> class TT>
  void g(TT<int, Alloc<int>>);
g(v);               // OK: TT = vector

 — end example ]

However, if the template-id is dependent, subsequent template argument substitution still applies to the template-id. [ Example:

template<typename...> using void_t = void;
template<typename T> void_t<typename T::foo> f();
f<int>();           // error, int does not have a nested type foo

 — end example ]

The type-id in an alias template declaration shall not refer to the alias template being declared. The type produced by an alias template specialization shall not directly or indirectly make use of that specialization. [ Example:

template <class T> struct A;
template <class T> using B = typename A<T>::U;
template <class T> struct A {
  typedef B<T> U;
};
B<short> b;         // error: instantiation of B<short> uses own type via A<short>​::​U

 — end example ]

Like normal (non-template) classes, class templates have an injected-class-name. The injected-class-name can be used as a template-name or a type-name. When it is used with a template-argument-list, as a template-argument for a template template-parameter, or as the final identifier in the elaborated-type-specifier of a friend class template declaration, it refers to the class template itself. Otherwise, it is equivalent to the template-name followed by the template-parameters of the class template enclosed in <>.

Within the scope of a class template specialization or partial specialization, when the injected-class-name is used as a type-name, it is equivalent to the template-name followed by the template-arguments of the class template specialization or partial specialization enclosed in <>. [ Example:

template<template<class> class T> class A { };
template<class T> class Y;
template<> class Y<int> {
  Y* p;                               // meaning Y<int>
  Y<char>* q;                         // meaning Y<char>
  A<Y>* a;                            // meaning A<​::​Y>
  class B {
    template<class> friend class Y;   // meaning ​::​Y
  };
};

 — end example ]

The injected-class-name of a class template or class template specialization can be used either as a template-name or a type-name wherever it is in scope. [ Example:

template <class T> struct Base {
  Base* p;
};

template <class T> struct Derived: public Base<T> {
  typename Derived::Base* p;    // meaning Derived​::​Base<T>
};

template<class T, template<class> class U = T::template Base> struct Third { };
Third<Base<int> > t;            // OK: default argument uses injected-class-name as a template

 — end example ]

A lookup that finds an injected-class-name ([class.member.lookup]) can result in an ambiguity in certain cases (for example, if it is found in more than one base class). If all of the injected-class-names that are found refer to specializations of the same class template, and if the name is used as a template-name, the reference refers to the class template itself and not a specialization thereof, and is not ambiguous. [ Example:

template <class T> struct Base { };
template <class T> struct Derived: Base<int>, Base<char> {
  typename Derived::Base b;             // error: ambiguous
  typename Derived::Base<double> d;     // OK
};

 — end example ]

When the normal name of the template (i.e., the name from the enclosing scope, not the injected-class-name) is used, it always refers to the class template itself and not a specialization of the template. [ Example:

template<class T> class X {
  X* p;             // meaning X<T>
  X<T>* p2;
  X<int>* p3;
  ::X* p4;          // error: missing template argument list
                    // ​::​X does not refer to the injected-class-name
};

 — end example ]

A template-parameter shall not be redeclared within its scope (including nested scopes). A template-parameter shall not have the same name as the template name. [ Example:

template<class T, int i> class Y {
  int T;            // error: template-parameter redeclared
  void f() {
    char T;         // error: template-parameter redeclared
  }
};

template<class X> class X;      // error: template-parameter redeclared

 — end example ]

In the definition of a member of a class template that appears outside of the class template definition, the name of a member of the class template hides the name of a template-parameter of any enclosing class templates (but not a template-parameter of the member if the member is a class or function template). [ Example:

template<class T> struct A {
  struct B { /* ... */ };
  typedef void C;
  void f();
  template<class U> void g(U);
};

template<class B> void A<B>::f() {
  B b;              // A's B, not the template parameter
}

template<class B> template<class C> void A<B>::g(C) {
  B b;              // A's B, not the template parameter
  C c;              // the template parameter C, not A's C
}

 — end example ]

In the definition of a member of a class template that appears outside of the namespace containing the class template definition, the name of a template-parameter hides the name of a member of this namespace. [ Example:

namespace N {
  class C { };
  template<class T> class B {
    void f(T);
  };
}
template<class C> void N::B<C>::f(C) {
  C b;              // C is the template parameter, not N​::​C
}

 — end example ]

In the definition of a class template or in the definition of a member of such a template that appears outside of the template definition, for each non-dependent base class ([temp.dep.type]), if the name of the base class or the name of a member of the base class is the same as the name of a template-parameter, the base class name or member name hides the template-parameter name. [ Example:

struct A {
  struct B { /* ... */ };
  int a;
  int Y;
};

template<class B, class a> struct X : A {
  B b;              // A's B
  a b;              // error: A's a isn't a type name
};

 — end example ]

Inside a template, some constructs have semantics which may differ from one instantiation to another. Such a construct depends on the template parameters. In particular, types and expressions may depend on the type and/or value of template parameters (as determined by the template arguments) and this determines the context for name lookup for certain names. An expressions may be type-dependent (that is, its type may depend on a template parameter) or value-dependent (that is, its value when evaluated as a constant expression ([expr.const]) may depend on a template parameter) as described in this subclause. In an expression of the form:

postfix-expression ( expression-list${}_{opt}$ )

where the postfix-expression is an unqualified-id, the unqualified-id denotes a dependent name if

• (1.1)

  any of the expressions in the expression-list is a pack expansion,

• (1.2)

  any of the expressions or braced-init-lists in the expression-list is type-dependent, or

• (1.3)

  the unqualified-id is a template-id in which any of the template arguments depends on a template parameter.

If an operand of an operator is a type-dependent expression, the operator also denotes a dependent name. Such names are unbound and are looked up at the point of the template instantiation ([temp.point]) in both the context of the template definition and the context of the point of instantiation.

[ Example:

template<class T> struct X : B<T> {
  typename T::A* pa;
  void f(B<T>* pb) {
    static int i = B<T>::i;
    pb->j++;
  }
};

the base class name B<T>, the type name T​::​A, the names B<T>​::​i and pb->j explicitly depend on the template-parameter.  — end example ]

In the definition of a class or class template, the scope of a dependent base class is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member. [ Example:

typedef double A;
template<class T> class B {
  typedef int A;
};
template<class T> struct X : B<T> {
  A a;              // a has type double
};

The type name A in the definition of X<T> binds to the typedef name defined in the global namespace scope, not to the typedef name defined in the base class B<T>.  — end example ] [ Example:

struct A {
  struct B { /* ... */ };
  int a;
  int Y;
};

int a;

template<class T> struct Y : T {
  struct B { /* ... */ };
  B b;                          // The B defined in Y
  void f(int i) { a = i; }      // ​::​a
  Y* p;                         // Y<T>
};

Y<A> ya;

The members A​::​B, A​::​a, and A​::​Y of the template argument A do not affect the binding of names in Y<A>.  — end example ]

Non-dependent names used in a template definition are found using the usual name lookup and bound at the point they are used. [ Example:

void g(double);
void h();

template<class T> class Z {
public:
  void f() {
    g(1);           // calls g(double)
    h++;            // ill-formed: cannot increment function; this could be diagnosed
                    // either here or at the point of instantiation
  }
};

void g(int);        // not in scope at the point of the template definition, not considered for the call g(1)

 — end example ]

In resolving dependent names, names from the following sources are considered:

• (1.1)

  Declarations that are visible at the point of definition of the template.

• (1.2)

  Declarations from namespaces associated with the types of the function arguments both from the instantiation context ([temp.point]) and from the definition context.

Friend classes or functions can be declared within a class template. When a template is instantiated, the names of its friends are treated as if the specialization had been explicitly declared at its point of instantiation.

As with non-template classes, the names of namespace-scope friend functions of a class template specialization are not visible during an ordinary lookup unless explicitly declared at namespace scope ([class.friend]). Such names may be found under the rules for associated classes ([basic.lookup.argdep]).140 [ Example:

template<typename T> struct number {
  number(int);
  friend number gcd(number x, number y) { return 0; };
};

void g() {
  number<double> a(3), b(4);
  a = gcd(a,b);     // finds gcd because number<double> is an associated class,
                    // making gcd visible in its namespace (global scope)
  b = gcd(3,4);     // ill-formed; gcd is not visible
}

 — end example ]

Unless a class template specialization has been explicitly instantiated or explicitly specialized, the class template specialization is implicitly instantiated when the specialization is referenced in a context that requires a completely-defined object type or when the completeness of the class type affects the semantics of the program. [ Note: In particular, if the semantics of an expression depend on the member or base class lists of a class template specialization, the class template specialization is implicitly generated. For instance, deleting a pointer to class type depends on whether or not the class declares a destructor, and a conversion between pointers to class type depends on the inheritance relationship between the two classes involved.  — end note ] [ Example:

template<class T> class B { /* ... */ };
template<class T> class D : public B<T> { /* ... */ };

void f(void*);
void f(B<int>*);

void g(D<int>* p, D<char>* pp, D<double>* ppp) {
  f(p);             // instantiation of D<int> required: call f(B<int>*)
  B<char>* q = pp;  // instantiation of D<char> required: convert D<char>* to B<char>*
  delete ppp;       // instantiation of D<double> required
}

 — end example ] If a class template has been declared, but not defined, at the point of instantiation, the instantiation yields an incomplete class type ([basic.types]). [ Example:

template<class T> class X;
X<char> ch;         // error: incomplete type X<char>

 — end example ] [ Note: Within a template declaration, a local class or enumeration and the members of a local class are never considered to be entities that can be separately instantiated (this includes their default arguments, noexcept-specifiers, and non-static data member initializers, if any). As a result, the dependent names are looked up, the semantic constraints are checked, and any templates used are instantiated as part of the instantiation of the entity within which the local class or enumeration is declared.  — end note ]

The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions, default arguments, or noexcept-specifiers of the class member functions, member classes, scoped member enumerations, static data members, member templates, and friends; and it causes the implicit instantiation of the definitions of unscoped member enumerations and member anonymous unions. However, for the purpose of determining whether an instantiated redeclaration is valid according to [basic.def.odr] and [class.mem], a declaration that corresponds to a definition in the template is considered to be a definition. [ Example:

template<class T, class U>
struct Outer {
  template<class X, class Y> struct Inner;
  template<class Y> struct Inner<T, Y>;         // #1a
  template<class Y> struct Inner<T, Y> { };     // #1b; OK: valid redeclaration of #1a
  template<class Y> struct Inner<U, Y> { };     // #2
};

Outer<int, int> outer;                          // error at #2

Outer<int, int>​::​Inner<int, Y> is redeclared at #1b. (It is not defined but noted as being associated with a definition in Outer<T, U>.) #2 is also a redeclaration of #1a. It is noted as associated with a definition, so it is an invalid redeclaration of the same partial specialization.

template<typename T> struct Friendly {
  template<typename U> friend int f(U) { return sizeof(T); }
};
Friendly<char> fc;
Friendly<float> ff;                             // ill-formed: produces second definition of f(U)

 — end example ]

Unless a member of a class template or a member template has been explicitly instantiated or explicitly specialized, the specialization of the member is implicitly instantiated when the specialization is referenced in a context that requires the member definition to exist; in particular, the initialization (and any associated side effects) of a static data member does not occur unless the static data member is itself used in a way that requires the definition of the static data member to exist.

Unless a function template specialization has been explicitly instantiated or explicitly specialized, the function template specialization is implicitly instantiated when the specialization is referenced in a context that requires a function definition to exist. A function whose declaration was instantiated from a friend function definition is implicitly instantiated when it is referenced in a context that requires a function definition to exist. Unless a call is to a function template explicit specialization or to a member function of an explicitly specialized class template, a default argument for a function template or a member function of a class template is implicitly instantiated when the function is called in a context that requires the value of the default argument.

[ Example:

template<class T> struct Z {
  void f();
  void g();
};

void h() {
  Z<int> a;         // instantiation of class Z<int> required
  Z<char>* p;       // instantiation of class Z<char> not required
  Z<double>* q;     // instantiation of class Z<double> not required

  a.f();            // instantiation of Z<int>​::​f() required
  p->g();           // instantiation of class Z<char> required, and
                    // instantiation of Z<char>​::​g() required
}

Nothing in this example requires class Z<double>, Z<int>​::​g(), or Z<char>​::​f() to be implicitly instantiated.  — end example ]

Unless a variable template specialization has been explicitly instantiated or explicitly specialized, the variable template specialization is implicitly instantiated when the specialization is used. A default template argument for a variable template is implicitly instantiated when the variable template is referenced in a context that requires the value of the default argument.

If the function selected by overload resolution can be determined without instantiating a class template definition, it is unspecified whether that instantiation actually takes place. [ Example:

template <class T> struct S {
  operator int();
};

void f(int);
void f(S<int>&);
void f(S<float>);

void g(S<int>& sr) {
  f(sr);            // instantiation of S<int> allowed but not required
                    // instantiation of S<float> allowed but not required
};

 — end example ]

If a function template or a member function template specialization is used in a way that involves overload resolution, a declaration of the specialization is implicitly instantiated ([temp.over]).

An implementation shall not implicitly instantiate a function template, a variable template, a member template, a non-virtual member function, a member class, a static data member of a class template, or a substatement of a constexpr if statement, unless such instantiation is required. It is unspecified whether or not an implementation implicitly instantiates a virtual member function of a class template if the virtual member function would not otherwise be instantiated. The use of a template specialization in a default argument shall not cause the template to be implicitly instantiated except that a class template may be instantiated where its complete type is needed to determine the correctness of the default argument. The use of a default argument in a function call causes specializations in the default argument to be implicitly instantiated.

Implicitly instantiated class, function, and variable template specializations are placed in the namespace where the template is defined. Implicitly instantiated specializations for members of a class template are placed in the namespace where the enclosing class template is defined. Implicitly instantiated member templates are placed in the namespace where the enclosing class or class template is defined. [ Example:

namespace N {
  template<class T> class List {
  public:
    T* get();
  };
}

template<class K, class V> class Map {
public:
  N::List<V> lt;
  V get(K);
};

void g(Map<const char*,int>& m) {
  int i = m.get("Nicholas");
}

a call of lt.get() from Map<const char*,int>​::​get() would place List<int>​::​get() in the namespace N rather than in the global namespace.  — end example ]

If a function template f is called in a way that requires a default argument to be used, the dependent names are looked up, the semantics constraints are checked, and the instantiation of any template used in the default argument is done as if the default argument had been an initializer used in a function template specialization with the same scope, the same template parameters and the same access as that of the function template f used at that point, except that the scope in which a closure type is declared ([expr.prim.lambda.closure]) – and therefore its associated namespaces – remain as determined from the context of the definition for the default argument. This analysis is called default argument instantiation. The instantiated default argument is then used as the argument of f.

Each default argument is instantiated independently. [ Example:

template<class T> void f(T x, T y = ydef(T()), T z = zdef(T()));

class  A { };

A zdef(A);

void g(A a, A b, A c) {
  f(a, b, c);       // no default argument instantiation
  f(a, b);          // default argument z = zdef(T()) instantiated
  f(a);             // ill-formed; ydef is not declared
}

 — end example ]

The noexcept-specifier of a function template specialization is not instantiated along with the function declaration; it is instantiated when needed ([except.spec]). If such an noexcept-specifier is needed but has not yet been instantiated, the dependent names are looked up, the semantics constraints are checked, and the instantiation of any template used in the noexcept-specifier is done as if it were being done as part of instantiating the declaration of the specialization at that point.

[ Note: [temp.point] defines the point of instantiation of a template specialization.  — end note ]

There is an implementation-defined quantity that specifies the limit on the total depth of recursive instantiations ([implimits]), which could involve more than one template. The result of an infinite recursion in instantiation is undefined. [ Example:

template<class T> class X {
  X<T>* p;          // OK
  X<T*> a;          // implicit generation of X<T> requires
                    // the implicit instantiation of X<T*> which requires
                    // the implicit instantiation of X<T**> which …
};

 — end example ]

A class, function, variable, or member template specialization can be explicitly instantiated from its template. A member function, member class or static data member of a class template can be explicitly instantiated from the member definition associated with its class template. An explicit instantiation of a function template or member function of a class template shall not use the inline or constexpr specifiers.

The syntax for explicit instantiation is:

explicit-instantiation:
	extern${}_{opt}$ template declaration

There are two forms of explicit instantiation: an explicit instantiation definition and an explicit instantiation declaration. An explicit instantiation declaration begins with the extern keyword.

If the explicit instantiation is for a class or member class, the elaborated-type-specifier in the declaration shall include a simple-template-id. If the explicit instantiation is for a function or member function, the unqualified-id in the declaration shall be either a template-id or, where all template arguments can be deduced, a template-name or operator-function-id. [ Note: The declaration may declare a qualified-id, in which case the unqualified-id of the qualified-id must be a template-id.  — end note ] If the explicit instantiation is for a member function, a member class or a static data member of a class template specialization, the name of the class template specialization in the qualified-id for the member name shall be a simple-template-id. If the explicit instantiation is for a variable, the unqualified-id in the declaration shall be a template-id. An explicit instantiation shall appear in an enclosing namespace of its template. If the name declared in the explicit instantiation is an unqualified name, the explicit instantiation shall appear in the namespace where its template is declared or, if that namespace is inline ([namespace.def]), any namespace from its enclosing namespace set. [ Note: Regarding qualified names in declarators, see [dcl.meaning].  — end note ] [ Example:

template<class T> class Array { void mf(); };
template class Array<char>;
template void Array<int>::mf();

template<class T> void sort(Array<T>& v) { /* ... */ }
template void sort(Array<char>&);       // argument is deduced here

namespace N {
  template<class T> void f(T&) { }
}
template void N::f<int>(int&);

 — end example ]

A declaration of a function template, a variable template, a member function or static data member of a class template, or a member function template of a class or class template shall precede an explicit instantiation of that entity. A definition of a class template, a member class of a class template, or a member class template of a class or class template shall precede an explicit instantiation of that entity unless the explicit instantiation is preceded by an explicit specialization of the entity with the same template arguments. If the declaration of the explicit instantiation names an implicitly-declared special member function, the program is ill-formed.

For a given set of template arguments, if an explicit instantiation of a template appears after a declaration of an explicit specialization for that template, the explicit instantiation has no effect. Otherwise, for an explicit instantiation definition the definition of a function template, a variable template, a member function template, or a member function or static data member of a class template shall be present in every translation unit in which it is explicitly instantiated.

An explicit instantiation of a class, function template, or variable template specialization is placed in the namespace in which the template is defined. An explicit instantiation for a member of a class template is placed in the namespace where the enclosing class template is defined. An explicit instantiation for a member template is placed in the namespace where the enclosing class or class template is defined. [ Example:

namespace N {
  template<class T> class Y { void mf() { } };
}

template class Y<int>;          // error: class template Y not visible in the global namespace

using N::Y;
template class Y<int>;          // error: explicit instantiation outside of the namespace of the template

template class N::Y<char*>;             // OK: explicit instantiation in namespace N
template void N::Y<double>::mf();       // OK: explicit instantiation in namespace N

 — end example ]

A trailing template-argument can be left unspecified in an explicit instantiation of a function template specialization or of a member function template specialization provided it can be deduced from the type of a function parameter ([temp.deduct]). [ Example:

template<class T> class Array { /* ... */ };
template<class T> void sort(Array<T>& v) { /* ... */ }

// instantiate sort(Array<int>&) – template-argument deduced
template void sort<>(Array<int>&);

 — end example ]

An explicit instantiation that names a class template specialization is also an explicit instantiation of the same kind (declaration or definition) of each of its members (not including members inherited from base classes and members that are templates) that has not been previously explicitly specialized in the translation unit containing the explicit instantiation, except as described below. [ Note: In addition, it will typically be an explicit instantiation of certain implementation-dependent data about the class.  — end note ]

An explicit instantiation definition that names a class template specialization explicitly instantiates the class template specialization and is an explicit instantiation definition of only those members that have been defined at the point of instantiation.

Except for inline functions and variables, declarations with types deduced from their initializer or return value ([dcl.spec.auto]), const variables of literal types, variables of reference types, and class template specializations, explicit instantiation declarations have the effect of suppressing the implicit instantiation of the entity to which they refer. [ Note: The intent is that an inline function that is the subject of an explicit instantiation declaration will still be implicitly instantiated when odr-used so that the body can be considered for inlining, but that no out-of-line copy of the inline function would be generated in the translation unit. — end note ]

If an entity is the subject of both an explicit instantiation declaration and an explicit instantiation definition in the same translation unit, the definition shall follow the declaration. An entity that is the subject of an explicit instantiation declaration and that is also used in a way that would otherwise cause an implicit instantiation in the translation unit shall be the subject of an explicit instantiation definition somewhere in the program; otherwise the program is ill-formed, no diagnostic required. [ Note: This rule does apply to inline functions even though an explicit instantiation declaration of such an entity has no other normative effect. This is needed to ensure that if the address of an inline function is taken in a translation unit in which the implementation chose to suppress the out-of-line body, another translation unit will supply the body. — end note ] An explicit instantiation declaration shall not name a specialization of a template with internal linkage.

The usual access checking rules do not apply to names used to specify explicit instantiations. [ Note: In particular, the template arguments and names used in the function declarator (including parameter types, return types and exception specifications) may be private types or objects which would normally not be accessible and the template may be a member template or member function which would not normally be accessible.  — end note ]

An explicit instantiation does not constitute a use of a default argument, so default argument instantiation is not done. [ Example:

char* p = 0;
template<class T> T g(T x = &p) { return x; }
template int g<int>(int);       // OK even though &p isn't an int.

 — end example ]

An explicit specialization of any of the following:

• (1.1)

  function template

• (1.2)

  class template

• (1.3)

  variable template

• (1.4)

  member function of a class template

• (1.5)

  static data member of a class template

• (1.6)

  member class of a class template

• (1.7)

  member enumeration of a class template

• (1.8)

  member class template of a class or class template

• (1.9)

  member function template of a class or class template

can be declared by a declaration introduced by template<>; that is:

explicit-specialization:
	template < > declaration

[ Example:

template<class T> class stream;

template<> class stream<char> { /* ... */ };

template<class T> class Array { /* ... */ };
template<class T> void sort(Array<T>& v) { /* ... */ }

template<> void sort<char*>(Array<char*>&);

Given these declarations, stream<char> will be used as the definition of streams of chars; other streams will be handled by class template specializations instantiated from the class template. Similarly, sort<char*> will be used as the sort function for arguments of type Array<char*>; other Array types will be sorted by functions generated from the template.  — end example ]

An explicit specialization may be declared in any scope in which the corresponding primary template may be defined ([namespace.memdef], [class.mem], [temp.mem]).

A declaration of a function template, class template, or variable template being explicitly specialized shall precede the declaration of the explicit specialization. [ Note: A declaration, but not a definition of the template is required.  — end note ] The definition of a class or class template shall precede the declaration of an explicit specialization for a member template of the class or class template. [ Example:

template<> class X<int> { /* ... */ };          // error: X not a template

template<class T> class X;

template<> class X<char*> { /* ... */ };        // OK: X is a template

 — end example ]

A member function, a member function template, a member class, a member enumeration, a member class template, a static data member, or a static data member template of a class template may be explicitly specialized for a class specialization that is implicitly instantiated; in this case, the definition of the class template shall precede the explicit specialization for the member of the class template. If such an explicit specialization for the member of a class template names an implicitly-declared special member function (Clause [special]), the program is ill-formed.

A member of an explicitly specialized class is not implicitly instantiated from the member declaration of the class template; instead, the member of the class template specialization shall itself be explicitly defined if its definition is required. In this case, the definition of the class template explicit specialization shall be in scope at the point at which the member is defined. The definition of an explicitly specialized class is unrelated to the definition of a generated specialization. That is, its members need not have the same names, types, etc. as the members of a generated specialization. Members of an explicitly specialized class template are defined in the same manner as members of normal classes, and not using the template<> syntax. The same is true when defining a member of an explicitly specialized member class. However, template<> is used in defining a member of an explicitly specialized member class template that is specialized as a class template. [ Example:

template<class T> struct A {
  struct B { };
  template<class U> struct C { };
};

template<> struct A<int> {
  void f(int);
};

void h() {
  A<int> a;
  a.f(16);          // A<int>​::​f must be defined somewhere
}

// template<> not used for a member of an explicitly specialized class template
void A<int>::f(int) { /* ... */ }

template<> struct A<char>::B {
  void f();
};
// template<> also not used when defining a member of an explicitly specialized member class
void A<char>::B::f() { /* ... */ }

template<> template<class U> struct A<char>::C {
  void f();
};
// template<> is used when defining a member of an explicitly specialized member class template
// specialized as a class template
template<>
template<class U> void A<char>::C<U>::f() { /* ... */ }

template<> struct A<short>::B {
  void f();
};
template<> void A<short>::B::f() { /* ... */ }              // error: template<> not permitted

template<> template<class U> struct A<short>::C {
  void f();
};
template<class U> void A<short>::C<U>::f() { /* ... */ }    // error: template<> required

 — end example ]

If a template, a member template or a member of a class template is explicitly specialized then that specialization shall be declared before the first use of that specialization that would cause an implicit instantiation to take place, in every translation unit in which such a use occurs; no diagnostic is required. If the program does not provide a definition for an explicit specialization and either the specialization is used in a way that would cause an implicit instantiation to take place or the member is a virtual member function, the program is ill-formed, no diagnostic required. An implicit instantiation is never generated for an explicit specialization that is declared but not defined. [ Example:

class String { };
template<class T> class Array { /* ... */ };
template<class T> void sort(Array<T>& v) { /* ... */ }

void f(Array<String>& v) {
  sort(v);          // use primary template sort(Array<T>&), T is String
}

template<> void sort<String>(Array<String>& v);     // error: specialization after use of primary template
template<> void sort<>(Array<char*>& v);            // OK: sort<char*> not yet used
template<class T> struct A {
  enum E : T;
  enum class S : T;
};
template<> enum A<int>::E : int { eint };           // OK
template<> enum class A<int>::S : int { sint };     // OK
template<class T> enum A<T>::E : T { eT };
template<class T> enum class A<T>::S : T { sT };
template<> enum A<char>::E : char { echar };        // ill-formed, A<char>​::​E was instantiated
                                                    // when A<char> was instantiated
template<> enum class A<char>::S : char { schar };  // OK

 — end example ]

The placement of explicit specialization declarations for function templates, class templates, variable templates, member functions of class templates, static data members of class templates, member classes of class templates, member enumerations of class templates, member class templates of class templates, member function templates of class templates, static data member templates of class templates, member functions of member templates of class templates, member functions of member templates of non-template classes, static data member templates of non-template classes, member function templates of member classes of class templates, etc., and the placement of partial specialization declarations of class templates, variable templates, member class templates of non-template classes, static data member templates of non-template classes, member class templates of class templates, etc., can affect whether a program is well-formed according to the relative positioning of the explicit specialization declarations and their points of instantiation in the translation unit as specified above and below. When writing a specialization, be careful about its location; or to make it compile will be such a trial as to kindle its self-immolation.

A template explicit specialization is in the scope of the namespace in which the template was defined. [ Example:

namespace N {
  template<class T> class X { /* ... */ };
  template<class T> class Y { /* ... */ };

  template<> class X<int> { /* ... */ };        // OK: specialization in same namespace
  template<> class Y<double>;                   // forward-declare intent to specialize for double
}

template<> class N::Y<double> { /* ... */ };    // OK: specialization in enclosing namespace
template<> class N::Y<short> { /* ... */ };     // OK: specialization in enclosing namespace

 — end example ]

A simple-template-id that names a class template explicit specialization that has been declared but not defined can be used exactly like the names of other incompletely-defined classes ([basic.types]). [ Example:

template<class T> class X;                      // X is a class template
template<> class X<int>;

X<int>* p;                                      // OK: pointer to declared class X<int>
X<int> x;                                       // error: object of incomplete class X<int>

 — end example ]

A trailing template-argument can be left unspecified in the template-id naming an explicit function template specialization provided it can be deduced from the function argument type. [ Example:

template<class T> class Array { /* ... */ };
template<class T> void sort(Array<T>& v);

// explicit specialization for sort(Array<int>&)
// with deduced template-argument of type int
template<> void sort(Array<int>&);

 — end example ]

A function with the same name as a template and a type that exactly matches that of a template specialization is not an explicit specialization ([temp.fct]).

An explicit specialization of a function or variable template is inline only if it is declared with the inline specifier or defined as deleted, and independently of whether its function or variable template is inline. [ Example:

template<class T> void f(T) { /* ... */ }
template<class T> inline T g(T) { /* ... */ }

template<> inline void f<>(int) { /* ... */ }   // OK: inline
template<> int g<>(int) { /* ... */ }           // OK: not inline

 — end example ]

An explicit specialization of a static data member of a template or an explicit specialization of a static data member template is a definition if the declaration includes an initializer; otherwise, it is a declaration. [ Note: The definition of a static data member of a template that requires default-initialization must use a braced-init-list:

template<> X Q<int>::x;                         // declaration
template<> X Q<int>::x ();                      // error: declares a function
template<> X Q<int>::x { };                     // definition

 — end note ]

A member or a member template of a class template may be explicitly specialized for a given implicit instantiation of the class template, even if the member or member template is defined in the class template definition. An explicit specialization of a member or member template is specified using the syntax for explicit specialization. [ Example:

template<class T> struct A {
  void f(T);
  template<class X1> void g1(T, X1);
  template<class X2> void g2(T, X2);
  void h(T) { }
};

// specialization
template<> void A<int>::f(int);

// out of class member template definition
template<class T> template<class X1> void A<T>::g1(T, X1) { }

// member template specialization
template<> template<class X1> void A<int>::g1(int, X1);

// member template specialization
template<> template<>
  void A<int>::g1(int, char);           // X1 deduced as char
template<> template<>
  void A<int>::g2<char>(int, char);     // X2 specified as char

// member specialization even if defined in class definition
template<> void A<int>::h(int) { }

 — end example ]

A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a template<> for each enclosing class template that is explicitly specialized. [ Example:

template<class T1> class A {
  template<class T2> class B {
    void mf();
  };
};
template<> template<> class A<int>::B<double>;
template<> template<> void A<char>::B<char>::mf();

 — end example ]

In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well. In such explicit specialization declaration, the keyword template followed by a template-parameter-list shall be provided instead of the template<> preceding the explicit specialization declaration of the member. The types of the template-parameters in the template-parameter-list shall be the same as those specified in the primary template definition. [ Example:

template <class T1> class A {
  template<class T2> class B {
    template<class T3> void mf1(T3);
    void mf2();
  };
};
template <> template <class X>
  class A<int>::B {
      template <class T> void mf1(T);
  };
template <> template <> template<class T>
  void A<int>::B<double>::mf1(T t) { }
template <class Y> template <>
  void A<Y>::B<double>::mf2() { }       // ill-formed; B<double> is specialized but
                                        // its enclosing class template A is not

 — end example ]

A specialization of a member function template, member class template, or static data member template of a non-specialized class template is itself a template.

An explicit specialization declaration shall not be a friend declaration.

Default function arguments shall not be specified in a declaration or a definition for one of the following explicit specializations:

• (19.1)

  the explicit specialization of a function template;

• (19.2)

  the explicit specialization of a member function template;

• (19.3)

  the explicit specialization of a member function of a class template where the class template specialization to which the member function specialization belongs is implicitly instantiated. [ Note: Default function arguments may be specified in the declaration or definition of a member function of a class template specialization that is explicitly specialized.  — end note ]

Template arguments can be specified when referring to a function template specialization by qualifying the function template name with the list of template-arguments in the same way as template-arguments are specified in uses of a class template specialization. [ Example:

template<class T> void sort(Array<T>& v);
void f(Array<dcomplex>& cv, Array<int>& ci) {
  sort<dcomplex>(cv);                   // sort(Array<dcomplex>&)
  sort<int>(ci);                        // sort(Array<int>&)
}

and

template<class U, class V> U convert(V v);

void g(double d) {
  int i = convert<int,double>(d);       // int convert(double)
  char c = convert<char,double>(d);     // char convert(double)
}

 — end example ]

A template argument list may be specified when referring to a specialization of a function template

• (2.1)

  when a function is called,

• (2.2)

  when the address of a function is taken, when a function initializes a reference to function, or when a pointer to member function is formed,

• (2.3)

  in an explicit specialization,

• (2.4)

  in an explicit instantiation, or

• (2.5)

  in a friend declaration.

Trailing template arguments that can be deduced or obtained from default template-arguments may be omitted from the list of explicit template-arguments. A trailing template parameter pack not otherwise deduced will be deduced to an empty sequence of template arguments. If all of the template arguments can be deduced, they may all be omitted; in this case, the empty template argument list <> itself may also be omitted. In contexts where deduction is done and fails, or in contexts where deduction is not done, if a template argument list is specified and it, along with any default template arguments, identifies a single function template specialization, then the template-id is an lvalue for the function template specialization. [ Example:

template<class X, class Y> X f(Y);
template<class X, class Y, class ... Z> X g(Y);
void h() {
  int i = f<int>(5.6);          // Y is deduced to be double
  int j = f(5.6);               // ill-formed: X cannot be deduced
  f<void>(f<int, bool>);        // Y for outer f deduced to be int (*)(bool)
  f<void>(f<int>);              // ill-formed: f<int> does not denote a single function template specialization
  int k = g<int>(5.6);          // Y is deduced to be double, Z is deduced to an empty sequence
  f<void>(g<int, bool>);        // Y for outer f is deduced to be int (*)(bool),
                                // Z is deduced to an empty sequence
}

 — end example ]

[ Note: An empty template argument list can be used to indicate that a given use refers to a specialization of a function template even when a non-template function ([dcl.fct]) is visible that would otherwise be used. For example:

template <class T> int f(T);    // #1
int f(int);                     // #2
int k = f(1);                   // uses #2
int l = f<>(1);                 // uses #1

 — end note ]

Template arguments that are present shall be specified in the declaration order of their corresponding template-parameters. The template argument list shall not specify more template-arguments than there are corresponding template-parameters unless one of the template-parameters is a template parameter pack. [ Example:

template<class X, class Y, class Z> X f(Y,Z);
template<class ... Args> void f2();
void g() {
  f<int,const char*,double>("aa",3.0);
  f<int,const char*>("aa",3.0); // Z is deduced to be double
  f<int>("aa",3.0);             // Y is deduced to be const char*, and Z is deduced to be double
  f("aa",3.0);                  // error: X cannot be deduced
  f2<char, short, int, long>(); // OK
}

 — end example ]

Implicit conversions (Clause [conv]) will be performed on a function argument to convert it to the type of the corresponding function parameter if the parameter type contains no template-parameters that participate in template argument deduction. [ Note: Template parameters do not participate in template argument deduction if they are explicitly specified. For example,

template<class T> void f(T);

class Complex {
  Complex(double);
};

void g() {
  f<Complex>(1);    // OK, means f<Complex>(Complex(1))
}

 — end note ]

[ Note: Because the explicit template argument list follows the function template name, and because conversion member function templates and constructor member function templates are called without using a function name, there is no way to provide an explicit template argument list for these function templates.  — end note ]

[ Note: For simple function names, argument dependent lookup applies even when the function name is not visible within the scope of the call. This is because the call still has the syntactic form of a function call ([basic.lookup.unqual]). But when a function template with explicit template arguments is used, the call does not have the correct syntactic form unless there is a function template with that name visible at the point of the call. If no such name is visible, the call is not syntactically well-formed and argument-dependent lookup does not apply. If some such name is visible, argument dependent lookup applies and additional function templates may be found in other namespaces. [ Example:

namespace A {
  struct B { };
  template<int X> void f(B);
}
namespace C {
  template<class T> void f(T t);
}
void g(A::B b) {
  f<3>(b);          // ill-formed: not a function call
  A::f<3>(b);       // well-formed
  C::f<3>(b);       // ill-formed; argument dependent lookup applies only to unqualified names
  using C::f;
  f<3>(b);          // well-formed because C​::​f is visible; then A​::​f is found by argument dependent lookup
}

 — end example ]  — end note ]

Template argument deduction can extend the sequence of template arguments corresponding to a template parameter pack, even when the sequence contains explicitly specified template arguments. [ Example:

template<class ... Types> void f(Types ... values);

void g() {
  f<int*, float*>(0, 0, 0);     // Types is deduced to the sequence int*, float*, int
}

 — end example ]

When a function template specialization is referenced, all of the template arguments shall have values. The values can be explicitly specified or, in some cases, be deduced from the use or obtained from default template-arguments. [ Example:

void f(Array<dcomplex>& cv, Array<int>& ci) {
  sort(cv);                     // calls sort(Array<dcomplex>&)
  sort(ci);                     // calls sort(Array<int>&)
}

and

void g(double d) {
  int i = convert<int>(d);      // calls convert<int,double>(double)
  int c = convert<char>(d);     // calls convert<char,double>(double)
}

 — end example ]

When an explicit template argument list is specified, the template arguments must be compatible with the template parameter list and must result in a valid function type as described below; otherwise type deduction fails. Specifically, the following steps are performed when evaluating an explicitly specified template argument list with respect to a given function template:

• (2.1)

  The specified template arguments must match the template parameters in kind (i.e., type, non-type, template). There must not be more arguments than there are parameters unless at least one parameter is a template parameter pack, and there shall be an argument for each non-pack parameter. Otherwise, type deduction fails.

• (2.2)

  Non-type arguments must match the types of the corresponding non-type template parameters, or must be convertible to the types of the corresponding non-type parameters as specified in [temp.arg.nontype], otherwise type deduction fails.

• (2.3)

  The specified template argument values are substituted for the corresponding template parameters as specified below.

After this substitution is performed, the function parameter type adjustments described in [dcl.fct] are performed. [ Example: A parameter type of “void (const int, int[5])” becomes “void(*)(int,int*)”.  — end example ] [ Note: A top-level qualifier in a function parameter declaration does not affect the function type but still affects the type of the function parameter variable within the function.  — end note ] [ Example:

template <class T> void f(T t);
template <class X> void g(const X x);
template <class Z> void h(Z, Z*);

int main() {
  // #1: function type is f(int), t is non const
  f<int>(1);

  // #2: function type is f(int), t is const
  f<const int>(1);

  // #3: function type is g(int), x is const
  g<int>(1);

  // #4: function type is g(int), x is const
  g<const int>(1);

  // #5: function type is h(int, const int*)
  h<const int>(1,0);
}

 — end example ]

[ Note: f<int>(1) and f<const int>(1) call distinct functions even though both of the functions called have the same function type.  — end note ]

The resulting substituted and adjusted function type is used as the type of the function template for template argument deduction. If a template argument has not been deduced and its corresponding template parameter has a default argument, the template argument is determined by substituting the template arguments determined for preceding template parameters into the default argument. If the substitution results in an invalid type, as described above, type deduction fails. [ Example:

template <class T, class U = double>
void f(T t = 0, U u = 0);

void g() {
  f(1, 'c');        // f<int,char>(1,'c')
  f(1);             // f<int,double>(1,0)
  f();              // error: T cannot be deduced
  f<int>();         // f<int,double>(0,0)
  f<int,char>();    // f<int,char>(0,0)
}

 — end example ]

When all template arguments have been deduced or obtained from default template arguments, all uses of template parameters in the template parameter list of the template and the function type are replaced with the corresponding deduced or default argument values. If the substitution results in an invalid type, as described above, type deduction fails.

At certain points in the template argument deduction process it is necessary to take a function type that makes use of template parameters and replace those template parameters with the corresponding template arguments. This is done at the beginning of template argument deduction when any explicitly specified template arguments are substituted into the function type, and again at the end of template argument deduction when any template arguments that were deduced or obtained from default arguments are substituted.

The substitution occurs in all types and expressions that are used in the function type and in template parameter declarations. The expressions include not only constant expressions such as those that appear in array bounds or as nontype template arguments but also general expressions (i.e., non-constant expressions) inside sizeof, decltype, and other contexts that allow non-constant expressions. The substitution proceeds in lexical order and stops when a condition that causes deduction to fail is encountered. [ Note: The equivalent substitution in exception specifications is done only when the noexcept-specifier is instantiated, at which point a program is ill-formed if the substitution results in an invalid type or expression.  — end note ] [ Example:

template <class T> struct A { using X = typename T::X; };
template <class T> typename T::X f(typename A<T>::X);
template <class T> void f(...) { }
template <class T> auto g(typename A<T>::X) -> typename T::X;
template <class T> void g(...) { }

void h() {
  f<int>(0);        // OK, substituting return type causes deduction to fail
  g<int>(0);        // error, substituting parameter type instantiates A<int>
}

 — end example ]

If a substitution results in an invalid type or expression, type deduction fails. An invalid type or expression is one that would be ill-formed, with a diagnostic required, if written using the substituted arguments. [ Note: If no diagnostic is required, the program is still ill-formed. Access checking is done as part of the substitution process.  — end note ] Only invalid types and expressions in the immediate context of the function type and its template parameter types can result in a deduction failure. [ Note: The substitution into types and expressions can result in effects such as the instantiation of class template specializations and/or function template specializations, the generation of implicitly-defined functions, etc. Such effects are not in the “immediate context” and can result in the program being ill-formed. — end note ]

[ Example:

struct X { };
struct Y {
  Y(X){}
};

template <class T> auto f(T t1, T t2) -> decltype(t1 + t2);     // #1
X f(Y, Y);          // #2

X x1, x2;
X x3 = f(x1, x2);   // deduction fails on #1 (cannot add X+X), calls #2

 — end example ]

[ Note: Type deduction may fail for the following reasons:

• (8.1)

  Attempting to instantiate a pack expansion containing multiple parameter packs of differing lengths.

• (8.2)

  Attempting to create an array with an element type that is void, a function type, a reference type, or an abstract class type, or attempting to create an array with a size that is zero or negative. [ Example:

  template <class T> int f(T[5]);
  int I = f<int>(0);
  int j = f<void>(0);             // invalid array
  

   — end example ]

• (8.3)

  Attempting to use a type that is not a class or enumeration type in a qualified name. [ Example:

  template <class T> int f(typename T::B*);
  int i = f<int>(0);

   — end example ]

• (8.4)

  Attempting to use a type in a nested-name-specifier of a qualified-id when that type does not contain the specified member, or

  • (8.4.1)

    the specified member is not a type where a type is required, or

  • (8.4.2)

    the specified member is not a template where a template is required, or

  • (8.4.3)

    the specified member is not a non-type where a non-type is required.

  [ Example:

  template <int I> struct X { };
  template <template <class T> class> struct Z { };
  template <class T> void f(typename T::Y*){}
  template <class T> void g(X<T::N>*){}
  template <class T> void h(Z<T::template TT>*){}
  struct A {};
  struct B { int Y; };
  struct C {
    typedef int N;
  };
  struct D {
    typedef int TT;
  };
  
  int main() {
    // Deduction fails in each of these cases:
    f<A>(0);          // A does not contain a member Y
    f<B>(0);          // The Y member of B is not a type
    g<C>(0);          // The N member of C is not a non-type
    h<D>(0);          // The TT member of D is not a template
  }

   — end example ]

• (8.5)

  Attempting to create a pointer to reference type.

• (8.6)

  Attempting to create a reference to void.

• (8.7)

  Attempting to create “pointer to member of T” when T is not a class type. [ Example:

  template <class T> int f(int T::*);
  int i = f<int>(0);

   — end example ]

• (8.8)

  Attempting to give an invalid type to a non-type template parameter. [ Example:

  template <class T, T> struct S {};
  template <class T> int f(S<T, T()>*);
  struct X {};
  int i0 = f<X>(0);

   — end example ]

• (8.9)

  Attempting to perform an invalid conversion in either a template argument expression, or an expression used in the function declaration. [ Example:

  template <class T, T*> int f(int);
  int i2 = f<int,1>(0);           // can't conv 1 to int*
  

   — end example ]

• (8.10)

  Attempting to create a function type in which a parameter has a type of void, or in which the return type is a function type or array type.

• (8.11)

  Attempting to create a function type in which a parameter type or the return type is an abstract class type.

 — end note ]

[ Example: In the following example, assuming a signed char cannot represent the value 1000, a narrowing conversion would be required to convert the template-argument of type int to signed char, therefore substitution fails for the second template ([temp.arg.nontype]).

template <int> int f(int);
template <signed char> int f(int);
int i1 = f<1000>(0);            // OK
int i2 = f<1>(0);               // ambiguous; not narrowing

 — end example ]