C++中利用模板实现instanceof和is_super_of
最近看了一点模板元编程的东西，忽然灵光一闪，便想出了一个方法来判断一个类是否是另一个类的基类is_base_of。稍加改变也可以实现类似于java的instanceof
虽然平时写程序很少会需要这两个东西，不过万一用到呢
预备知识：
1. 如果将一个T类型临时对象传递给一个函数，那么这个函数接受的参数类型必须是：const T& 或 T
2. 函数模板匹配的原则：见http://www.cnblogs.com/hdtianfu/archive/2011/09/05/2167973.html
利用上述两点，可以制作出下面这个模板函数 is_supe_of<T1, T2>()
注意：这个函数有一点缺陷： T2必须具有缺省构造函数。
代码如下：
1 template<typename T>
2 class CTestSuper{
3 public:
4
5 static bool testsuper(const T&){return true;}
6 template<typename U>
7 static bool testsuper(U&){return false;}
8 };
9
10 template<typename T1, typename T2>
11 bool is_super_of(){
12
13 return CTestSuper<T1>::testsuper(T2());
14 }
15
16 template<typename T1, typename T2>
17 bool instanceof(const T2&){
18 return CTestSuper<T1>::testsuper(T2());
19 }
2 class CTestSuper{
3 public:
4
5 static bool testsuper(const T&){return true;}
6 template<typename U>
7 static bool testsuper(U&){return false;}
8 };
9
10 template<typename T1, typename T2>
11 bool is_super_of(){
12
13 return CTestSuper<T1>::testsuper(T2());
14 }
15
16 template<typename T1, typename T2>
17 bool instanceof(const T2&){
18 return CTestSuper<T1>::testsuper(T2());
19 }
注：instanceof 的实现是有缺陷的， 它只能按照静态类型进行判定，如果想要判断一个基类指针是否指向某个子类，这种方法便无能为力了，这时你需要的是dynamic_cast。
测试代码如下：
1 class A{};
2 class B : public A{};
3 class C:public B{};
4
5 int main(){
6
7 cout << "A is Super of B : " << (is_super_of<A, B>() ? "true" : "false") << endl;
8 cout << "A is Super of C : " << (is_super_of<A, C>() ? "true" : "false") << endl;
9 cout << "B is Super of A : " << (is_super_of<B, A>() ? "true" : "false") << endl;
10 cout << "C is Super of A : " << (is_super_of<C, A>() ? "true" : "false") << endl;
11
12 B b;
13 cout << "b is instance of A : " << (instanceof<A>(b) ? "true" : "false") << endl;
14 cout << "b is instance of B : " << (instanceof<B>(b) ? "true" : "false") << endl;
15 cout << "b is instance of C : " << (instanceof<C>(b) ? "true" : "false") << endl;
16
17 }
2 class B : public A{};
3 class C:public B{};
4
5 int main(){
6
7 cout << "A is Super of B : " << (is_super_of<A, B>() ? "true" : "false") << endl;
8 cout << "A is Super of C : " << (is_super_of<A, C>() ? "true" : "false") << endl;
9 cout << "B is Super of A : " << (is_super_of<B, A>() ? "true" : "false") << endl;
10 cout << "C is Super of A : " << (is_super_of<C, A>() ? "true" : "false") << endl;
11
12 B b;
13 cout << "b is instance of A : " << (instanceof<A>(b) ? "true" : "false") << endl;
14 cout << "b is instance of B : " << (instanceof<B>(b) ? "true" : "false") << endl;
15 cout << "b is instance of C : " << (instanceof<C>(b) ? "true" : "false") << endl;
16
17 }
输出如下：
A is Super of B : true
A is Super of C : true
B is Super of A : false
C is Super of A : false
b is instance of A : true
b is instance of B : true
b is instance of C : false
