Question

How print format string passed as argument ?

example.cpp:

#include <iostream> 
int main(int ac, char* av[]) 
{
     printf(av[1],"anything");
     return 0;
}

try:

example.exe "print this\non newline"

output is:

print this\non newline

instead I want:

print this
on newline

Answer 1

No, do not do that! That is a very severe vulnerability. You should never accept format strings as input. If you would like to print a newline whenever you see a “\n”, a better approach would be:

#include <iostream>
#include <cstdlib>

int main(int argc, char* argv[])
{
     if ( argc != 2 ){
         std::cerr << "Exactly one parameter required!" << std::endl;
         return 1;
     }

     int idx = 0;
     const char* str = argv[1];
     while ( str[idx] != '\0' ){
          if ( (str[idx]=='\\') && (str[idx+1]=='n') ){
                 std::cout << std::endl;
                 idx+=2;
          }else{
                 std::cout << str[idx];
                 idx++;
          }
     }
     return 0;
}

Or, if you are including the Boost C++ Libraries in your project, you can use the boost::replace_all function to replace instances of “\\n” with “\n”, as suggested by Pukku.