< cpp‎ | utility‎ | variadic
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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.


[edit] Parameters

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

[edit] Expanded value


[edit] Notes

va_start is required to support parm_n with overloaded operator&.

[edit] 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);
    return result;
int main() 
    std::cout << add_nums(4, 25, 25, 50, 50) << '\n';



[edit] 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

[edit] See also

accesses the next variadic function argument
(function macro) [edit]
ends traversal of the variadic function arguments
(function macro) [edit]
C documentation for va_start