c++ - 是否有可能在运行时从 C++ 类声明中知道继承的数量？

Question

给定这些类示例，

class A {};
class B : A {};
class C {};
class D : A, C {};
class E : D {};

该函数应返回类 A 的继承数为 0，而对于类 B、C、D 和 E，该函数应分别返回 1、0、2、1。

Note: The function shouldn't take account the whole inherited classes (for example, E takes D that is inherited from A and C indirectly, so it would has 3 but in my case I want to know the number of inheritances of the class in its declaration)

Answer 1

不幸的是，这还没有进入语言。有 std::direct_bases 提案但被拒绝:What is the status of N2965 - std::bases and std::direct_bases?

所以你需要自己动手 - 有很多方法可以解决这个问题，但例如它可能类似于以下内容:

#include <tuple>

    class A{
        public:
      using bases = std::tuple<>;
    };

    class B:A{
        public:
      using bases = std::tuple<A>;
    };

    class C{
        public:
      using bases = std::tuple<>;
    };

    class D:A,C{
        public:
      using bases = std::tuple<A,C>;
    };

    class E:D{
        public:
      using bases = std::tuple<D>;
    };

    template <class T>
    size_t count_bases() {
        return std::tuple_size<typename T::bases>::value;
    }

    int main() {
      return count_bases<D>();//returns '2'
    }

自己试试:https://godbolt.org/z/u0qoZa