The function declarator includes the list of parameters that can be passed to the function when it is called by another function, or by itself.

In C++, the parameter list of a function is referred to as its signature. The name and signature of a function uniquely identify it. As the word itself suggests, the function signature is used by the compiler to distinguish among the different instances of overloaded functions.

Function parameter declaration syntax
 
      .-,-------------.
      V               |
>>-(----+-----------+-+--+--------+--)-------------------------><
        '-parameter-'    '-,--...-'
 

parameter
 
>>-+----------+--type_specifier--+------------+----------------><
   '-register-'                  '-declarator-'
 

An empty argument list in a function declaration or definition indicates a function that takes no arguments. To explicitly indicate that a function does not take any arguments, you can declare the function in two ways: with an empty parameter list, or with the keyword void:

      int f(void);
      int f();

An empty argument list in a function definition indicates a function that takes no arguments. An empty argument list in a function declaration indicates that a function may take any number or type of arguments. Thus,

int f()
{
...
}

indicates that function f takes no arguments. However,

int f();

simply indicates that the number and type of parameters is not known. To explicitly indicate that a function does not take any arguments, you should define the function with the keyword void.

An ellipsis at the end of the parameter specifications is used to specify that a function has a variable number of parameters. The number of parameters is equal to, or greater than, the number of parameter specifications.

      int f(int, ...);

The comma before the ellipsis is optional. In addition, a parameter declaration is not required before the ellipsis.

At least one parameter declaration, as well as a comma before the ellipsis, are both required in C.

Related information

• The void type
• Type specifiers
• Type qualifiers
• Exception specifications (C++ only)