Carrying type information in class template instantiation

Question

I need to access type information from a class I used to instantiate another class.

Specifically void Beta<T>::do_something() needs to accept parameters of type W, S that were used to instantiate class Alpha<W, S>.

template<typename W, S> 
class Alpha {
public:
  using carry_W = W;
  using carry_S = S;
};

template<typename T> 
class Beta {};
template<typename T>
void Beta<T>::do_something(typename T::carry_W p1, typename T::carry_S p2) {}

Beta<Alpha<int, double>> b;

The solution above works fine but is there any other way to do this without aliasing the types as class members? Is there a more "C++" way of doing this?

possible duplicate of [Can one access the template parameter outside of a template without a typedef?](http://stackoverflow.com/questions/3696286/can-one-access-the-template-parameter-outside-of-a-template-without-a-typedef) — m.s., Sep 07 '15 at 08:31
Purely stylistic but I'd rather use something like [this](http://coliru.stacked-crooked.com/a/2dbf6b21987a5a7b) — Marco A., Sep 07 '15 at 08:32
Why don't you just take all arguments with forwarding references and let validity inside `do_something` do the rest? — Columbo, Sep 07 '15 at 08:33
@Columbo not sure I get it, would it be possible to develop this more? — Florin Gogianu, Sep 07 '15 at 08:41

tahsmith · Accepted Answer · 2015-09-12T23:19:30.650

1

You can create a class template that consists only of a forward declaration and a partial specialisation.

#include <iostream>

using namespace std;

template<typename W, typename S> 
class Alpha {
};

template<typename>
class Beta;

template<typename W, typename S, template<typename, typename> class T>
class Beta<T<W,S>> {
public:
  void do_something(W w, S s) {
      cout << w << ", " << s << '\n';
  }
};

int main() { 
    Beta<Alpha<int, double>> b;
    b.do_something(0, 0.0);
}

edited Sep 12 '15 at 23:19

answered Sep 07 '15 at 11:19

tahsmith

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Yes, but it would only match a specific template. – Florin Gogianu Sep 12 '15 at 18:51
@FlorinGogianu Ah K, I though this was for some specific `Alpha`. – tahsmith Sep 12 '15 at 23:12
I guess this is the best we get. Thanks! – Florin Gogianu Sep 20 '15 at 10:00
@FlorinGogianu, Is there any way I can expand on this for you? It seems to be a fairly exact match for your question. – tahsmith Sep 20 '15 at 11:54
I think I got it. Marked your take as answer. – Florin Gogianu Sep 20 '15 at 11:59

xtofl · Answer 2 · 2015-09-07T09:09:56.140

0

Do you mean something like the following (template 'pattern matching')?

template<typename T<W, S>>
void Beta<T>::do_something(W, S) {...}

While I think your question is totally legit, I fear that the current C++ does not allow this shortcut...

edited Sep 07 '15 at 09:09

answered Sep 07 '15 at 08:26

xtofl

40,723
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Yes, that would be something I guess. I was asking because the way I wrote it seems to me a bit of a hack and I was not sure if this is the accepted way of doing it. – Florin Gogianu Sep 07 '15 at 10:08

Carrying type information in class template instantiation

2 Answers2