C++ missing std::string printf

gistfile1.cpp

#include <cstdio>
#include <cstdarg>
#include <string>
//missing string printf
//this is safe and convenient but not exactly efficient
inline std::string format(const char* fmt, ...){
    int size = 512;
    char* buffer = 0;
    buffer = new char[size];
    va_list vl;
    va_start(vl,fmt);
    int nsize = vsnprintf(buffer,size,fmt,vl);
    if(size<=nsize){//fail delete buffer and try again
        delete buffer; buffer = 0;
        buffer = new char[nsize+1];//+1 for /0
        nsize = vsnprintf(buffer,size,fmt,vl);
    }
    std::string ret(buffer);
    va_end(vl);
    delete buffer;
    return ret;
}