Recursively Pass Template Template To a Template Template Function

Question

It seems to me, that since in C++ we can pass a template template, then a function accepting a template template should be able to pass a (its) template template into itself. However this appears not to be the case.

template< typename PREVIOUS_TYPE_T, typename TYPE_T >
struct Node
{
    typedef TYPE_T TYPE;
    typedef PREVIOUS_TYPE_T PREVIOUS_TYPE;
    PREVIOUS_TYPE_T* previous;
    template< template< typename... > typename FUNCTOR_T, typename... CURRENT_ARGUMENTS_T >
    void DoCall() {
        previous->DoCall< FUNCTOR_T, TYPE_T, CURRENT_ARGUMENTS_T... >();
    }
};

Is there anyway to pass FUNCTOR_T above, or any work around to doing so?

I'm not sure, what was wrong with your original revision? The revisions could affect the answer. Could you clarify which one you actually mean and why? — , Aug 15 '19 at 21:37
@Chipster Thank you for your response. The original version was copy/pasted from an old file, not with the one I am currently working with which is what you see above. The old version had `template< typename... ARGUMENTS_T > typename FUNCTOR_T` and passes `ARGUMENTS_T` into the last (variadic) parameter of `DoCall` in the recursive call, which causes an obvious problem and isent what I am asking. What I am asking is: is there a way or work around to pass a template template parameter to a function called inside another function (or functor, etc.) to the effect of what is above? — The Floating Brain, Aug 15 '19 at 21:45

Guillaume Racicot · Accepted Answer · 2019-08-15T22:14:35.040

1

You cannot use ARGUMENTS_T. That name has no effect.

When you're passing a template template around, it's still a template with no parameters passed to it. There is no template parameter to extract from std::plus, as opposed to std::plus<int>.

Instead, use CURRENT_ARGUMENTS_T, which is what you intended to forward.

Also, you previous is a dependent type. You must desambiguate the template:

previous->template DoCall< FUNCTOR_T, TYPE_T, CURRENT_ARGUMENTS_T... >();

edited Aug 15 '19 at 22:14

answered Aug 15 '19 at 21:04

Guillaume Racicot

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Thank you for the reply. Sorry, I had removed `ARGUMENTS_T` before but copy/pasted from the wrong place. However it still fails compilation on GCC and MSVC with the code you posted. – The Floating Brain Aug 15 '19 at 21:14
1

@TheFloatingBrain see my edit – Guillaume Racicot Aug 15 '19 at 22:14
Sorry for the late reply, had to go, still reviewing your answer. – The Floating Brain Aug 16 '19 at 22:30
Thank you so much, this was very helpful!! – The Floating Brain Aug 17 '19 at 04:24

Recursively Pass Template Template To a Template Template Function

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