[dcl.fct.def.default]

An explicitly defaulted special member function

If the type of

A function explicitly defaulted on its first declaration is implicitly inline ([dcl.inline]), and is implicitly constexpr ([dcl.constexpr]) if it is constexpr-suitable.

[Note 1:

Other defaulted functions are not implicitly constexpr.

— end note]

[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.

[Note 2:

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]

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 3:

The implicit definition of a non-user-provided defaulted function does not bind any names.

— 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]