A function definition of the form:
attribute-specifier-seqopt decl-specifier-seqopt declarator virt-specifier-seqopt = 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 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, and,
it is implicitly considered to have the same exception-specification as if it had been implicitly declared ([except.spec]).
If a function that is explicitly defaulted has an explicit exception-specification that is not compatible ([except.spec]) with the exception-specification on the implicit declaration, then
if the function is explicitly defaulted on its first declaration, it is defined as deleted;
otherwise, the program is ill-formed.
[ Example:
struct S {
constexpr S() = default; S(int a = 0) = default; void operator=(const S&) = default; ~S() throw(int) = default; private:
int i;
S(S&); };
S::S(S&) = default;
— 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;
— end example ]
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