The __if_exists Statement

The __if_exists Statement

__if_exists allows you to conditionally include code depending on whether the specified symbol exists.

__if_exists ( variable ) { 
statements 
}

where:

variable

The symbol whose existence you want to test for.

statements

One or more statements to execute if variable exists.

 Remarks

__if_exists can be used to test for the existence of both members and non-members identifiers and can be used to test for the existence of an overloaded function but not for a specific form of the overload.

__if_exists should only be used within the body of a function. Outside of the body of a function __if_exists can only refer to fully defined types. The Visual C++ compiler must know what members a class template has before any instantiations.

__if_not_exists allows you to conditionally include code depending on whether the specified symbol does not exist.

 Example

// 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;
}

 Output

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