Here's one you can use with (pretty much) any version of C++:

    __attribute__ ((format (printf, 1, 2))) 
    static inline std::string format(const char* fmt, ...) {
        va_list args;
        va_start(args, fmt);
        size_t bytes = vsnprintf(0, 0, fmt, args);
        va_end(args);
    
        va_list again;
        va_start(again, fmt);
        std::string result(bytes, '\0');
        vsnprintf(&result[0], bytes + 1, fmt, again);
        va_end(again);
        return result;
    }