C++编程中__if_exists与__if_not_exists语句的用法

__if_exists ( identifier ) { 
statements
};

// the__if_exists_statement.cpp
// compile with: /EHsc
#include <iostream>

template<typename T>
class X : public T {
public:
  void Dump() {
   std::cout << "In X<T>::Dump()" << std::endl;

   __if_exists(T::Dump) {
     T::Dump();
   }

   __if_not_exists(T::Dump) {
     std::cout << "T::Dump does not exist" << std::endl;
   }
  }  
};

class A {
public:
  void Dump() {
   std::cout << "In A::Dump()" << std::endl;
  }
};

class B {};

bool g_bFlag = true;

class C {
public:
  void f(int);
  void f(double);
};

int main() { 
  X<A> x1;
  X<B> x2;

  x1.Dump();
  x2.Dump();

  __if_exists(::g_bFlag) {
   std::cout << "g_bFlag = " << g_bFlag << std::endl;
  }

  __if_exists(C::f) {
   std::cout << "C::f exists" << std::endl;
  }

  return 0;
}

In X<T>::Dump()
In A::Dump()
In X<T>::Dump()
T::Dump does not exist
g_bFlag = 1
C::f exists

__if_not_exists ( identifier ) { 
statements
};