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va_start

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

Contents

[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

Run this code
#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: 

150

[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 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

[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

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