A variadic function that accepts Strings and Ints, Format the latter and concatenate all?

Question

I'm trying to use the answer from DanielKO in this question for my needs but i'm not familiar with templates and variadic functions, and i don't get what should i do.

What i'd need is a variadic c++(11) function which i can call like this:

 String NewMsg = CreateMessage("SET",16,1,17,0,"RED",47);

and have NewMsg= "SET,0010,0001,0011,0000,RED,002F".

I'm not even able to get where should i add the comma between the arguments. And then: How could i distinguish between integers and string while parsing the args, so to format each integer to hexadecimal strings?

Answer 1

You use recursion and function overloading

std::string CreateMessage(int i)
{
    return /* i formatted as hex */;
}

std::string CreateMessage(const char* s)
{
    return s;
}

template<typename T, typename... Ts>
std::string CreateMessage(T t, Ts... ts)
{
    return CreateMessage(t) + "," + CreateMessage(ts...);
}

Answer 2

One option is to use recursion with templates, as Passer By did in his answer. However, in my opinion a more elegant solution (if you are able to use C++17 lanugage features) is to use a fold expression to avoid recursion. The expression is expanded such that Append is called directly for each argument, sort of like a for-loop across the arguments evaluated at compile time.

template <class T>
void Append(std::ostringstream &out, T &&arg) {
    out << "," << std::forward<T>(arg);
}

template <class... TArgs>
std::string CreateMessage(TArgs &&...args) {
    std::ostringstream out;
    (Append(out, std::forward<TArgs>(args)), ...);
    return out.str().substr(1);
}

Live demo here.