Constraining values of non-type template parameters

Asked Oct 10 '22 at 16:15

Active Oct 10 '22 at 16:15

Viewed 36 times

I'm trying to introduce a check where a non-type template parameter for a class is smaller than some maximum. Currently I have the following code which compiles:

template<std::size_t Size>
requires (Size < SIZE_MAXIMUM)
class arena_allocator
{
...

I have the following concept:

template<std::size_t Size> concept allocation_size =
requires{
    Size < SIZE_MAXIMUM;
};

When applying the concept as follows:

template<allocation_size Size>
class arena_allocator
{
...

I get the following error: error C7600: 'hp::mem::allocation_size': the concept designated by a type constraint shall be a type concept

Based on this I would guess that constraining non-type template parameters isn't possible using concepts, or am I missing something? If not, how would I constrain the values of a non-type template parameter?

asked Oct 10 '22 at 16:15

Milan Dierick

note: the concept `requires {Size < SIZE_MAXIMUM;}` only check the expression is valid. – apple apple Oct 10 '22 at 16:34
1

Just to make sure I understand the distinction, it checks if the expression is valid, it doesn't check if the result of that expression equals true? – Milan Dierick Oct 10 '22 at 16:36
1

yes, it should be `concept allocation_size = Size < SIZE_MAXIMUM;` – apple apple Oct 10 '22 at 16:42
@YSC yes the `requires(...)` is correct, but the `requires{...}` is not – apple apple Oct 10 '22 at 16:44

Constraining values of non-type template parameters

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