__if_exists Statement

The __if_exists statement tests whether the specified identifier exists. If the identifier exists, the specified statement block is executed.

__if_exists ( identifier ) { 
statements
};

Parameters

Remarks

• Apply the __if_exists statement to only simple types, not templates.

• Apply the __if_exists statement to identifiers both inside or outside a class. Do not apply the __if_exists statement to local variables.

• Use the __if_exists statement only in the body of a function. Outside of the body of a function, the __if_exists statement can test only fully defined types.

• When you test for overloaded functions, you cannot test for a specific form of the overload.

The complement to the __if_exists statement is the __if_not_exists statement.

Example

Notice that this example uses templates, which is not advised.

// 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

See Also

Reference

Selection Statements (C++)

C++ Keywords

__if_not_exists Statement