Formatting strings using safe printf-like functions

The printf family of functions is a threat to security. It is a very bad design to allow users to put their own strings as a type and format the specifiers. So what do we do when user-defined format is required? How shall we implement the std::string to_string(const std::string& format_specifier) const; member function of the following class?

class i_hold_some_internals 
{
    int i;
    std::string s;
    char c;
    // ...
}; 

Getting ready

Basic knowledge of standard library is more than enough for this recipe.

How to do it...

We wish to allow users to specify their own output format for a string:

1. To do that in a safe manner, we need the following header:

#include <boost/format.hpp>

2. Now, we add some comments for the user:

    // `fmt` parameter may contain the following:
    // $1$ for outputting integer 'i'.
    // $2$ for outputting string 's'.
    // $3$ for outputting character 'c'.
    std::string to_string(const std::string&amp...