va_start

< cpp‎ | utility‎ | variadic

Defined in header `<cstdarg>`
void va_start( std::va_list ap, parm_n );

The va_start macro enables access to the variable arguments following the named argument parm_n.

va_start should be invoked with an instance to a valid va_list object ap before any calls to va_arg.

If the parm_n is a pack expansion or an entity resulting from a lambda capture, the program is ill-formed, no diagnostic required.

(since C++11)

If parm_n is declared with reference type or with a type not compatible with the type that results from default argument promotions, the behavior is undefined.

Parameters

ap	-	an instance of the va_list type
parm_n	-	the named parameter preceding the first variable parameter

Expanded value

(none)

Notes

va_start is required to support parm_n with overloaded operator&.

Example

#include <iostream>
#include <cstdarg>
 
int add_nums(int count, ...) 
{
    int result = 0;
    std::va_list args;
    va_start(args, count);
    for (int i = 0; i < count; ++i) {
        result += va_arg(args, int);
    }
    va_end(args);
    return result;
}
 
int main() 
{
    std::cout << add_nums(4, 25, 25, 50, 50) << '\n';
}

Output:

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 273	C++98	`va_start` may not work if unary `operator&` is overloaded	required to work correctly even if `operator&` is overloaded