14 Templates [temp]

A template specialization ([temp.spec]) can be referred to by a template-id:

simple-template-id:
  template-name < template-argument-listopt >

template-id:
  simple-template-id
  operator-function-id < template-argument-listopt >
  literal-operator-id < template-argument-listopt >

template-name:
  identifier

template-argument-list:
  template-argument ...opt
  template-argument-list , template-argument ...opt

template-argument:
  constant-expression
  type-id
  id-expression

[ Note: The name lookup rules ([basic.lookup]) are used to associate the use of a name with a template declaration; that is, to identify a name as a template-name.  — end note ]

For a template-name to be explicitly qualified by the template arguments, the name must be known to refer to a template.

After name lookup ([basic.lookup]) finds that a name is a template-name or that an operator-function-id or a literal-operator-id refers to a set of overloaded functions any member of which is a function template, if this is followed by a <, the < is always taken as the delimiter of a template-argument-list and never as the less-than operator. When parsing a template-argument-list, the first non-nested >138 is taken as the ending delimiter rather than a greater-than operator. Similarly, the first non-nested >> is treated as two consecutive but distinct > tokens, the first of which is taken as the end of the template-argument-list and completes the template-id. [ Note: The second > token produced by this replacement rule may terminate an enclosing template-id construct or it may be part of a different construct (e.g. a cast). — end note ] [ Example:

template<int i> class X { /* ... */ };

X< 1>2 > x1;                        // syntax error
X<(1>2)> x2;                        // OK

template<class T> class Y { /* ... */ };
Y<X<1>> x3;                         // OK, same as Y<X<1> > x3;
Y<X<6>>1>> x4;                      // syntax error
Y<X<(6>>1)>> x5;                    // OK

 — end example ]

When the name of a member template specialization appears after . or -> in a postfix-expression or after a nested-name-specifier in a qualified-id, and the object expression of the postfix-expression is type-dependent or the nested-name-specifier in the qualified-id refers to a dependent type, but the name is not a member of the current instantiation ([temp.dep.type]), the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template. [ Example:

struct X {
  template<std::size_t> X* alloc();
  template<std::size_t> static X* adjust();
};
template<class T> void f(T* p) {
  T* p1 = p->alloc<200>();          // ill-formed: < means less than
  T* p2 = p->template alloc<200>(); // OK: < starts template argument list
  T::adjust<100>();                 // ill-formed: < means less than
  T::template adjust<100>();        // OK: < starts template argument list
}

 — end example ]

A name prefixed by the keyword template shall be a template-id or the name shall refer to a class template. [ Note: The keyword template may not be applied to non-template members of class templates.  — end note ] [ Note: As is the case with the typename prefix, the template prefix is allowed in cases where it is not strictly necessary; i.e., when the nested-name-specifier or the expression on the left of the -> or . is not dependent on a template-parameter, or the use does not appear in the scope of a template.  — end note ] [ Example:

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

template <class T> void f(T t) {
  A<T> a;
  a.template f<>(t);                // OK: calls template
  a.template f(t);                  // error: not a template-id
}

template <class T> struct B {
  template <class T2> struct C { };
};

// OK: T::template C names a class template:
template <class T, template <class X> class TT = T::template C> struct D { };
D<B<int> > db;

 — end example ]

A simple-template-id that names a class template specialization is a class-name (Clause [class]).

A template-id that names an alias template specialization is a type-name.