Using contrained-type-specifier as a type of constrained variable

[Anton Bikineev]

Hi SG8,

I haven't found an answer in N4553,but is it possible by design to use constrained-type-specifier to declare constrained variable, like

template <class T>

concept bool callable = ...;

template <class T>

concept bool button = requires (T t, callable c)

{

    {t.on_click(c)};

};

Anyway, it would be of a good use.

[Anton Bikineev]

trying to compile this code with gcc-trunk gives the following:

Antons-MacBook-Air:c++ bikineev$ g++ -o 41 41.cc -std=c++1z -fconcepts

41.cc:21:52: internal compiler error: in synthesize_implicit_template_parm, at cp/parser.c:37830

 concept bool button = requires (T t, callable c)

                                    ^

41.cc:21:52: internal compiler error: Abort trap: 6

среда, 15 июня 2016 г., 14:45:52 UTC+3 пользователь Anton Bikineev написал:

[Andrew Sutton]

It isn't specifically allowed and therefore ill-formed (7.1.6.4p8). I think this might be useful, but how would you go determine if a constraint using a placeholder was valid?

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Andrew Sutton

[Tom Honermann]

On 6/15/2016 7:48 AM, Anton Bikineev wrote:

trying to compile this code with gcc-trunk gives the following:

Antons-MacBook-Air:c++ bikineev$ g++ -o 41 41.cc -std=c++1z -fconcepts

41.cc:21:52: internal compiler error: in synthesize_implicit_template_parm, at cp/parser.c:37830

 concept bool button = requires (T t, callable c)

                                    ^

41.cc:21:52: internal compiler error: Abort trap: 6

I filed a bug to address the ICE: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71543

среда, 15 июня 2016 г., 14:45:52 UTC+3 пользователь Anton Bikineev написал:

Hi SG8,

I haven't found an answer in N4553,but is it possible by design to use constrained-type-specifier to declare constrained variable, like

template <class T>

concept bool callable = ...;

template <class T>

concept bool button = requires (T t, callable c)

{

    {t.on_click(c)};

};

Anyway, it would be of a good use.

I have wanted to do something like this as well, but I don't have any ideas on how to make it workable.

The typical workaround is to add an additional template parameter for the additional type:

template <class T, class C>

concept bool button =
    callable<C> &&
    requires (T t, C c) {

        {t.on_click(c)};

    };

Tom.

[Andrew Sutton]

I have wanted to do something like this as well, but I don't have any ideas on how to make it workable.

The typical workaround is to add an additional template parameter for the additional type:

template <class T, class C>

concept bool button =
    callable<C> &&
    requires (T t, C c) {

        {t.on_click(c)};

    };

But this isn't the same thing. This requires on_click to be defined for a specific type C. Using a placeholder would require on_click to be defined for *all types* satisfying Callable.

--

Andrew Sutton

[Tom Honermann]

Yes, I know.  This is why I referred to this approach as a workaround and not a solution.  As far as I know, there is no way to accomplish the stated goal in the current design.  The best approximation I've found has been to define the concept as I did above, and then to static assert that the concept is satisfied by a given T with an archetype that models C.  This at least ensures that T isn't restricted to a specific set of types.  But, this also means you have to know what T is, so is only a useful practice for the author of T.

Tom.