How to use variable length parameter list with only one reference parameter?

Question

Recently I meet an annoying problem that I want to define functions like this:

std::string my_sprintf(const char* format, ...)
{
    va_list args;
    va_start(args, format);
    ...
}
std::string my_sprintf(const std::string& format, ...)
{
    va_list args;
    va_start(args, format); // error
    ...
}

But it seems reference value can't be the last parameter when using variable length parameter list.

Is that a way that I can let user use std::string as format string?

Maybe factors or something else that detect std::string and convert it as c_str() would work, but I don't know how to deal with the following variable length parameter list.

Edit: I'm not using variadic template because i'm using vsprintf inside. Maybe avoid using vsprintf and using std::stringstream is an option?

Answer 1

The behaviour is undefined: See C++ Standard 18.10/3 (for C++11 and C++14), or cppreference.com: Variadic Arguments.

Essentially you can only use types that are available in C with va_start, although an exception is made for the std::nullptr_t type.

Answer 2

In C++, the way to pass "unknown" arguments is to use variadic template:

template <typename ... Ts>
std::string my_sprintf(const std::string& format, Ts&&...args)
{
    return my_sprintf(format.c_str(), std::forward<Ts>(args)...);
}

and you version with const char* should probably also be variadic template, by using sprintf instead of vsprintf.