Dynamic Class Members

Question

I am new to C++11's variadic templates, and do not know if what I am doing wrong is a simple syntax error, or is impossible. I would like to write something like the following:

template< typename... Args >
struct Test
{
    Args... args;
    Test( Args... args_ ) : args( args_... ) {
    }
};

Basically this class would have "pseudo dynamic members". I have tried this with:

struct A {};
template< typename... Args >
A* MakeB( Args... args )
{
    struct B : public A
    {
        Args... args;
        B( Args... args_ ) : args( args_... ) {
        }
    };
    return new B;
};

Is it impossible, or am I just not good with variadic templates?

score 4 · Accepted Answer · answered Oct 25 '13 at 21:48

Packs are not types, and cannot be used like types. They are a special construction only for use in templates. You cannot have a "pack member".

The typical solution is to have a tuple member:

#include <tuple>

template< typename... Args >
struct Test
{
    std::tuple<Args...> args_;

    Test(Args const &... args)
    : args_(args_...)
    { }
};

Your little maker would look like this:

template <typename ...Args>
Test<Args...> make_test<Args const &... args)
{
    return Test<Args>(args...);
}

Usage:

auto t = make_test(1, true, 'x');   // makes a Test<int, bool, char>

Is there a way to extract the data so I could use the expansion syntax to put the members into a function call effectivly i.e func( args... ); ? — The Floating Brain, Oct 25 '13 at 22:09

Dynamic Class Members

1 Answers1