11 Declarators [dcl.decl]

11.4 Function definitions [dcl.fct.def]

11.4.2 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,
  • have the same declared function type (except for possibly differing ref-qualifiers and except that in the case of a copy constructor or copy assignment operator, the parameter type may be “reference to non-const T”, where T is the name of the member function's class) as if it had been implicitly declared, and
  • not have default arguments.
An explicitly-defaulted function that is not defined as deleted may be declared constexpr only if it would have been implicitly declared as constexpr.
If a function is explicitly defaulted on its first declaration, it is implicitly considered to be constexpr if the implicit declaration would be.
If a function that is explicitly defaulted is declared with a noexcept-specifier that does not produce the same exception specification as the implicit declaration ([except.spec]), then
  • if the function is explicitly defaulted on its first declaration, it is defined as deleted;
  • otherwise, the program is ill-formed.
struct S {
  constexpr S() = default;              // ill-formed: implicit S() is not constexpr
  S(int a = 0) = default;               // ill-formed: default argument
  void operator=(const S&) = default;   // ill-formed: non-matching return type
  ~S() noexcept(false) = default;       // deleted: exception specification does not match
  int i;
  S(S&);                                // OK: private copy constructor
S::S(S&) = default;                     // OK: defines copy constructor
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]), which might mean defining them 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 defined at the point where it is explicitly defaulted; if such a function is implicitly defined as deleted, the program is ill-formed.
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
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() = default;   // not first declaration
end example