va_start - cppreference.com (original) (raw)

| | | | | -------------------------------------------- | | | | 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.

[edit] Parameters

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

[edit] Expanded value

(none)

[edit] Notes

va_start is required to support parm_n with overloaded operator&.

[edit] Example

#include #include 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:

[edit] Defect reports

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