Conversion SUPPERSUBCLASS 判断 T 是否为 U 的父类

本文链接：https://blog.youkuaiyun.com/zhangxiaonanwin/article/details/6080556

本文介绍了一个C++模板元编程示例，用于检查一种类型是否可以通过类型转换派生于另一种类型。通过定义`Conversion`类和宏`SUPPERSUBCLASS`来实现这一功能。

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#include <cstdlib>
#include <iostream>

using namespace std;
template<class T,class U>
class Conversion
{
typedef char small;
class Big{char dummy[2];};
static small Test(T);
static Big Test(...);
static T MakeT();
static U MakeU();
public:
enum{exists = sizeof(Test(MakeU())) == sizeof(small)};
enum{exist2ways = exists && Conversion<U,T>::exists};
enum{sametype = false};
};
template<class T>
class Conversion<T,T>
{public:enum{exists = 1,exist2ways = 1,sametype = 1};};
#define SUPPERSUBCLASS(T,U)/
        (Conversion<const T*,const U*>::exists)&&/
        !(Conversion<const T*,const void*>::sametype)//防止 T* 是void*类型
#define SUPPERSUBCLASS_STRICT(T,U)/
        (Conversion<const T,const U>::exists)&&/
        !(Conversion<const T,const void>::sametype)//防止 T* 是void*类型
class Base
{};
class Derived:public Base
{};
int main(int argc, char *argv[])
{
    cout<<"SUPPERSUBCLASS(T,U)结果 (1 = T是U的父类)(0 = T不是U的父类)"<<endl;
    cout<<"----------------------------------------------------"<<endl;
    bool bRes = SUPPERSUBCLASS(Base,Derived);
    cout<<"SUPPERSUBCLASS(Base,Derived)的关系:"<<bRes<<endl;
    bRes = SUPPERSUBCLASS(Derived,Base);
    cout<<"SUPPERSUBCLASS(Derived,Base)的关系:"<<bRes<<endl;
    bRes = SUPPERSUBCLASS(int,Base);
    cout<<"SUPPERSUBCLASS(int,Base)的关系:"<<bRes<<endl;
    bRes = SUPPERSUBCLASS(int,float);
    cout<<"SUPPERSUBCLASS(int,float)的关系:"<<bRes<<endl;
    bRes = SUPPERSUBCLASS(int,void);
    cout<<"SUPPERSUBCLASS(int,void)的关系:"<<bRes<<endl;
    bRes = SUPPERSUBCLASS(Derived,void);
    cout<<"SUPPERSUBCLASS(Derived,void)的关系:"<<bRes<<endl;
    bRes = SUPPERSUBCLASS(void,Derived);
    cout<<"SUPPERSUBCLASS(void,Derived)的关系:"<<bRes<<endl;
    cout<<"----------------------------------------------------"<<endl;
    system("PAUSE");
    return EXIT_SUCCESS;
}