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va_start - cppreference.com

From cppreference.com

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

Parameters

ap - an object 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 <cstdarg>
#include <iostream>

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 it was unclear whether va_start is required to
support parm_ns with overloaded operator& 		required
LWG 2099 C++98 the behavior was undefined if parm_n is
declared with a function, array, or reference type 		the behavior is undefined if
parm_n is of reference type

See also