主页 / coding / 问题 / 79005790
Accepted
cppBeginner
Asked: 2024-09-20 15:50:31 +0800 CST

concept<A,B> 检查“A”是否源自“B<...something...>”

  • 772

MCVE神栓链接

我有一个概念Trait_AActuallyBTemplate<A,B>来检查是否A= B<...something...>。
它有效。

如何创建一个新概念来检查是否A继承B<...something...>？

#include <vector>
#include <iostream>
namespace hid{
    template <typename T,template<typename... Ty> typename B>
    struct is_mytype : std::false_type { };

    template <typename... A,template<typename... Ty> typename B>
    struct is_mytype<B<A...> ,B> : std::true_type { };

};

template <typename T,template<typename... Ty> typename B>
concept Trait_AActuallyBTemplate = hid::is_mytype<T,B>::value;

//vvv please modify here
template <typename T,template<typename... Ty> typename B>
concept Trait_AInheritFromBTemplate = hid::is_mytype<T,B>::value; 
//^^^ please modify here

int main() {
    static_assert(Trait_AActuallyBTemplate<std::vector<int>,std::vector>,"hehe1");
    static_assert(!Trait_AActuallyBTemplate<int,std::vector>,"hehe2");
    struct MyVector : public std::vector<int> {};
    static_assert(Trait_AInheritFromBTemplate<MyVector,std::vector>,"hehe3"); 
    //^^^ Assert fail only here. (It should pass)  I am sad.
}
c++

1 个回答

  1. Best Answer

    您可以通过尝试调用接受指向基址的指针的虚拟（模板化）函数来检查这一点。这将拒绝无法访问或模棱两可的基址，但这通常是可取的。

    namespace detail
{
    template <template <typename...> typename T, typename ...P>
    constexpr void foo(const T<P...> *) {}
}

template <typename T, template <typename...> typename U>
concept InheritsFromTemplate = requires(T *t){detail::foo<U>(t);};

    template <typename ...P> struct A {};
struct B : A<int, float> {};
struct C {};
struct D : virtual A<int, float> {};
struct E : private A<int, float> {};
struct F : A<int, float>, A<double> {};

static_assert(InheritsFromTemplate<A<int, float>, A>); // Exact match.
static_assert(InheritsFromTemplate<B, A>); // Inherits.
static_assert(!InheritsFromTemplate<C, A>); // Doesn't inherit.
static_assert(InheritsFromTemplate<D, A>); // Virtual inheritance is ok.
static_assert(!InheritsFromTemplate<E, A>); // Inaccessible base is rejected.
static_assert(!InheritsFromTemplate<F, A>); // Ambiguous base is rejected.
    • 8

相关问题