c++ - 在编译时查找基类

Question

标题几乎说明了一切:在 C++ 中有没有一种方法可以在编译时获取类的基类型？ IE。是否可以将一个类传递给一个模板，然后让该模板使用它传递给定类的基类的其他模板？

我的问题不是我是否可以自己实现这样的功能，毫无疑问我可以(使用特征等)。我的问题是是否有一些(晦涩的)内置功能可用于此目的。

Answer 1

gcc支持这一点。见

• Kerrek's answer
• tr2/type_traits
• Andy Prowl's code example
• n2965
• What is the status of N2965 - std::bases and std::direct_bases?
• How to query for all base classes of a class at compile time?

n2965 提供了一个例子。

This simple examples illustrates the results of these type traits. In the Suppose we have the following class hierarchy:

class E {};
class D {};
class C : virtual public D, private E {};
class B : virtual public D, public E {};
class A : public B, public C {};

It follows that bases<A>::type is tuple<D, B, E, C, E>

Similarly, direct_bases<A>::type is tuple<B, C>

Andy Prowl的代码示例如下:

#include <tr2/type_traits>
#include <tuple>

template<typename T>
struct dbc_as_tuple { };

template<typename... Ts>
struct dbc_as_tuple<std::tr2::__reflection_typelist<Ts...>>
{
    typedef std::tuple<Ts...> type;
};

struct A {};
struct B {};
struct C : A, B {};

int main()
{
    using namespace std;

    using direct_base_classes = dbc_as_tuple<tr2::direct_bases<C>::type>::type;

    using first = tuple_element<0, direct_base_classes>::type;
    using second = tuple_element<1, direct_base_classes>::type;

    static_assert(is_same<first, A>::value, "Error!");   // Will not fire
    static_assert(is_same<second, B>::value, "Error!");  // Will not fire
}