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用成员函数重载实现is_base_of

std::is_base_of是一个C++ 11标准中用于判断某个类是否是另一个类父类的类模板。

#include "killCmake.h"#include<string>using namespace std;class A
{
};class B : public A
{
public:B(int x): x_(x){}
private:int x_;
};//template<typename Base,typename Derived>
//struct is_base_of {...};int main()
{std::cout << std::is_base_of<A, A>::value << std::endl;std::cout << std::is_base_of<B, A>::value << std::endl;std::cout << std::is_base_of<A, B>::value << std::endl;return 0;
}

C++ 17标准中又引入了变量模板简化std::is_base_of的书写。

#include "killCmake.h"#include<string>using namespace std;class A
{
};class B : public A
{
public:B(int x): x_(x){}
private:int x_;
};//template<typename Base,typename Derived>
//struct is_base_of {...};template<class Base,class Derived>
inline constexpr bool is_base_of_v_v = std::is_base_of<Base, Derived>::value;int main()
{std::cout << std::is_base_of<A, A>::value << std::endl;std::cout << std::is_base_of<B, A>::value << std::endl;std::cout << std::is_base_of<A, B>::value << std::endl;std::cout << std::endl;// 简化版本std::cout << is_base_of_v_v<A, A> << std::endl;std::cout << is_base_of_v_v<B, A> << std::endl;std::cout << is_base_of_v_v<A, B> << std::endl;return 0;
}

std::is_base_of的实现代码，写一个IsBaseOf类模板来实现，代码如下。

#include "killCmake.h"#include<string>using namespace std;class A
{
};class B : public A
{
public:B(int x): x_(x){}
private:int x_;
};//template<typename Base,typename Derived>
//struct is_base_of {...};template<class Base,class Derived>
inline constexpr bool is_base_of_v_v = std::is_base_of<Base, Derived>::value;template<typename Base,typename Derived> // <父类，子类>
class IsBaseOf
{
private:template<typename T>static std::true_type test(T*);template<typename>static std::false_type test(void*);template<typename B,typename D>static auto test_middle() -> decltype(test<B>(static_cast<D*>(nullptr)));// 调用test()public:static constexpr bool value = IsSameType < std::integral_constant<bool, std::is_class_v<Base>&& std::is_class_v<Derived>&& decltype(test_middle<Base, Derived>())::value, std::integral_constant<bool, true>>::value;
};int main()
{std::cout << std::is_base_of<A, A>::value << std::endl;std::cout << std::is_base_of<B, A>::value << std::endl;std::cout << std::is_base_of<A, B>::value << std::endl;std::cout << std::endl;// 简化版本std::cout << is_base_of_v_v<A, A> << std::endl;std::cout << is_base_of_v_v<B, A> << std::endl;std::cout << is_base_of_v_v<A, B> << std::endl;return 0;
}

未完待续，干他菊花就对了