What is the use of variadic constructor in C++?

Question

Consider following program:

#include <iostream>
struct Test
{
    Test(...)
    {
        std::cout<<"Variadic constructor\n";
    }
};
int main()
{
    Test t;
    t={3,4,5};
}

I think it is variadic constructor. Does the C++ standard says that constructor can be variadic? What is the use of such constructor? What is the rationale for allowing variadic constructor?

So you don't have to write multiple constructors. This one isn't very useful, by the way, it doesn't provide any info as to what the parameters actually are. — Mr Lister, Oct 13 '15 at 16:45
For the first question: functions can be variadic, constructor is a function. Why can't it be variadic? — lisyarus, Oct 13 '15 at 16:45
@MrLister: It would be better if you write an answer that explains its usefulness using an example. — Destructor, Oct 13 '15 at 16:46
Now that we have variadic templates, the varargs method is pretty much useless. Lack of type safety and all. — Ben Voigt, Oct 13 '15 at 16:47
Pravasi Meet, but as @lisyarus says, a variadic constructor is exactly as useful as any other variadic function. Are you really inquiring about what variadic functions are for at all? — Mr Lister, Oct 13 '15 at 16:51

score 7 · Accepted Answer · edited Nov 12 '15 at 06:31

Let's try to answer your questions one by one:

I think it is variadic constructor.

You are correct.

Does the C++ standard says that constructor can be variadic?

IANALL, but, I think so. Why not? Constructor is just a (member) function.

What is the use of such constructor?

Like any other variadic function - to pass a variable number of arguments. It also has the same problems, mostly no type safety as any other variadic function. For example, let's say you need a list of (C) strings, you could do something like See live demo here.

#include <iostream>
#include <cstdarg>
struct Test
{
    Test(int n,...)
    {
        va_list va;
        va_start(va, n);
        for (int i = 0; i < n; ++i) {
             char const *s = va_arg(va, char*);
             std::cout<<"s=" << s << std::endl;
        }
        va_end(va);
    }
};
int main()
{
     Test t{3, "3","4","5"};
}

Keep in mind that for this to work, you need at least one "non-variadic" parameter. So a "pure variadic" constructor, like the one you show, doesn't make much sense in portable C++ code. For any particular platform, you may know how to access the parameters even without the non-variadic parameter, so, this might work:

     Test t={"3","4","5", NULL};

What is the rationale for allowing variadic constructor?

"It's C compatible and someone might have a use of it", I guess. If you know your way around <cstdarg>, it can be an effective tool. Of course, with C++11, you should most probably use variadic templates / perfect forwarding and initialization lists instead. But, with C++98 you didn't have these tools.

Your code contains compilation errors. (See live demo http://ideone.com/VkQmwx ) variable va should be of va_list not va_arg. (See working code here: http://ideone.com/nS2Kuw ) — Destructor, Nov 12 '15 at 06:30
@PravasiMeet thanks, I should have checked it, of course, but it was more of an illustration than a demo, that's why it slipped... — srdjan.veljkovic, Nov 12 '15 at 23:38

What is the use of variadic constructor in C++?

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