C++ <cstdarg> - va_copy

The C++ <cstdarg> va_copy macro copies src to dest. After the call, it carries the information needed to retrieve the same additional arguments as src.

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).

Syntax

C++11

void va_copy( va_list dest, va_list src );

Parameters

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

Return Value

None.

Example:

The example below shows the usage of va_copy macro function.

#include <iostream>
#include <cstdarg>
#include <cmath>

using namespace std;

double sample_stddev(int count, ...) {
  double sum = 0;
  va_list args1;
  va_list args2;

  va_start(args1, count);
  va_copy(args2, args1);

  //calculating mean
  for (int i = 0; i < count; ++i) {
    sum += va_arg(args1, double);
  }
  va_end(args1);
  double mean = sum / count;

  //calculating sum of square of difference
  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);
  
  //returning sample standard deviation
  return sqrt(sum_sq_diff / count);
}

int main () {
  cout<<"Std. deviation is: "<<sample_stddev(2, 25.0, 30.0)<<"\n";
  cout<<"Std. deviation is: "<<sample_stddev(3, 25.0, 30.0, 34.0)<<"\n";
  cout<<"Std. deviation is: "<<sample_stddev(4, 25.0, 30.0, 34.0, 40.0)<<"\n";
  return 0;
}

The output of the above code will be:

Std. deviation is: 2.5
Std. deviation is: 3.68179
Std. deviation is: 5.49432

❮ C++ <cstdarg> Library