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# va_copy

From cppreference.com
< cpp‎ | utility‎ | variadic

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).

### [edit] Parameters

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

### [edit] Expanded value

(none)

### [edit] 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

### [edit] See also

va_start | enables access to variadic function arguments (function macro) [edit]
va_arg | accesses the next variadic function argument (function macro) [edit]
va_end | ends traversal of the variadic function arguments (function macro) [edit]
C documentation for va_copy

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