va_start

 

C++

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Variadic functions

va_start

va_arg

va_copy

va_end

va_list

 

Defined in header <cstdarg>
void va_start( 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 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

Run this code

#include <iostream>
#include <cstdarg>
 
int add_nums(int count, ...) 
{
    int result = 0;
    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:

150

See also

va_arg	accesses the next variadic function argument (function macro)
va_end	ends traversal of the variadic function arguments (function macro)
C documentation for va_start