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va_arg

From cppreference.com
< cpp‎ | utility‎ | variadic
Revision as of 10:33, 15 September 2013 by 24.246.29.51 (Talk)

Defined in header <cstdarg>
T va_arg( va_list ap, T );

The va_arg macro expands to an expression of type T that corresponds to the next parameter from the va_list ap.

Prior to calling va_arg, ap must be initialized by a call to either va_start or va_copy, with no intervening call to va_end. Each invocation of the va_arg macro modifies ap to point to the next variable argument.

If va_arg is called when there are no more arguments in ap, or if the type of the next argument in ap (after promotions) is not compatible with T, the behavior is undefined, unless:

  • one type is a signed integer type, the other type is the corresponding unsigned integer type, and the value is representable in both types; or
  • one type is pointer to void and the other is a pointer to a character type.

Contents

Parameters

ap - an instance of the va_list type
T - the type of the next parameter in ap

Expanded value

the next variable parameter in ap

Example

#include <iostream>
#include <cstdarg>
#include <cmath>
 
double stddev(int count, ...) 
{
    double sum = 0;
    double sum_sq = 0;
    va_list args;
    va_start(args, count);
    for (int i = 0; i < count; ++i) {
        double num = va_arg(args, double);
        sum += num;
        sum_sq += num*num;
    }
    va_end(args);
    return std::sqrt(sum_sq/count - (sum/count)*(sum/count));
}
 
int main() 
{
    std::cout << stddev(4, 25.0, 27.3, 26.9, 25.7) << '\n';
}

Output:

0.920258

See also

enables access to variadic function arguments
(function macro) [edit]
(C++11)
makes a copy of the variadic function arguments
(function macro) [edit]
ends traversal of the variadic function arguments
(function macro) [edit]