StringUtil.h

#include <memory>
#include <iostream>
#include <string>
#include <cstdio>

using namespace std; //Don't if you're in a header-file

template<typename ... Args>
string string_format( const std::string& format, Args ... args )
{
    size_t size = snprintf( nullptr, 0, format.c_str(), args ... ) + 1; // Extra space for '\0'
    unique_ptr<char[]> buf( new char[ size ] ); 
    snprintf( buf.get(), size, format.c_str(), args ... );
    return string( buf.get(), buf.get() + size - 1 ); // We don't want the '\0' inside
}
//////////////////////////////////////////////////////////////

#include <stdarg.h>  // For va_start, etc.
#include <memory>    // For std::unique_ptr

std::string string_format(const std::string fmt_str, ...) 
{
    int final_n, n = ((int)fmt_str.size()) * 2; /* Reserve two times as much as the length of the fmt_str */
    std::unique_ptr<char[]> formatted;
    va_list ap;
    while(1) 
    {
        formatted.reset(new char[n]); /* Wrap the plain char array into the unique_ptr */
        strcpy(&formatted[0], fmt_str.c_str());
        va_start(ap, fmt_str);
        final_n = vsnprintf(&formatted[0], n, fmt_str.c_str(), ap);
        va_end(ap);
        if (final_n < 0 || final_n >= n)
            n += abs(final_n - n + 1);
        else
            break;
    }
    return std::string(formatted.get());
}