va_start

 

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Variadic functions

va_start

va_arg

va_copy (C++11)

va_end

va_list

 

Defined in header <cstdarg>
void va_start( std::va_list ap, parmN );		(until C++26)
void va_start( std::va_list ap, ... );			(since C++26)

The va_start macro enables access to the variable argumentsfollowing the named argument parmN(until C++26).

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

If 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.	(until C++26)
If more than one argument is present for va_start, the preprocessing tokens comprising the second and subsequent arguments are discarded. If any of those arguments expands to include unbalanced parentheses, or a preprocessing token that does not convert to a token, the program is ill-formed, no diagnostic required.	(since C++26)

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&.

(until C++26)

Example

Run this code

#include <cstdarg>
#include <iostream>

int add_nums(int count...)
{
    int result = 0;
    std::va_list args;
    va_start(args, count); // count can be omitted since C++26
    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

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

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