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.local]).

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.102

[ Example:

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

 — end example ]