CODEWITHSUNDEEP
c++c++-conceptsstatic-assertbuild-time
Joseph Garvin
Joseph Garvin

Reputation: 21974

Is there any reason to prefer asserting type traits over concepts?

As far as I can tell the code below asserts the same thing in two different ways:

#include <type_traits>

template<class T, class U>
concept nothrow_assignable = requires(T a, U b)
{
    requires noexcept(a = b);
};

template<class T>
void foo()
{
    // new way
    static_assert(nothrow_assignable<T, T>);

    // old way
    static_assert(std::is_nothrow_assignable_v<T, T>);
}

is there any reason to ever prefer the old way? The type_traits header is 3K+ lines of code, and this particular trait ends up invoking a deep tree of other traits (the definition is surprisingly involved, checking if the type is an array, a reference, a pointer, a function, etc and each of those checks instantiates a few more) so I would prefer not to use it for the sake of keeping my compile times low. But at the same time I've noticed that there don't appear to be standard concepts for every standard type trait (I have to implement nothrow_assignable myself) which seems like an obvious oversight so I'm wondering if there is a trade off here.

Upvotes: 0

Views: 228

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