sprintf C++ Reference

Return Value
On success, the total number of characters written is returned. This count does not include the additional null-character automatically appended at the end of the string.

On failure, a negative number is returned.

Example
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/* sprintf example */
#include <stdio.h>

int main ()
{
  char buffer [50];
  int n, a=5, b=3;
  n=sprintf (buffer, "%d plus %d is %d", a, b, a+b);
  printf ("[%s] is a %d char long string\n",buffer,n);
  return 0;
}
Output:

[5 plus 3 is 8] is a 13 char long string