8 Declarators [dcl.decl]

Function definitions have the form

function-definition:
    attribute-specifier-seqopt decl-specifier-seqopt declarator virt-specifier-seqopt function-body

function-body:
    ctor-initializeropt compound-statement
    function-try-block
    = default ;
    = delete ;

Any informal reference to the body of a function should be interpreted as a reference to the non-terminal function-body. The optional attribute-specifier-seq in a function-definition appertains to the function. A virt-specifier-seq can be part of a function-definition only if it is a member-declaration ([class.mem]).

The declarator in a function-definition shall have the form

D1 ( parameter-declaration-clause ) cv-qualifier-seqopt   
    ref-qualifieropt exception-specificationopt attribute-specifier-seqopt trailing-return-typeopt

as described in [dcl.fct]. A function shall be defined only in namespace or class scope.

[ Example: a simple example of a complete function definition is

int max(int a, int b, int c) {
  int m = (a > b) ? a : b;
  return (m > c) ? m : c;
}

Here int is the decl-specifier-seq; max(int a, int b, int c) is the declarator; { /* ... */ } is the function-body.  — end example ]

A ctor-initializer is used only in a constructor; see [class.ctor] and [class.init].

A cv-qualifier-seq or a ref-qualifier (or both) can be part of a non-static member function declaration, non-static member function definition, or pointer to member function only ([dcl.fct]); see [class.this].

[ Note: Unused parameters need not be named. For example,

void print(int a, int) {
  std::printf("a = %d\n",a);
}

 — end note ]

In the function-body, a function-local predefined variable denotes a block-scope object of static storage duration that is implicitly defined (see [basic.scope.block]).

The function-local predefined variable __func__ is defined as if a definition of the form

static const char __func__[] = "function-name";

had been provided, where function-name is an implementation-defined string. It is unspecified whether such a variable has an address distinct from that of any other object in the program.104

[ Example:

struct S {
  S() : s(__func__) { }             // OK
  const char* s;
};
void f(const char* s = __func__);  // error: __func__ is undeclared

 — end example ]

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

• (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 may be declared constexpr only if it would have been implicitly declared as constexpr. If a function is explicitly defaulted on its first declaration,

• (2.1)

  it is implicitly considered to be constexpr if the implicit declaration would be, and,

• (2.2)

  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

• (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() throw(int) = 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 ]

A function definition of the form:

attribute-specifier-seqopt decl-specifier-seqopt declarator virt-specifier-seqopt  = delete ;

is called a deleted definition. A function with a deleted definition is also called a deleted function.

A program that refers to a deleted function implicitly or explicitly, other than to declare it, is ill-formed. [ Note: This includes calling the function implicitly or explicitly and forming a pointer or pointer-to-member to the function. It applies even for references in expressions that are not potentially-evaluated. If a function is overloaded, it is referenced only if the function is selected by overload resolution.  — end note ]

[ Example: One can enforce non-default initialization and non-integral initialization with

struct onlydouble {
  onlydouble() = delete;              // OK, but redundant
  onlydouble(std::intmax_t) = delete;
  onlydouble(double);
};

 — end example ]

[ Example: One can prevent use of a class in certain new expressions by using deleted definitions of a user-declared operator new for that class.

struct sometype {
  void* operator new(std::size_t) = delete;
  void* operator new[](std::size_t) = delete;
};
sometype* p = new sometype;     // error, deleted class operator new
sometype* q = new sometype[3];  // error, deleted class operator new[]

 — end example ]

[ Example: One can make a class uncopyable, i.e. move-only, by using deleted definitions of the copy constructor and copy assignment operator, and then providing defaulted definitions of the move constructor and move assignment operator.

struct moveonly {
  moveonly() = default;
  moveonly(const moveonly&) = delete;
  moveonly(moveonly&&) = default;
  moveonly& operator=(const moveonly&) = delete;
  moveonly& operator=(moveonly&&) = default;
  ~moveonly() = default;
};
moveonly* p;
moveonly q(*p); // error, deleted copy constructor

 — end example ]

A deleted function is implicitly inline. [ Note: The one-definition rule ([basic.def.odr]) applies to deleted definitions.  — end note ] A deleted definition of a function shall be the first declaration of the function or, for an explicit specialization of a function template, the first declaration of that specialization. [ Example:

struct sometype {
  sometype();
};
sometype::sometype() = delete;      // ill-formed; not first declaration

 — end example ]