Reasonable string formatting for C++ (requires C++11)

stringf.cpp

#include <stdarg.c>
#include <string>
#include <vector>

std::string stringf(const char *fmt, ...) {
    std::vector<char> buffer(256);
    va_list args, cp;
    va_start(args, fmt);
    va_copy(cp, args);
    int sz = vsnprintf(&buffer[0], buffer.size(), fmt, args);
    if (sz >= int(buffer.size())) {
        buffer.resize(sz + 1);
        vsnprintf(&buffer[0], buffer.size(), fmt, cp);
    }
    va_end(cp);
    va_end(args);
    return &buffer[0];
}