C++ variable arguments using initializer lists

Question

Assume the following code, which is a tiny sprintf substitute. (The _itoa and the likes was just used to keep the code short.)

#include <cstdlib>
#include <string>
class Arg {
public:
    Arg(const std::string& s) :m_str(s) {}
    Arg(const char* s) : m_str(s) {}
    Arg(int digi, double number)  {char buf[128]; m_str = _gcvt(number, digi, buf);}
    operator const std::string& ()const { return m_str; }
private:
    std::string m_str;
};

class Format {
public:
    Format(/*const char* format, */std::initializer_list<Arg> args); // see below
    const std::string& str()const { return m_str; }
private:
    std::string m_str;
};

Format::Format(/*const char* format, */std::initializer_list<Arg> args) {
    auto arg = args.begin();
    auto format = std::string(*arg++);
    for(const char* c = format.c_str(); *c!='\0'; ++c) {
        if(*c=='%') { m_str+=*arg++; }
        else { m_str+=*c; }
    }
}


int main() {

    std::string test1 = Format{"test Double:% String:%", {5, 456.78}, "foo"}.str();

    // I want to make this work. See the braces.
    std::string test2 = Format("test Double:% String:%", {5, 456.78}, "foo").str();

    return 0;
}

You see, I want to pass arguments, limited to type "Arg", but use a constructor that uses e.g. varadic templates instead of the initializer_list<> for better readability.

I tried:

    template<typename... T>
    Format(T&& ... args) : Format(std::forward<Args>(args)...) {}

But i get:

error C2440: '<function-style-cast>': cannot convert from 'initializer list' to 'Format'
note: No constructor could take the source type, or constructor overload resolution was ambiguous

Answer 1

std::initializer_list requires {} not ().

{5, 456.78} has no type and couldn't be deduced for template.

The way to keep your syntax in the old overload way:

Format(Arg arg0) : Format(std::initializer_list{arg0});
Format(Arg arg0, Arg arg1) : Format({arg0, arg1});
Format(Arg arg0, Arg arg1, Arg arg2) : Format({arg0, arg1, arg2});
// ... Up to some limit