va_copy

 

C++

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Technical Specifications

 

Utilities library

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Variadic functions

va_start

va_arg

va_copy

va_end

va_list

 

Defined in header <cstdarg>
void va_copy( va_list dest, va_list src );		(since C++11)

The va_copy macro copies src to dest.

va_end should be called on dest before the function returns or any subsequent re-initialization of dest (via calls to va_start or va_copy).

Parameters

dest	-	an instance of the va_list type to initialize
src	-	the source va_list that will be used to initialize dest

Expanded value

(none)

Example

Run this code

#include <iostream>
#include <cstdarg>
#include <cmath>
 
double sample_stddev(int count, ...) 
{
    double sum = 0;
    va_list args1;
    va_start(args1, count);
    va_list args2;
    va_copy(args2, args1);
    for (int i = 0; i < count; ++i) {
        double num = va_arg(args1, double);
        sum += num;
    }
    va_end(args1);
    double mean = sum / count;
 
    double sum_sq_diff = 0;
    for (int i = 0; i < count; ++i) {
        double num = va_arg(args2, double);
        sum_sq_diff += (num-mean) * (num-mean);
    }
    va_end(args2);
    return std::sqrt(sum_sq_diff / count);
}
 
int main() 
{
    std::cout << sample_stddev(4, 25.0, 27.3, 26.9, 25.7) << '\n';
}

Output:

0.920258

See also

va_start	enables access to variadic function arguments (function macro)
va_arg	accesses the next variadic function argument (function macro)
va_end	ends traversal of the variadic function arguments (function macro)