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concepts question
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Bonita Montero
Jun 9, 2021, 11:26:37 AM6/9/21
to
If you have a class / struct with a template-parameter that
is specified to have a concept-constraint with requires - why
it is necessary to re-define this dependency in the external
definition of all members ? Is there a deeper necessity in
this ?
template<typename T>
requires regular<T>
struct S
{
void f();
// ...
};
template<typename T>
requires regular<T> // why is this duplicate check necessary
void S<T>::f()
{
// ...
}
is specified to have a concept-constraint with requires - why
it is necessary to re-define this dependency in the external
definition of all members ? Is there a deeper necessity in
this ?
template<typename T>
requires regular<T>
struct S
{
void f();
// ...
};
template<typename T>
requires regular<T> // why is this duplicate check necessary
void S<T>::f()
{
// ...
}
David Brown
Jun 9, 2021, 11:48:04 AM6/9/21
to
getting from your compiler (and what compiler is it) ?
<https://godbolt.org> testing is always useful for that kind of thing.
"requires" adds restrictions on how a template can be used, thus giving
clearer error messages at an earlier stage. I don't see why they would
be needed in the implementation of a function if they are given in the
declaration.
Bonita Montero
Jun 9, 2021, 12:10:31 PM6/9/21
to
error C3864: 'S<T>': requires clause is incompatible with the declaration
With clang I get the error:
error: requires clause differs in template redeclaration
David Brown
Jun 9, 2021, 12:50:38 PM6/9/21
to
two missing lines:
#include <concepts>
using namespace std;
Current gcc and clang with "--std=c++20 -Wpedantic" are happy with it,
as is MSVC v.19 with "/std:c++latest".
Are you missing some compiler options, or using a different version?
Again - go to <https://godbolt.org>, try it out on different compiler
versions and you might get an idea of what is going on.
Bonita Montero
Jun 9, 2021, 1:51:31 PM6/9/21
to
> I just tried it on <https://godbolt.org>, with the code you gave and the
> two missing lines:
>
> #include <concepts>
> using namespace std;
>
> Current gcc and clang with "--std=c++20 -Wpedantic" are happy with it,
> as is MSVC v.19 with "/std:c++latest".
clang 12 doesn't work: https://godbolt.org/z/jbzq8nP5G
> two missing lines:
>
> #include <concepts>
> using namespace std;
>
> Current gcc and clang with "--std=c++20 -Wpedantic" are happy with it,
> as is MSVC v.19 with "/std:c++latest".
g++ 11 does work.
-Wpedantic doesn't make a difference either.
David Brown
Jun 9, 2021, 2:49:03 PM6/9/21
to
and clang and MSVC, I had managed to undo the comment marker I had in
the second "requires". Sorry for the noise.
It seems it is only gcc that is happy with your code. It looked fine to
me, but I haven't studied the standards here.
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