解决静态断言中的不完整类型 [英] Work around incomplete type in static assert

问题描述

当表达式依赖于类类型本身时，有没有办法在类内部进行 static_assert ?也许延迟评估直到类型完成或模板实例化之后?

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?

示例代码:

#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;
}

推荐答案

这是一个使用辅助类和间接类型别名的解决方案.我相信这没有缺点.

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;

或者可以将 TestHelper 移到 TestImpl 中:

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;

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