9 Declarations [dcl.dcl]

9.5 Function definitions [dcl.fct.def]

9.5.2 Explicitly-defaulted functions [dcl.fct.def.default]

A function definition whose function-body is of the form = default ; is called an explicitly-defaulted definition.
A function that is explicitly defaulted shall
  • be a special member function or a comparison operator function ([over.binary]), and
  • not have default arguments.
An explicitly defaulted special member function is allowed to differ from the corresponding special member function that would have been implicitly declared, as follows:
  • and may have differing ref-qualifiers;
  • if has an implicit object parameter of type “reference to C”, may be an explicit object member function whose explicit object parameter is of type “reference to C”, in which case the type of would differ from the type of in that the type of has an additional parameter;
  • and may have differing exception specifications; and
  • if has a non-object parameter of type const C&, the corresponding non-object parameter of may be of type C&.
If the type of differs from the type of in a way other than as allowed by the preceding rules, then:
  • if is an assignment operator, and the return type of differs from the return type of or 's non-object parameter type is not a reference, the program is ill-formed;
  • otherwise, if is explicitly defaulted on its first declaration, it is defined as deleted;
  • otherwise, the program is ill-formed.
A function explicitly defaulted on its first declaration is implicitly inline ([dcl.inline]), and is implicitly constexpr ([dcl.constexpr]) if it satisfies the requirements for a constexpr function.
[Example 1: struct S { S(int a = 0) = default; // error: default argument void operator=(const S&) = default; // error: non-matching return type ~S() noexcept(false) = default; // OK, despite mismatched exception specification private: int i; S(S&); // OK, private copy constructor }; S::S(S&) = default; // OK, defines copy constructor struct T { T(); T(T &&) noexcept(false); }; struct U { T t; U(); U(U &&) noexcept = default; }; U u1; U u2 = static_cast<U&&>(u1); // OK, calls std​::​terminate if T​::​T(T&&) throws — end example]
Explicitly-defaulted functions and implicitly-declared functions are collectively called defaulted functions, and the implementation shall provide implicit definitions for them ([class.ctor], [class.dtor], [class.copy.ctor], [class.copy.assign]) as described below, including possibly defining them as deleted.
A defaulted prospective destructor ([class.dtor]) that is not a destructor is defined as deleted.
A defaulted special member function that is neither a prospective destructor nor an eligible special member function ([special]) is defined as deleted.
A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration.
A user-provided explicitly-defaulted function (i.e., explicitly defaulted after its first declaration) is implicitly defined at the point where it is explicitly defaulted; if such a function is implicitly defined as deleted, the program is ill-formed.
A non-user-provided defaulted function (i.e. implicitly declared or explicitly defaulted in the class) that is not defined as deleted is implicitly defined when it is odr-used ([basic.def.odr]) or needed for constant evaluation ([expr.const]).
[Note 1:
Declaring a function as defaulted after its first declaration can provide efficient execution and concise definition while enabling a stable binary interface to an evolving code base.
— end note]
[Example 2: struct trivial { trivial() = default; trivial(const trivial&) = default; trivial(trivial&&) = default; trivial& operator=(const trivial&) = default; trivial& operator=(trivial&&) = default; ~trivial() = default; }; struct nontrivial1 { nontrivial1(); }; nontrivial1::nontrivial1() = default; // not first declaration — end example]