The default constructor ([class.ctor]), copy constructor and copy assignment operator ([class.copy]), move constructor and move assignment operator ([class.copy]), and destructor ([class.dtor]) are special member functions. [ Note: The implementation will implicitly declare these member functions for some class types when the program does not explicitly declare them. The implementation will implicitly define them if they are odr-used ([basic.def.odr]). See [class.ctor], [class.dtor] and [class.copy]. — end note ] An implicitly-declared special member function is declared at the closing } of the class-specifier. Programs shall not define implicitly-declared special member functions.
Programs may explicitly refer to implicitly-declared special member functions. [ Example: a program may explicitly call, take the address of or form a pointer to member to an implicitly-declared special member function.
struct A { }; // implicitly declared A::operator=
struct B : A {
B& operator=(const B &);
};
B& B::operator=(const B& s) {
this->A::operator=(s); // well formed
return *this;
}
— end example ]
[ Note: The special member functions affect the way objects of class type are created, copied, moved, and destroyed, and how values can be converted to values of other types. Often such special member functions are called implicitly. — end note ]
Special member functions obey the usual access rules (Clause [class.access]). [ Example: declaring a constructor protected ensures that only derived classes and friends can create objects using it. — end example ]
For a class, its non-static data members, its non-virtual direct base classes, and, if the class is not abstract ([class.abstract]), its virtual base classes are called its potentially constructed subobjects.
Constructors do not have names. A declaration of a constructor uses a function declarator ([dcl.fct]) of the form
ptr-declarator ( parameter-declaration-clause ) exception-specificationopt attribute-specifier-seqopt
where the ptr-declarator consists solely of an id-expression, an optional attribute-specifier-seq, and optional surrounding parentheses, and the id-expression has one of the following forms:
in a member-declaration that belongs to the member-specification of a class but is not a friend declaration ([class.friend]), the id-expression is the injected-class-name (Clause [class]) of the immediately-enclosing class;
in a member-declaration that belongs to the member-specification of a class template but is not a friend declaration, the id-expression is a class-name that names the current instantiation ([temp.dep.type]) of the immediately-enclosing class template; or
in a declaration at namespace scope or in a friend declaration, the id-expression is a qualified-id that names a constructor ([class.qual]).
The class-name shall not be a typedef-name. In a constructor declaration, each decl-specifier in the optional decl-specifier-seq shall be friend, inline, explicit, or constexpr. [ Example:
struct S {
S(); // declares the constructor
};
S::S() { } // defines the constructor
— end example ]
A constructor is used to initialize objects of its class type. Because constructors do not have names, they are never found during name lookup; however an explicit type conversion using the functional notation ([expr.type.conv]) will cause a constructor to be called to initialize an object. [ Note: For initialization of objects of class type see [class.init]. — end note ]
A constructor can be invoked for a const, volatile or const volatile object. const and volatile semantics ([dcl.type.cv]) are not applied on an object under construction. They come into effect when the constructor for the most derived object ([intro.object]) ends.
A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X, a constructor having no parameters is implicitly declared as defaulted ([dcl.fct.def]). An implicitly-declared default constructor is an inline public member of its class. A defaulted default constructor for class X is defined as deleted if:
X is a union-like class that has a variant member with a non-trivial default constructor,
any non-static data member with no brace-or-equal-initializer is of reference type,
any non-variant non-static data member of const-qualified type (or array thereof) with no brace-or-equal-initializer does not have a user-provided default constructor,
X is a union and all of its variant members are of const-qualified type (or array thereof),
X is a non-union class and all members of any anonymous union member are of const-qualified type (or array thereof),
any potentially constructed subobject, except for a non-static data member with a brace-or-equal-initializer, has class type M (or array thereof) and either M has no default constructor or overload resolution ([over.match]) as applied to M's default constructor results in an ambiguity or in a function that is deleted or inaccessible from the defaulted default constructor, or
any potentially constructed subobject has a type with a destructor that is deleted or inaccessible from the defaulted default constructor.
A default constructor is trivial if it is not user-provided and if:
its class has no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
no non-static data member of its class has a brace-or-equal-initializer, and
all the direct base classes of its class have trivial default constructors, and
for all the non-static data members of its class that are of class type (or array thereof), each such class has a trivial default constructor.
Otherwise, the default constructor is non-trivial.
A default constructor that is defaulted and not defined as deleted is implicitly defined when it is odr-used ([basic.def.odr]) to create an object of its class type ([intro.object]) or when it is explicitly defaulted after its first declaration. The implicitly-defined default constructor performs the set of initializations of the class that would be performed by a user-written default constructor for that class with no ctor-initializer ([class.base.init]) and an empty compound-statement. If that user-written default constructor would be ill-formed, the program is ill-formed. If that user-written default constructor would satisfy the requirements of a constexpr constructor ([dcl.constexpr]), the implicitly-defined default constructor is constexpr. Before the defaulted default constructor for a class is implicitly defined, all the non-user-provided default constructors for its base classes and its non-static data members shall have been implicitly defined. [ Note: An implicitly-declared default constructor has an exception-specification ([except.spec]). An explicitly-defaulted definition might have an implicit exception-specification, see [dcl.fct.def]. — end note ]
Default constructors are called implicitly to create class objects of static, thread, or automatic storage duration ([basic.stc.static], [basic.stc.thread], [basic.stc.auto]) defined without an initializer ([dcl.init]), are called to create class objects of dynamic storage duration ([basic.stc.dynamic]) created by a new-expression in which the new-initializer is omitted ([expr.new]), or are called when the explicit type conversion syntax ([expr.type.conv]) is used. A program is ill-formed if the default constructor for an object is implicitly used and the constructor is not accessible (Clause [class.access]).
[ Note: [class.base.init] describes the order in which constructors for base classes and non-static data members are called and describes how arguments can be specified for the calls to these constructors. — end note ]
A return statement in the body of a constructor shall not specify a return value. The address of a constructor shall not be taken.
A functional notation type conversion ([expr.type.conv]) can be used to create new objects of its type. [ Note: The syntax looks like an explicit call of the constructor. — end note ] [ Example:
complex zz = complex(1,2.3); cprint( complex(7.8,1.2) );
— end example ]
An object created in this way is unnamed. [ Note: [class.temporary] describes the lifetime of temporary objects. — end note ] [ Note: Explicit constructor calls do not yield lvalues, see [basic.lval]. — end note ]
[ Note: some language constructs have special semantics when used during construction; see [class.base.init] and [class.cdtor]. — end note ]
During the construction of a const object, if the value of the object or any of its subobjects is accessed through a glvalue that is not obtained, directly or indirectly, from the constructor's this pointer, the value of the object or subobject thus obtained is unspecified. [ Example:
struct C;
void no_opt(C*);
struct C {
int c;
C() : c(0) { no_opt(this); }
};
const C cobj;
void no_opt(C* cptr) {
int i = cobj.c * 100; // value of cobj.c is unspecified
cptr->c = 1;
cout << cobj.c * 100 // value of cobj.c is unspecified
<< '\n';
}
— end example ]
Temporaries of class type are created in various contexts: binding a reference to a prvalue ([dcl.init.ref]), returning a prvalue ([stmt.return]), a conversion that creates a prvalue ([conv.lval], [expr.static.cast], [expr.const.cast], [expr.cast]), throwing an exception ([except.throw]), and in some initializations ([dcl.init]). [ Note: The lifetime of exception objects is described in [except.throw]. — end note ] Even when the creation of the temporary object is unevaluated (Clause [expr]) or otherwise avoided ([class.copy]), all the semantic restrictions shall be respected as if the temporary object had been created and later destroyed. [ Note: This includes accessibility ([class.access]) and whether it is deleted, for the constructor selected and for the destructor. However, in the special case of a function call used as the operand of a decltype-specifier ([expr.call]), no temporary is introduced, so the foregoing does not apply to the prvalue of any such function call. — end note ]
[ Example: Consider the following code:
class X {
public:
X(int);
X(const X&);
X& operator=(const X&);
~X();
};
class Y {
public:
Y(int);
Y(Y&&);
~Y();
};
X f(X);
Y g(Y);
void h() {
X a(1);
X b = f(X(2));
Y c = g(Y(3));
a = f(a);
}
An implementation might use a temporary in which to construct X(2) before passing it to f() using X's copy constructor; alternatively, X(2) might be constructed in the space used to hold the argument. Likewise, an implementation might use a temporary in which to construct Y(3) before passing it to g() using Y's move constructor; alternatively, Y(3) might be constructed in the space used to hold the argument. Also, a temporary might be used to hold the result of f(X(2)) before copying it to b using X's copy constructor; alternatively, f()'s result might be constructed in b. Likewise, a temporary might be used to hold the result of g(Y(3)) before moving it to c using Y's move constructor; alternatively, g()'s result might be constructed in c. On the other hand, the expression a=f(a) requires a temporary for the result of f(a), which is then assigned to a. — end example ]
When an implementation introduces a temporary object of a class that has a non-trivial constructor ([class.ctor], [class.copy]), it shall ensure that a constructor is called for the temporary object. Similarly, the destructor shall be called for a temporary with a non-trivial destructor ([class.dtor]). Temporary objects are destroyed as the last step in evaluating the full-expression ([intro.execution]) that (lexically) contains the point where they were created. This is true even if that evaluation ends in throwing an exception. The value computations and side effects of destroying a temporary object are associated only with the full-expression, not with any specific subexpression.
There are two contexts in which temporaries are destroyed at a different point than the end of the full-expression. The first context is when a default constructor is called to initialize an element of an array. If the constructor has one or more default arguments, the destruction of every temporary created in a default argument is sequenced before the construction of the next array element, if any.
The second context is when a reference is bound to a temporary.117 The temporary to which the reference is bound or the temporary that is the complete object of a subobject to which the reference is bound persists for the lifetime of the reference except:
A temporary bound to a reference member in a constructor's ctor-initializer ([class.base.init]) persists until the constructor exits.
A temporary bound to a reference parameter in a function call ([expr.call]) persists until the completion of the full-expression containing the call.
The lifetime of a temporary bound to the returned value in a function return statement ([stmt.return]) is not extended; the temporary is destroyed at the end of the full-expression in the return statement.
A temporary bound to a reference in a new-initializer ([expr.new]) persists until the completion of the full-expression containing the new-initializer. [ Example:
struct S { int mi; const std::pair<int,int>& mp; };
S a { 1, {2,3} };
S* p = new S{ 1, {2,3} }; // Creates dangling reference
— end example ] [ Note: This may introduce a dangling reference, and implementations are encouraged to issue a warning in such a case. — end note ]
The destruction of a temporary whose lifetime is not extended by being bound to a reference is sequenced before the destruction of every temporary which is constructed earlier in the same full-expression. If the lifetime of two or more temporaries to which references are bound ends at the same point, these temporaries are destroyed at that point in the reverse order of the completion of their construction. In addition, the destruction of temporaries bound to references shall take into account the ordering of destruction of objects with static, thread, or automatic storage duration ([basic.stc.static], [basic.stc.thread], [basic.stc.auto]); that is, if obj1 is an object with the same storage duration as the temporary and created before the temporary is created the temporary shall be destroyed before obj1 is destroyed; if obj2 is an object with the same storage duration as the temporary and created after the temporary is created the temporary shall be destroyed after obj2 is destroyed. [ Example:
struct S {
S();
S(int);
friend S operator+(const S&, const S&);
~S();
};
S obj1;
const S& cr = S(16)+S(23);
S obj2;
the expression S(16) + S(23) creates three temporaries: a first temporary T1 to hold the result of the expression S(16), a second temporary T2 to hold the result of the expression S(23), and a third temporary T3 to hold the result of the addition of these two expressions. The temporary T3 is then bound to the reference cr. It is unspecified whether T1 or T2 is created first. On an implementation where T1 is created before T2, T2 shall be destroyed before T1. The temporaries T1 and T2 are bound to the reference parameters of operator+; these temporaries are destroyed at the end of the full-expression containing the call to operator+. The temporary T3 bound to the reference cr is destroyed at the end of cr's lifetime, that is, at the end of the program. In addition, the order in which T3 is destroyed takes into account the destruction order of other objects with static storage duration. That is, because obj1 is constructed before T3, and T3 is constructed before obj2, obj2 shall be destroyed before T3, and T3 shall be destroyed before obj1. — end example ]
The same rules apply to initialization of an initializer_list object ([dcl.init.list]) with its underlying temporary array
Type conversions of class objects can be specified by constructors and by conversion functions. These conversions are called user-defined conversions and are used for implicit type conversions (Clause [conv]), for initialization ([dcl.init]), and for explicit type conversions ([expr.cast], [expr.static.cast]).
User-defined conversions are applied only where they are unambiguous ([class.member.lookup], [class.conv.fct]). Conversions obey the access control rules (Clause [class.access]). Access control is applied after ambiguity resolution ([basic.lookup]).
[ Note: See [over.match] for a discussion of the use of conversions in function calls as well as examples below. — end note ]
At most one user-defined conversion (constructor or conversion function) is implicitly applied to a single value.
[ Example:
struct X {
operator int();
};
struct Y {
operator X();
};
Y a;
int b = a; // error
// a.operator X().operator int() not tried
int c = X(a); // OK: a.operator X().operator int()
— end example ]
User-defined conversions are used implicitly only if they are unambiguous. A conversion function in a derived class does not hide a conversion function in a base class unless the two functions convert to the same type. Function overload resolution ([over.match.best]) selects the best conversion function to perform the conversion. [ Example:
struct X {
operator int();
};
struct Y : X {
operator char();
};
void f(Y& a) {
if (a) { // ill-formed:
// X::operator int() or Y::operator char()
}
}
— end example ]
A constructor declared without the function-specifier explicit specifies a conversion from the types of its parameters to the type of its class. Such a constructor is called a converting constructor. [ Example:
struct X {
X(int);
X(const char*, int =0);
X(int, int);
};
void f(X arg) {
X a = 1; // a = X(1)
X b = "Jessie"; // b = X("Jessie",0)
a = 2; // a = X(2)
f(3); // f(X(3))
f({1, 2}); // f(X(1,2))
}
— end example ]
An explicit constructor constructs objects just like non-explicit constructors, but does so only where the direct-initialization syntax ([dcl.init]) or where casts ([expr.static.cast], [expr.cast]) are explicitly used. A default constructor may be an explicit constructor; such a constructor will be used to perform default-initialization or value-initialization ([dcl.init]). [ Example:
struct Z {
explicit Z();
explicit Z(int);
explicit Z(int, int);
};
Z a; // OK: default-initialization performed
Z a1 = 1; // error: no implicit conversion
Z a3 = Z(1); // OK: direct initialization syntax used
Z a2(1); // OK: direct initialization syntax used
Z* p = new Z(1); // OK: direct initialization syntax used
Z a4 = (Z)1; // OK: explicit cast used
Z a5 = static_cast<Z>(1); // OK: explicit cast used
Z a6 = { 3, 4 }; // error: no implicit conversion
— end example ]
A non-explicit copy/move constructor ([class.copy]) is a converting constructor. An implicitly-declared copy/move constructor is not an explicit constructor; it may be called for implicit type conversions.
A member function of a class X having no parameters with a name of the form
conversion-function-id: operator conversion-type-id
conversion-type-id: type-specifier-seq conversion-declaratoropt
conversion-declarator: ptr-operator conversion-declaratoropt
specifies a conversion from X to the type specified by the conversion-type-id. Such functions are called conversion functions. No return type can be specified. If a conversion function is a member function, the type of the conversion function ([dcl.fct]) is “function taking no parameter returning conversion-type-id”. A conversion function is never used to convert a (possibly cv-qualified) object to the (possibly cv-qualified) same object type (or a reference to it), to a (possibly cv-qualified) base class of that type (or a reference to it), or to (possibly cv-qualified) void.118
[ Example:
struct X {
operator int();
};
void f(X a) {
int i = int(a);
i = (int)a;
i = a;
}
In all three cases the value assigned will be converted by X::operator int(). — end example ]
A conversion function may be explicit ([dcl.fct.spec]), in which case it is only considered as a user-defined conversion for direct-initialization ([dcl.init]). Otherwise, user-defined conversions are not restricted to use in assignments and initializations. [ Example:
class Y { };
struct Z {
explicit operator Y() const;
};
void h(Z z) {
Y y1(z); // OK: direct-initialization
Y y2 = z; // ill-formed: copy-initialization
Y y3 = (Y)z; // OK: cast notation
}
void g(X a, X b) {
int i = (a) ? 1+a : 0;
int j = (a&&b) ? a+b : i;
if (a) {
}
}
— end example ]
The conversion-type-id shall not represent a function type nor an array type. The conversion-type-id in a conversion-function-id is the longest possible sequence of conversion-declarators. [ Note: This prevents ambiguities between the declarator operator * and its expression counterparts. [ Example:
&ac.operator int*i; // syntax error: // parsed as: &(ac.operator int *)i // not as: &(ac.operator int)*i
The * is the pointer declarator and not the multiplication operator. — end example ] — end note ]
These conversions are considered as standard conversions for the purposes of overload resolution ([over.best.ics], [over.ics.ref]) and therefore initialization ([dcl.init]) and explicit casts ([expr.static.cast]). A conversion to void does not invoke any conversion function ([expr.static.cast]). Even though never directly called to perform a conversion, such conversion functions can be declared and can potentially be reached through a call to a virtual conversion function in a base class.
A declaration of a destructor uses a function declarator ([dcl.fct]) of the form
ptr-declarator ( parameter-declaration-clause ) exception-specificationopt attribute-specifier-seqopt
where the ptr-declarator consists solely of an id-expression, an optional attribute-specifier-seq, and optional surrounding parentheses, and the id-expression has one of the following forms:
in a member-declaration that belongs to the member-specification of a class but is not a friend declaration ([class.friend]), the id-expression is ~class-name and the class-name is the injected-class-name (Clause [class]) of the immediately-enclosing class;
in a member-declaration that belongs to the member-specification of a class template but is not a friend declaration, the id-expression is ~class-name and the class-name names the current instantiation ([temp.dep.type]) of the immediately-enclosing class template; or
in a declaration at namespace scope or in a friend declaration, the id-expression is nested-name-specifier ~class-name and the class-name names the same class as the nested-name-specifier.
The class-name shall not be a typedef-name. A destructor shall take no arguments ([dcl.fct]). In a destructor declaration, each decl-specifier of the optional decl-specifier-seq shall be friend, inline, or virtual.
A destructor is used to destroy objects of its class type. The address of a destructor shall not be taken. A destructor can be invoked for a const, volatile or const volatile object. const and volatile semantics ([dcl.type.cv]) are not applied on an object under destruction. They stop being in effect when the destructor for the most derived object ([intro.object]) starts.
A declaration of a destructor that does not have an exception-specification is implicitly considered to have the same exception-specification as an implicit declaration ([except.spec]).
If a class has no user-declared destructor, a destructor is implicitly declared as defaulted ([dcl.fct.def]). An implicitly-declared destructor is an inline public member of its class.
A defaulted destructor for a class X is defined as deleted if:
X is a union-like class that has a variant member with a non-trivial destructor,
any potentially constructed subobject has class type M (or array thereof) and M has a deleted destructor or a destructor that is inaccessible from the defaulted destructor,
or, for a virtual destructor, lookup of the non-array deallocation function results in an ambiguity or in a function that is deleted or inaccessible from the defaulted destructor.
A destructor is trivial if it is not user-provided and if:
the destructor is not virtual,
all of the direct base classes of its class have trivial destructors, and
for all of the non-static data members of its class that are of class type (or array thereof), each such class has a trivial destructor.
Otherwise, the destructor is non-trivial.
A destructor that is defaulted and not defined as deleted is implicitly defined when it is odr-used ([basic.def.odr]) to destroy an object of its class type ([basic.stc]) or when it is explicitly defaulted after its first declaration.
Before the defaulted destructor for a class is implicitly defined, all the non-user-provided destructors for its base classes and its non-static data members shall have been implicitly defined.
After executing the body of the destructor and destroying any automatic objects allocated within the body, a destructor for class X calls the destructors for X's direct non-variant non-static data members, the destructors for X's direct base classes and, if X is the type of the most derived class ([class.base.init]), its destructor calls the destructors for X's virtual base classes. All destructors are called as if they were referenced with a qualified name, that is, ignoring any possible virtual overriding destructors in more derived classes. Bases and members are destroyed in the reverse order of the completion of their constructor (see [class.base.init]). A return statement ([stmt.return]) in a destructor might not directly return to the caller; before transferring control to the caller, the destructors for the members and bases are called. Destructors for elements of an array are called in reverse order of their construction (see [class.init]).
A destructor can be declared virtual ([class.virtual]) or pure virtual ([class.abstract]); if any objects of that class or any derived class are created in the program, the destructor shall be defined. If a class has a base class with a virtual destructor, its destructor (whether user- or implicitly-declared) is virtual.
[ Note: some language constructs have special semantics when used during destruction; see [class.cdtor]. — end note ]
A destructor is invoked implicitly
for a constructed object with static storage duration ([basic.stc.static]) at program termination ([basic.start.term]),
for a constructed object with thread storage duration ([basic.stc.thread]) at thread exit,
for a constructed object with automatic storage duration ([basic.stc.auto]) when the block in which an object is created exits ([stmt.dcl]),
for a constructed temporary object when its lifetime ends ([class.temporary]).
In each case, the context of the invocation is the context of the construction of the object. A destructor is also invoked implicitly through use of a delete-expression ([expr.delete]) for a constructed object allocated by a new-expression ([expr.new]); the context of the invocation is the delete-expression. [ Note: An array of class type contains several subobjects for each of which the destructor is invoked. — end note ] A destructor can also be invoked explicitly. A destructor is potentially invoked if it is invoked or as specified in [expr.new] and [class.base.init]. A program is ill-formed if a destructor that is potentially invoked is deleted or not accessible from the context of the invocation.
At the point of definition of a virtual destructor (including an implicit definition ([class.copy])), the non-array deallocation function is looked up in the scope of the destructor's class ([class.member.lookup]), and, if no declaration is found, the function is looked up in the global scope. If the result of this lookup is ambiguous or inaccessible, or if the lookup selects a placement deallocation function or a function with a deleted definition ([dcl.fct.def]), the program is ill-formed. [ Note: This assures that a deallocation function corresponding to the dynamic type of an object is available for the delete-expression ([class.free]). — end note ]
In an explicit destructor call, the destructor name appears as a ~ followed by a type-name or decltype-specifier that denotes the destructor's class type. The invocation of a destructor is subject to the usual rules for member functions ([class.mfct]); that is, if the object is not of the destructor's class type and not of a class derived from the destructor's class type (including when the destructor is invoked via a null pointer value), the program has undefined behavior. [ Note: invoking delete on a null pointer does not call the destructor; see [expr.delete]. — end note ] [ Example:
struct B {
virtual ~B() { }
};
struct D : B {
~D() { }
};
D D_object;
typedef B B_alias;
B* B_ptr = &D_object;
void f() {
D_object.B::~B(); // calls B's destructor
B_ptr->~B(); // calls D's destructor
B_ptr->~B_alias(); // calls D's destructor
B_ptr->B_alias::~B(); // calls B's destructor
B_ptr->B_alias::~B_alias(); // calls B's destructor
}
— end example ] [ Note: An explicit destructor call must always be written using a member access operator ([expr.ref]) or a qualified-id ([expr.prim]); in particular, the unary-expression ~X() in a member function is not an explicit destructor call ([expr.unary.op]). — end note ]
[ Note: explicit calls of destructors are rarely needed. One use of such calls is for objects placed at specific addresses using a new-expression with the placement option. Such use of explicit placement and destruction of objects can be necessary to cope with dedicated hardware resources and for writing memory management facilities. For example,
void* operator new(std::size_t, void* p) { return p; }
struct X {
X(int);
~X();
};
void f(X* p);
void g() { // rare, specialized use:
char* buf = new char[sizeof(X)];
X* p = new(buf) X(222); // use buf[] and initialize
f(p);
p->X::~X(); // cleanup
}
— end note ]
Once a destructor is invoked for an object, the object no longer exists; the behavior is undefined if the destructor is invoked for an object whose lifetime has ended ([basic.life]). [ Example: if the destructor for an automatic object is explicitly invoked, and the block is subsequently left in a manner that would ordinarily invoke implicit destruction of the object, the behavior is undefined. — end example ]
[ Note: the notation for explicit call of a destructor can be used for any scalar type name ([expr.pseudo]). Allowing this makes it possible to write code without having to know if a destructor exists for a given type. For example,
typedef int I; I* p; p->I::~I();
— end note ]
[ Example:
class Arena;
struct B {
void* operator new(std::size_t, Arena*);
};
struct D1 : B {
};
Arena* ap;
void foo(int i) {
new (ap) D1; // calls B::operator new(std::size_t, Arena*)
new D1[i]; // calls ::operator new[](std::size_t)
new D1; // ill-formed: ::operator new(std::size_t) hidden
}
— end example ]
When an object is deleted with a delete-expression ([expr.delete]), a deallocation function (operator delete() for non-array objects or operator delete[]() for arrays) is (implicitly) called to reclaim the storage occupied by the object ([basic.stc.dynamic.deallocation]).
Class-specific deallocation function lookup is a part of general deallocation function lookup ([expr.delete]) and occurs as follows. If the delete-expression is used to deallocate a class object whose static type has a virtual destructor, the deallocation function is the one selected at the point of definition of the dynamic type's virtual destructor ([class.dtor]).119 Otherwise, if the delete-expression is used to deallocate an object of class T or array thereof, the static and dynamic types of the object shall be identical and the deallocation function's name is looked up in the scope of T. If this lookup fails to find the name, general deallocation function lookup ([expr.delete]) continues. If the result of the lookup is ambiguous or inaccessible, or if the lookup selects a placement deallocation function, the program is ill-formed.
Any deallocation function for a class X is a static member (even if not explicitly declared static). [ Example:
class X {
void operator delete(void*);
void operator delete[](void*, std::size_t);
};
class Y {
void operator delete(void*, std::size_t);
void operator delete[](void*);
};
— end example ]
Since member allocation and deallocation functions are static they cannot be virtual. [ Note: however, when the cast-expression of a delete-expression refers to an object of class type, because the deallocation function actually called is looked up in the scope of the class that is the dynamic type of the object, if the destructor is virtual, the effect is the same. For example,
struct B {
virtual ~B();
void operator delete(void*, std::size_t);
};
struct D : B {
void operator delete(void*);
};
void f() {
B* bp = new D;
delete bp; //1: uses D::operator delete(void*)
}
Here, storage for the non-array object of class D is deallocated by D::operator delete(), due to the virtual destructor. — end note ] [ Note: Virtual destructors have no effect on the deallocation function actually called when the cast-expression of a delete-expression refers to an array of objects of class type. For example,
struct B {
virtual ~B();
void operator delete[](void*, std::size_t);
};
struct D : B {
void operator delete[](void*, std::size_t);
};
void f(int i) {
D* dp = new D[i];
delete [] dp; // uses D::operator delete[](void*, std::size_t)
B* bp = new D[i];
delete[] bp; // undefined behavior
}
— end note ]
Access to the deallocation function is checked statically. Hence, even though a different one might actually be executed, the statically visible deallocation function is required to be accessible. [ Example: for the call on line //1 above, if B::operator delete() had been private, the delete expression would have been ill-formed. — end example ]
[ Note: If a deallocation function has no explicit exception-specification, it is treated as if it were specified with noexcept(true) ([except.spec]). — end note ]
A similar provision is not needed for the array version of operator delete because [expr.delete] requires that in this situation, the static type of the object to be deleted be the same as its dynamic type.
When no initializer is specified for an object of (possibly cv-qualified) class type (or array thereof), or the initializer has the form (), the object is initialized as specified in [dcl.init].
An object of class type (or array thereof) can be explicitly initialized; see [class.expl.init] and [class.base.init].
When an array of class objects is initialized (either explicitly or implicitly) and the elements are initialized by constructor, the constructor shall be called for each element of the array, following the subscript order; see [dcl.array]. [ Note: Destructors for the array elements are called in reverse order of their construction. — end note ]
An object of class type can be initialized with a parenthesized expression-list, where the expression-list is construed as an argument list for a constructor that is called to initialize the object. Alternatively, a single assignment-expression can be specified as an initializer using the = form of initialization. Either direct-initialization semantics or copy-initialization semantics apply; see [dcl.init]. [ Example:
struct complex {
complex();
complex(double);
complex(double,double);
};
complex sqrt(complex,complex);
complex a(1); // initialize by a call of
// complex(double)
complex b = a; // initialize by a copy of a
complex c = complex(1,2); // construct complex(1,2)
// using complex(double,double)
// copy/move it into c
complex d = sqrt(b,c); // call sqrt(complex,complex)
// and copy/move the result into d
complex e; // initialize by a call of
// complex()
complex f = 3; // construct complex(3) using
// complex(double)
// copy/move it into f
complex g = { 1, 2 }; // initialize by a call of
// complex(double, double)
— end example ] [ Note: overloading of the assignment operator ([over.ass]) has no effect on initialization. — end note ]
An object of class type can also be initialized by a braced-init-list. List-initialization semantics apply; see [dcl.init] and [dcl.init.list]. [ Example:
complex v[6] = { 1, complex(1,2), complex(), 2 };
Here, complex::complex(double) is called for the initialization of v[0] and v[3], complex::complex(double, double) is called for the initialization of v[1], complex::complex() is called for the initialization v[2], v[4], and v[5]. For another example,
struct X {
int i;
float f;
complex c;
} x = { 99, 88.8, 77.7 };
Here, x.i is initialized with 99, x.f is initialized with 88.8, and complex::complex(double) is called for the initialization of x.c. — end example ] [ Note: Braces can be elided in the initializer-list for any aggregate, even if the aggregate has members of a class type with user-defined type conversions; see [dcl.init.aggr]. — end note ]
[ Note: If T is a class type with no default constructor, any declaration of an object of type T (or array thereof) is ill-formed if no initializer is explicitly specified (see [class.init] and [dcl.init]). — end note ]
[ Note: the order in which objects with static or thread storage duration are initialized is described in [basic.start.init] and [stmt.dcl]. — end note ]
In the definition of a constructor for a class, initializers for direct and virtual base subobjects and non-static data members can be specified by a ctor-initializer, which has the form
ctor-initializer: : mem-initializer-list
mem-initializer-list: mem-initializer ...opt mem-initializer ...opt , mem-initializer-list
mem-initializer: mem-initializer-id ( expression-listopt ) mem-initializer-id braced-init-list
mem-initializer-id: class-or-decltype identifier
In a mem-initializer-id an initial unqualified identifier is looked up in the scope of the constructor's class and, if not found in that scope, it is looked up in the scope containing the constructor's definition. [ Note: If the constructor's class contains a member with the same name as a direct or virtual base class of the class, a mem-initializer-id naming the member or base class and composed of a single identifier refers to the class member. A mem-initializer-id for the hidden base class may be specified using a qualified name. — end note ] Unless the mem-initializer-id names the constructor's class, a non-static data member of the constructor's class, or a direct or virtual base of that class, the mem-initializer is ill-formed.
A mem-initializer-list can initialize a base class using any class-or-decltype that denotes that base class type. [ Example:
struct A { A(); };
typedef A global_A;
struct B { };
struct C: public A, public B { C(); };
C::C(): global_A() { } // mem-initializer for base A
— end example ]
If a mem-initializer-id is ambiguous because it designates both a direct non-virtual base class and an inherited virtual base class, the mem-initializer is ill-formed. [ Example:
struct A { A(); };
struct B: public virtual A { };
struct C: public A, public B { C(); };
C::C(): A() { } // ill-formed: which A?
— end example ]
A ctor-initializer may initialize a variant member of the constructor's class. If a ctor-initializer specifies more than one mem-initializer for the same member or for the same base class, the ctor-initializer is ill-formed.
A mem-initializer-list can delegate to another constructor of the constructor's class using any class-or-decltype that denotes the constructor's class itself. If a mem-initializer-id designates the constructor's class, it shall be the only mem-initializer; the constructor is a delegating constructor, and the constructor selected by the mem-initializer is the target constructor. The principal constructor is the first constructor invoked in the construction of an object (that is, not a target constructor for that object's construction). The target constructor is selected by overload resolution. Once the target constructor returns, the body of the delegating constructor is executed. If a constructor delegates to itself directly or indirectly, the program is ill-formed; no diagnostic is required. [ Example:
struct C {
C( int ) { } // #1: non-delegating constructor
C(): C(42) { } // #2: delegates to #1
C( char c ) : C(42.0) { } // #3: ill-formed due to recursion with #4
C( double d ) : C('a') { } // #4: ill-formed due to recursion with #3
};
— end example ]
The expression-list or braced-init-list in a mem-initializer is used to initialize the designated subobject (or, in the case of a delegating constructor, the complete class object) according to the initialization rules of [dcl.init] for direct-initialization.
[ Example:
struct B1 { B1(int); /* ... */ };
struct B2 { B2(int); /* ... */ };
struct D : B1, B2 {
D(int);
B1 b;
const int c;
};
D::D(int a) : B2(a+1), B1(a+2), c(a+3), b(a+4)
{ /* ... */ }
D d(10);
— end example ] The initialization performed by each mem-initializer constitutes a full-expression. Any expression in a mem-initializer is evaluated as part of the full-expression that performs the initialization. A mem-initializer where the mem-initializer-id denotes a virtual base class is ignored during execution of a constructor of any class that is not the most derived class.
In a non-delegating constructor, if a given potentially constructed subobject is not designated by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer), then
if the entity is a non-static data member that has a brace-or-equal-initializer and either
the constructor's class is a union ([class.union]), and no other variant member of that union is designated by a mem-initializer-id or
the constructor's class is not a union, and, if the entity is a member of an anonymous union, no other member of that union is designated by a mem-initializer-id,
the entity is initialized as specified in [dcl.init];
otherwise, if the entity is an anonymous union or a variant member ([class.union]), no initialization is performed;
otherwise, the entity is default-initialized ([dcl.init]).
[ Note: An abstract class ([class.abstract]) is never a most derived class, thus its constructors never initialize virtual base classes, therefore the corresponding mem-initializers may be omitted. — end note ] An attempt to initialize more than one non-static data member of a union renders the program ill-formed. [ Note: After the call to a constructor for class X for an object with automatic or dynamic storage duration has completed, if the constructor was not invoked as part of value-initialization and a member of X is neither initialized nor given a value during execution of the compound-statement of the body of the constructor, the member has an indeterminate value. — end note ] [ Example:
struct A {
A();
};
struct B {
B(int);
};
struct C {
C() { } // initializes members as follows:
A a; // OK: calls A::A()
const B b; // error: B has no default constructor
int i; // OK: i has indeterminate value
int j = 5; // OK: j has the value 5
};
— end example ]
If a given non-static data member has both a brace-or-equal-initializer and a mem-initializer, the initialization specified by the mem-initializer is performed, and the non-static data member's brace-or-equal-initializer is ignored. [ Example: Given
struct A {
int i = /* some integer expression with side effects */ ;
A(int arg) : i(arg) { }
// ...
};
the A(int) constructor will simply initialize i to the value of arg, and the side effects in i's brace-or-equal-initializer will not take place. — end example ]
In a non-delegating constructor, the destructor for each potentially constructed subobject of class type is potentially invoked ([class.dtor]). [ Note: This provision ensures that destructors can be called for fully-constructed sub-objects in case an exception is thrown ([except.ctor]). — end note ]
In a non-delegating constructor, initialization proceeds in the following order:
First, and only for the constructor of the most derived class ([intro.object]), virtual base classes are initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where “left-to-right” is the order of appearance of the base classes in the derived class base-specifier-list.
Then, direct base classes are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).
Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).
Finally, the compound-statement of the constructor body is executed.
[ Note: The declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization. — end note ]
[ Example:
struct V {
V();
V(int);
};
struct A : virtual V {
A();
A(int);
};
struct B : virtual V {
B();
B(int);
};
struct C : A, B, virtual V {
C();
C(int);
};
A::A(int i) : V(i) { /* ... */ }
B::B(int i) { /* ... */ }
C::C(int i) { /* ... */ }
V v(1); // use V(int)
A a(2); // use V(int)
B b(3); // use V()
C c(4); // use V()
— end example ]
Names in the expression-list or braced-init-list of a mem-initializer are evaluated in the scope of the constructor for which the mem-initializer is specified. [ Example:
class X {
int a;
int b;
int i;
int j;
public:
const int& r;
X(int i): r(a), b(i), i(i), j(this->i) { }
};
initializes X::r to refer to X::a, initializes X::b with the value of the constructor parameter i, initializes X::i with the value of the constructor parameter i, and initializes X::j with the value of X::i; this takes place each time an object of class X is created. — end example ] [ Note: Because the mem-initializer are evaluated in the scope of the constructor, the this pointer can be used in the expression-list of a mem-initializer to refer to the object being initialized. — end note ]
Member functions (including virtual member functions, [class.virtual]) can be called for an object under construction. Similarly, an object under construction can be the operand of the typeid operator ([expr.typeid]) or of a dynamic_cast ([expr.dynamic.cast]). However, if these operations are performed in a ctor-initializer (or in a function called directly or indirectly from a ctor-initializer) before all the mem-initializers for base classes have completed, the result of the operation is undefined. [ Example:
class A {
public:
A(int);
};
class B : public A {
int j;
public:
int f();
B() : A(f()), // undefined: calls member function
// but base A not yet initialized
j(f()) { } // well-defined: bases are all initialized
};
class C {
public:
C(int);
};
class D : public B, C {
int i;
public:
D() : C(f()), // undefined: calls member function
// but base C not yet initialized
i(f()) { } // well-defined: bases are all initialized
};
— end example ]
[ Note: [class.cdtor] describes the result of virtual function calls, typeid and dynamic_casts during construction for the well-defined cases; that is, describes the polymorphic behavior of an object under construction. — end note ]
A mem-initializer followed by an ellipsis is a pack expansion ([temp.variadic]) that initializes the base classes specified by a pack expansion in the base-specifier-list for the class. [ Example:
template<class... Mixins>
class X : public Mixins... {
public:
X(const Mixins&... mixins) : Mixins(mixins)... { }
};
For an object with a non-trivial constructor, referring to any non-static member or base class of the object before the constructor begins execution results in undefined behavior. For an object with a non-trivial destructor, referring to any non-static member or base class of the object after the destructor finishes execution results in undefined behavior. [ Example:
struct X { int i; };
struct Y : X { Y(); }; // non-trivial
struct A { int a; };
struct B : public A { int j; Y y; }; // non-trivial
extern B bobj;
B* pb = &bobj; // OK
int* p1 = &bobj.a; // undefined, refers to base class member
int* p2 = &bobj.y.i; // undefined, refers to member's member
A* pa = &bobj; // undefined, upcast to a base class type
B bobj; // definition of bobj
extern X xobj;
int* p3 = &xobj.i; //OK, X is a trivial class
X xobj;
For another example,
struct W { int j; };
struct X : public virtual W { };
struct Y {
int* p;
X x;
Y() : p(&x.j) { // undefined, x is not yet constructed
}
};
— end example ]
To explicitly or implicitly convert a pointer (a glvalue) referring to an object of class X to a pointer (reference) to a direct or indirect base class B of X, the construction of X and the construction of all of its direct or indirect bases that directly or indirectly derive from B shall have started and the destruction of these classes shall not have completed, otherwise the conversion results in undefined behavior. To form a pointer to (or access the value of) a direct non-static member of an object obj, the construction of obj shall have started and its destruction shall not have completed, otherwise the computation of the pointer value (or accessing the member value) results in undefined behavior. [ Example:
struct A { };
struct B : virtual A { };
struct C : B { };
struct D : virtual A { D(A*); };
struct X { X(A*); };
struct E : C, D, X {
E() : D(this), // undefined: upcast from E* to A*
// might use path E* → D* → A*
// but D is not constructed
// D((C*)this), // defined:
// E* → C* defined because E() has started
// and C* → A* defined because
// C fully constructed
X(this) { // defined: upon construction of X,
// C/B/D/A sublattice is fully constructed
}
};
— end example ]
Member functions, including virtual functions ([class.virtual]), can be called during construction or destruction ([class.base.init]). When a virtual function is called directly or indirectly from a constructor or from a destructor, including during the construction or destruction of the class's non-static data members, and the object to which the call applies is the object (call it x) under construction or destruction, the function called is the final overrider in the constructor's or destructor's class and not one overriding it in a more-derived class. If the virtual function call uses an explicit class member access ([expr.ref]) and the object expression refers to the complete object of x or one of that object's base class subobjects but not x or one of its base class subobjects, the behavior is undefined. [ Example:
struct V {
virtual void f();
virtual void g();
};
struct A : virtual V {
virtual void f();
};
struct B : virtual V {
virtual void g();
B(V*, A*);
};
struct D : A, B {
virtual void f();
virtual void g();
D() : B((A*)this, this) { }
};
B::B(V* v, A* a) {
f(); // calls V::f, not A::f
g(); // calls B::g, not D::g
v->g(); // v is base of B, the call is well-defined, calls B::g
a->f(); // undefined behavior, a's type not a base of B
}
— end example ]
The typeid operator ([expr.typeid]) can be used during construction or destruction ([class.base.init]). When typeid is used in a constructor (including the mem-initializer or brace-or-equal-initializer for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of typeid refers to the object under construction or destruction, typeid yields the std::type_info object representing the constructor or destructor's class. If the operand of typeid refers to the object under construction or destruction and the static type of the operand is neither the constructor or destructor's class nor one of its bases, the result of typeid is undefined.
dynamic_casts ([expr.dynamic.cast]) can be used during construction or destruction ([class.base.init]). When a dynamic_cast is used in a constructor (including the mem-initializer or brace-or-equal-initializer for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of the dynamic_cast refers to the object under construction or destruction, this object is considered to be a most derived object that has the type of the constructor or destructor's class. If the operand of the dynamic_cast refers to the object under construction or destruction and the static type of the operand is not a pointer to or object of the constructor or destructor's own class or one of its bases, the dynamic_cast results in undefined behavior.
[ Example:
struct V {
virtual void f();
};
struct A : virtual V { };
struct B : virtual V {
B(V*, A*);
};
struct D : A, B {
D() : B((A*)this, this) { }
};
B::B(V* v, A* a) {
typeid(*this); // type_info for B
typeid(*v); // well-defined: *v has type V, a base of B
// yields type_info for B
typeid(*a); // undefined behavior: type A not a base of B
dynamic_cast<B*>(v); // well-defined: v of type V*, V base of B
// results in B*
dynamic_cast<B*>(a); // undefined behavior,
// a has type A*, A not a base of B
}
A class object can be copied or moved in two ways: by initialization ([class.ctor], [dcl.init]), including for function argument passing ([expr.call]) and for function value return ([stmt.return]); and by assignment ([expr.ass]). Conceptually, these two operations are implemented by a copy/move constructor ([class.ctor]) and copy/move assignment operator ([over.ass]).
A non-template constructor for class X is a copy constructor if its first parameter is of type X&, const X&, volatile X& or const volatile X&, and either there are no other parameters or else all other parameters have default arguments ([dcl.fct.default]). [ Example: X::X(const X&) and X::X(X&,int=1) are copy constructors.
struct X {
X(int);
X(const X&, int = 1);
};
X a(1); // calls X(int);
X b(a, 0); // calls X(const X&, int);
X c = b; // calls X(const X&, int);
— end example ]
A non-template constructor for class X is a move constructor if its first parameter is of type X&&, const X&&, volatile X&&, or const volatile X&&, and either there are no other parameters or else all other parameters have default arguments ([dcl.fct.default]). [ Example: Y::Y(Y&&) is a move constructor.
struct Y {
Y(const Y&);
Y(Y&&);
};
extern Y f(int);
Y d(f(1)); // calls Y(Y&&)
Y e = d; // calls Y(const Y&)
— end example ]
[ Note: All forms of copy/move constructor may be declared for a class. [ Example:
struct X {
X(const X&);
X(X&); // OK
X(X&&);
X(const X&&); // OK, but possibly not sensible
};
— end example ] — end note ]
[ Note: If a class X only has a copy constructor with a parameter of type X&, an initializer of type const X or volatile X cannot initialize an object of type (possibly cv-qualified) X. [ Example:
struct X {
X(); // default constructor
X(X&); // copy constructor with a nonconst parameter
};
const X cx;
X x = cx; // error: X::X(X&) cannot copy cx into x
— end example ] — end note ]
A declaration of a constructor for a class X is ill-formed if its first parameter is of type (optionally cv-qualified) X and either there are no other parameters or else all other parameters have default arguments. A member function template is never instantiated to produce such a constructor signature. [ Example:
struct S {
template<typename T> S(T);
S();
};
S g;
void h() {
S a(g); // does not instantiate the member template to produce S::S<S>(S);
// uses the implicitly declared copy constructor
}
— end example ]
If the class definition does not explicitly declare a copy constructor, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted ([dcl.fct.def]). The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor.
The implicitly-declared copy constructor for a class X will have the form
X::X(const X&)
if each potentially constructed subobject of a class type M (or array thereof) has a copy constructor whose first parameter is of type const M& or const volatile M&.120 Otherwise, the implicitly-declared copy constructor will have the form
X::X(X&)
If the definition of a class X does not explicitly declare a move constructor, one will be implicitly declared as defaulted if and only if
X does not have a user-declared copy constructor,
X does not have a user-declared copy assignment operator,
X does not have a user-declared move assignment operator, and
X does not have a user-declared destructor.
[ Note: When the move constructor is not implicitly declared or explicitly supplied, expressions that otherwise would have invoked the move constructor may instead invoke a copy constructor. — end note ]
An implicitly-declared copy/move constructor is an inline public member of its class. A defaulted copy/move constructor for a class X is defined as deleted ([dcl.fct.def.delete]) if X has:
a variant member with a non-trivial corresponding constructor and X is a union-like class,
a potentially constructed subobject type M (or array thereof) that cannot be copied/moved because overload resolution ([over.match]), as applied to M's corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor,
any potentially constructed subobject of a type with a destructor that is deleted or inaccessible from the defaulted constructor, or,
for the copy constructor, a non-static data member of rvalue reference type.
A defaulted move constructor that is defined as deleted is ignored by overload resolution ([over.match], [over.over]). [ Note: A deleted move constructor would otherwise interfere with initialization from an rvalue which can use the copy constructor instead. — end note ]
A copy/move constructor for class X is trivial if it is not user-provided, its parameter-type-list is equivalent to the parameter-type-list of an implicit declaration, and if
class X has no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
class X has no non-static data members of volatile-qualified type, and
the constructor selected to copy/move each direct base class subobject is trivial, and
for each non-static data member of X that is of class type (or array thereof), the constructor selected to copy/move that member is trivial;
otherwise the copy/move constructor is non-trivial.
A copy/move constructor that is defaulted and not defined as deleted is implicitly defined if it is odr-used ([basic.def.odr]) or when it is explicitly defaulted after its first declaration. [ Note: The copy/move constructor is implicitly defined even if the implementation elided its odr-use ([basic.def.odr], [class.temporary]). — end note ] If the implicitly-defined constructor would satisfy the requirements of a constexpr constructor ([dcl.constexpr]), the implicitly-defined constructor is constexpr.
Before the defaulted copy/move constructor for a class is implicitly defined, all non-user-provided copy/move constructors for its potentially constructed subobjects shall have been implicitly defined. [ Note: An implicitly-declared copy/move constructor has an exception-specification ([except.spec]). — end note ]
The implicitly-defined copy/move constructor for a non-union class X performs a memberwise copy/move of its bases and members. [ Note: brace-or-equal-initializers of non-static data members are ignored. See also the example in [class.base.init]. — end note ] The order of initialization is the same as the order of initialization of bases and members in a user-defined constructor (see [class.base.init]). Let x be either the parameter of the constructor or, for the move constructor, an xvalue referring to the parameter. Each base or non-static data member is copied/moved in the manner appropriate to its type:
if the member is an array, each element is direct-initialized with the corresponding subobject of x;
if a member m has rvalue reference type T&&, it is direct-initialized with static_cast<T&&>(x.m);
otherwise, the base or member is direct-initialized with the corresponding base or member of x.
Virtual base class subobjects shall be initialized only once by the implicitly-defined copy/move constructor (see [class.base.init]).
The implicitly-defined copy/move constructor for a union X copies the object representation ([basic.types]) of X.
A user-declared copy assignment operator X::operator= is a non-static non-template member function of class X with exactly one parameter of type X, X&, const X&, volatile X& or const volatile X&.121 [ Note: An overloaded assignment operator must be declared to have only one parameter; see [over.ass]. — end note ] [ Note: More than one form of copy assignment operator may be declared for a class. — end note ] [ Note: If a class X only has a copy assignment operator with a parameter of type X&, an expression of type const X cannot be assigned to an object of type X. [ Example:
struct X {
X();
X& operator=(X&);
};
const X cx;
X x;
void f() {
x = cx; // error: X::operator=(X&) cannot assign cx into x
}
— end example ] — end note ]
If the class definition does not explicitly declare a copy assignment operator, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy assignment operator is defined as deleted; otherwise, it is defined as defaulted ([dcl.fct.def]). The latter case is deprecated if the class has a user-declared copy constructor or a user-declared destructor. The implicitly-declared copy assignment operator for a class X will have the form
X& X::operator=(const X&)
if
each direct base class B of X has a copy assignment operator whose parameter is of type const B&, const volatile B& or B, and
for all the non-static data members of X that are of a class type M (or array thereof), each such class type has a copy assignment operator whose parameter is of type const M&, const volatile M& or M.122
Otherwise, the implicitly-declared copy assignment operator will have the form
X& X::operator=(X&)
A user-declared move assignment operator X::operator= is a non-static non-template member function of class X with exactly one parameter of type X&&, const X&&, volatile X&&, or const volatile X&&. [ Note: An overloaded assignment operator must be declared to have only one parameter; see [over.ass]. — end note ] [ Note: More than one form of move assignment operator may be declared for a class. — end note ]
If the definition of a class X does not explicitly declare a move assignment operator, one will be implicitly declared as defaulted if and only if
X does not have a user-declared copy constructor,
X does not have a user-declared move constructor,
X does not have a user-declared copy assignment operator, and
X does not have a user-declared destructor.
[ Example: The class definition
struct S {
int a;
S& operator=(const S&) = default;
};
will not have a default move assignment operator implicitly declared because the copy assignment operator has been user-declared. The move assignment operator may be explicitly defaulted.
struct S {
int a;
S& operator=(const S&) = default;
S& operator=(S&&) = default;
};
— end example ]
The implicitly-declared move assignment operator for a class X will have the form
X& X::operator=(X&&);
The implicitly-declared copy/move assignment operator for class X has the return type X&; it returns the object for which the assignment operator is invoked, that is, the object assigned to. An implicitly-declared copy/move assignment operator is an inline public member of its class.
A defaulted copy/move assignment operator for class X is defined as deleted if X has:
a variant member with a non-trivial corresponding assignment operator and X is a union-like class, or
a non-static data member of const non-class type (or array thereof), or
a non-static data member of reference type, or
a potentially constructed subobject of class type M (or array thereof) that cannot be copied/moved because overload resolution ([over.match]), as applied to M's corresponding assignment operator, results in an ambiguity or a function that is deleted or inaccessible from the defaulted assignment operator.
A defaulted move assignment operator that is defined as deleted is ignored by overload resolution ([over.match], [over.over]).
Because a copy/move assignment operator is implicitly declared for a class if not declared by the user, a base class copy/move assignment operator is always hidden by the corresponding assignment operator of a derived class ([over.ass]). A using-declaration ([namespace.udecl]) that brings in from a base class an assignment operator with a parameter type that could be that of a copy/move assignment operator for the derived class is not considered an explicit declaration of such an operator and does not suppress the implicit declaration of the derived class operator; the operator introduced by the using-declaration is hidden by the implicitly-declared operator in the derived class.
A copy/move assignment operator for class X is trivial if it is not user-provided, its parameter-type-list is equivalent to the parameter-type-list of an implicit declaration, and if
class X has no virtual functions ([class.virtual]) and no virtual base classes ([class.mi]), and
class X has no non-static data members of volatile-qualified type, and
the assignment operator selected to copy/move each direct base class subobject is trivial, and
for each non-static data member of X that is of class type (or array thereof), the assignment operator selected to copy/move that member is trivial;
otherwise the copy/move assignment operator is non-trivial.
A copy/move assignment operator for a class X that is defaulted and not defined as deleted is implicitly defined when it is odr-used ([basic.def.odr]) (e.g., when it is selected by overload resolution to assign to an object of its class type) or when it is explicitly defaulted after its first declaration. The implicitly-defined copy/move assignment operator is constexpr if
X is a literal type, and
the assignment operator selected to copy/move each direct base class subobject is a constexpr function, and
for each non-static data member of X that is of class type (or array thereof), the assignment operator selected to copy/move that member is a constexpr function.
Before the defaulted copy/move assignment operator for a class is implicitly defined, all non-user-provided copy/move assignment operators for its direct base classes and its non-static data members shall have been implicitly defined. [ Note: An implicitly-declared copy/move assignment operator has an exception-specification ([except.spec]). — end note ]
The implicitly-defined copy/move assignment operator for a non-union class X performs memberwise copy/move assignment of its subobjects. The direct base classes of X are assigned first, in the order of their declaration in the base-specifier-list, and then the immediate non-static data members of X are assigned, in the order in which they were declared in the class definition. Let x be either the parameter of the function or, for the move operator, an xvalue referring to the parameter. Each subobject is assigned in the manner appropriate to its type:
if the subobject is of class type, as if by a call to operator= with the subobject as the object expression and the corresponding subobject of x as a single function argument (as if by explicit qualification; that is, ignoring any possible virtual overriding functions in more derived classes);
if the subobject is an array, each element is assigned, in the manner appropriate to the element type;
if the subobject is of scalar type, the built-in assignment operator is used.
It is unspecified whether subobjects representing virtual base classes are assigned more than once by the implicitly-defined copy assignment operator. [ Example:
struct V { };
struct A : virtual V { };
struct B : virtual V { };
struct C : B, A { };
It is unspecified whether the virtual base class subobject V is assigned twice by the implicitly-defined copy assignment operator for C. — end example ] [ Note: This does not apply to move assignment, as a defaulted move assignment operator is deleted if the class has virtual bases. — end note ]
The implicitly-defined copy assignment operator for a union X copies the object representation ([basic.types]) of X.
A program is ill-formed if the copy/move constructor or the copy/move assignment operator for an object is implicitly odr-used and the special member function is not accessible (Clause [class.access]). [ Note: Copying/moving one object into another using the copy/move constructor or the copy/move assignment operator does not change the layout or size of either object. — end note ]
When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the constructor selected for the copy/move operation and/or the destructor for the object have side effects. In such cases, the implementation treats the source and target of the omitted copy/move operation as simply two different ways of referring to the same object, and the destruction of that object occurs at the later of the times when the two objects would have been destroyed without the optimization.123 This elision of copy/move operations, called copy elision, is permitted in the following circumstances (which may be combined to eliminate multiple copies):
in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object (other than a function or catch-clause parameter) with the same cv-unqualified type as the function return type, the copy/move operation can be omitted by constructing the automatic object directly into the function's return value
in a throw-expression, when the operand is the name of a non-volatile automatic object (other than a function or catch-clause parameter) whose scope does not extend beyond the end of the innermost enclosing try-block (if there is one), the copy/move operation from the operand to the exception object ([except.throw]) can be omitted by constructing the automatic object directly into the exception object
when a temporary class object that has not been bound to a reference ([class.temporary]) would be copied/moved to a class object with the same cv-unqualified type, the copy/move operation can be omitted by constructing the temporary object directly into the target of the omitted copy/move
when the exception-declaration of an exception handler (Clause [except]) declares an object of the same type (except for cv-qualification) as the exception object ([except.throw]), the copy operation can be omitted by treating the exception-declaration as an alias for the exception object if the meaning of the program will be unchanged except for the execution of constructors and destructors for the object declared by the exception-declaration. [ Note: There cannot be a move from the exception object because it is always an lvalue. — end note ]
[ Example:
class Thing {
public:
Thing();
~Thing();
Thing(const Thing&);
};
Thing f() {
Thing t;
return t;
}
Thing t2 = f();
Here the criteria for elision can be combined to eliminate two calls to the copy constructor of class Thing: the copying of the local automatic object t into the temporary object for the return value of function f() and the copying of that temporary object into object t2. Effectively, the construction of the local object t can be viewed as directly initializing the global object t2, and that object's destruction will occur at program exit. Adding a move constructor to Thing has the same effect, but it is the move construction from the temporary object to t2 that is elided. — end example ]
When the criteria for elision of a copy/move operation are met, but not for an exception-declaration, and the object to be copied is designated by an lvalue, or when the expression in a return statement is a (possibly parenthesized) id-expression that names an object with automatic storage duration declared in the body or parameter-declaration-clause of the innermost enclosing function or lambda-expression, overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue. If the first overload resolution fails or was not performed, or if the type of the first parameter of the selected constructor is not an rvalue reference to the object's type (possibly cv-qualified), overload resolution is performed again, considering the object as an lvalue. [ Note: This two-stage overload resolution must be performed regardless of whether copy elision will occur. It determines the constructor to be called if elision is not performed, and the selected constructor must be accessible even if the call is elided. — end note ]
[ Example:
class Thing {
public:
Thing();
~Thing();
Thing(Thing&&);
private:
Thing(const Thing&);
};
Thing f(bool b) {
Thing t;
if (b)
throw t; // OK: Thing(Thing&&) used (or elided) to throw t
return t; // OK: Thing(Thing&&) used (or elided) to return t
}
Thing t2 = f(false); // OK: Thing(Thing&&) used (or elided) to construct t2
— end example ]
This implies that the reference parameter of the implicitly-declared copy constructor cannot bind to a volatile lvalue; see [diff.special].
Because a template assignment operator or an assignment operator taking an rvalue reference parameter is never a copy assignment operator, the presence of such an assignment operator does not suppress the implicit declaration of a copy assignment operator. Such assignment operators participate in overload resolution with other assignment operators, including copy assignment operators, and, if selected, will be used to assign an object.
This implies that the reference parameter of the implicitly-declared copy assignment operator cannot bind to a volatile lvalue; see [diff.special].
Because only one object is destroyed instead of two, and one copy/move constructor is not executed, there is still one object destroyed for each one constructed.
A using-declaration ([namespace.udecl]) that names a constructor implicitly declares a set of inheriting constructors. The candidate set of inherited constructors from the class X named in the using-declaration consists of actual constructors and notional constructors that result from the transformation of defaulted parameters as follows:
all non-template constructors of X, and
for each non-template constructor of X that has at least one parameter with a default argument, the set of constructors that results from omitting any ellipsis parameter specification and successively omitting parameters with a default argument from the end of the parameter-type-list, and
all constructor templates of X, and
for each constructor template of X that has at least one parameter with a default argument, the set of constructor templates that results from omitting any ellipsis parameter specification and successively omitting parameters with a default argument from the end of the parameter-type-list.
The constructor characteristics of a constructor or constructor template are
the template parameter list ([temp.param]), if any,
the parameter-type-list ([dcl.fct]),
absence or presence of explicit ([class.conv.ctor]), and
absence or presence of constexpr ([dcl.constexpr]).
For each non-template constructor in the candidate set of inherited constructors other than a constructor having no parameters or a copy/move constructor having a single parameter, a constructor is implicitly declared with the same constructor characteristics unless there is a user-declared constructor with the same signature in the complete class where the using-declaration appears or the constructor would be a default, copy, or move constructor for that class. Similarly, for each constructor template in the candidate set of inherited constructors, a constructor template is implicitly declared with the same constructor characteristics unless there is an equivalent user-declared constructor template ([temp.over.link]) in the complete class where the using-declaration appears. [ Note: Default arguments are not inherited. An exception-specification is implied as specified in [except.spec]. — end note ]
A constructor so declared has the same access as the corresponding constructor in X. It is deleted if the corresponding constructor in X is deleted ([dcl.fct.def]). An inheriting constructor shall not be explicitly instantiated ([temp.explicit]) or explicitly specialized ([temp.expl.spec]).
[ Note: Default and copy/move constructors may be implicitly declared as specified in [class.ctor] and [class.copy]. — end note ]
[ Example:
struct B1 {
B1(int);
};
struct B2 {
B2(int = 13, int = 42);
};
struct D1 : B1 {
using B1::B1;
};
struct D2 : B2 {
using B2::B2;
};
The candidate set of inherited constructors in D1 for B1 is
B1(const B1&)
B1(B1&&)
B1(int)
The set of constructors present in D1 is
D1(), implicitly-declared default constructor, ill-formed if odr-used
D1(const D1&), implicitly-declared copy constructor, not inherited
D1(D1&&), implicitly-declared move constructor, not inherited
D1(int), implicitly-declared inheriting constructor
The candidate set of inherited constructors in D2 for B2 is
B2(const B2&)
B2(B2&&)
B2(int = 13, int = 42)
B2(int = 13)
B2()
The set of constructors present in D2 is
D2(), implicitly-declared default constructor, not inherited
D2(const D2&), implicitly-declared copy constructor, not inherited
D2(D2&&), implicitly-declared move constructor, not inherited
D2(int, int), implicitly-declared inheriting constructor
D2(int), implicitly-declared inheriting constructor
— end example ]
[ Note: If two using-declarations declare inheriting constructors with the same signatures, the program is ill-formed ([class.mem], [over.load]), because an implicitly-declared constructor introduced by the first using-declaration is not a user-declared constructor and thus does not preclude another declaration of a constructor with the same signature by a subsequent using-declaration. [ Example:
struct B1 {
B1(int);
};
struct B2 {
B2(int);
};
struct D1 : B1, B2 {
using B1::B1;
using B2::B2;
}; // ill-formed: attempts to declare D1(int) twice
struct D2 : B1, B2 {
using B1::B1;
using B2::B2;
D2(int); // OK: user declaration supersedes both implicit declarations
};
— end example ] — end note ]
An inheriting constructor for a class is implicitly defined when it is odr-used ([basic.def.odr]) to create an object of its class type ([intro.object]). An implicitly-defined inheriting constructor performs the set of initializations of the class that would be performed by a user-written inline constructor for that class with a mem-initializer-list whose only mem-initializer has a mem-initializer-id that names the base class denoted in the nested-name-specifier of the using-declaration and an expression-list as specified below, and where the compound-statement in its function body is empty ([class.base.init]). If that user-written constructor would be ill-formed, the program is ill-formed. Each expression in the expression-list is of the form static_cast<T&&>(p), where p is the name of the corresponding constructor parameter and T is the declared type of p.
[ Example:
struct B1 {
B1(int) { }
};
struct B2 {
B2(double) { }
};
struct D1 : B1 {
using B1::B1; // implicitly declares D1(int)
int x;
};
void test() {
D1 d(6); // OK: d.x is not initialized
D1 e; // error: D1 has no default constructor
}
struct D2 : B2 {
using B2::B2; // OK: implicitly declares D2(double)
B1 b;
};
D2 f(1.0); // error: B1 has no default constructor
template< class T >
struct D : T {
using T::T; // declares all constructors from class T
~D() { std::clog << "Destroying wrapper" << std::endl; }
};
Class template D wraps any class and forwards all of its constructors, while writing a message to the standard log whenever an object of class D is destroyed. — end example ]