11 Declarators [dcl.decl]

11.4 Function definitions [dcl.fct.def]

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

A function definition of the form:

attribute-specifier-seq $_{o p t}$  decl-specifier-seq $_{o p t}$  declarator virt-specifier-seq $_{o p t}$   = default ;

is called an explicitly-defaulted definition. A function that is explicitly defaulted shall

(1.1)
be a special member function,
(1.2)
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
(1.3)
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

(3.1)
if the function is explicitly defaulted on its first declaration, it is defined as deleted;
(3.2)
otherwise, the program is ill-formed.

[ Example:

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
private:
  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. [ Note: 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:

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 ]