Work around incomplete type in static assert

问题

Is there a way to static_assert inside a class when the expression depends on the class type itself? Maybe delay the evaluation until the type is complete or after template instantiation?

Example code:

#include <type_traits>

template<typename T>
struct Test {
   T x = 0; // make non-trivial
   static_assert(std::is_trivial<Test<T>>::value, "");
};

int main() {
    // would like static assert failure, instead get 'incomplete type' error
    Test<int> test1;
    Test<float> test2;
    return 0;
}

回答1:

Here's a solution using a helper class and a type alias for indirection. I believe this has no drawbacks.

template<typename T>
struct TestImpl {
    T x = 0; // make non-trivial
};

template<typename T>
struct TestHelper {
    using type = TestImpl<T>;
    static_assert(std::is_trivial<type>::value, "");
};

template<typename T>
using Test = typename TestHelper<T>::type;

edit: Alternatively TestHelper can be moved into TestImpl:

template<typename T>
struct TestImpl {
    T x = 0; // make non-trivial

    struct Helper {
        using type = TestImpl;
        static_assert(std::is_trivial<type>::value, "");
    };
};

template<typename T>
using Test = typename TestImpl<T>::Helper::type;

回答2:

You can define a destructor and put the static_assert there, at least with g++ 5.4 -std=c++11 this works.

回答3:

(moving from the comments)

You can add an intermediate class to keep the logic, and let your clients instantiate a derived one, which just contains the static_asserts

#include <type_traits>

template<typename T>
struct TestImpl {
    T x = 0; // make non-trivial
};

template<typename T>
struct Test : TestImpl<T> {
    static_assert(std::is_trivial<TestImpl<T>>::value, "");
    using TestImpl<T>::TestImpl; // inherit constructors
};

来源：https://stackoverflow.com/questions/42779120/work-around-incomplete-type-in-static-assert

标签

c++

templates

typetraits

static-assert