__far and C++ Classes

If a member function gets the modifier __far, the this pointer is a __far pointer in the context of a call to that function. This is useful, for instance, if the owner class of the function is not allocated on the default data page. See the following listing.

class A {
public:

  void f_far(void) __far {

    /* __far version of member function A::f() */

  }

  void f(void) {

    /* normal version of member function A::f() */

  }

};

#pragma DATA_SEG MyDirectSeg // use direct addressing mode

A a_normal; // normal instance

#pragma DATA_SEG __FAR_SEG MyFarSeg // use extended addressing mode

A __far a_far;   // __far instance

void main(void)

  a_normal.f();  // call normal version of A::f() for normal instance

  a_far.f_far(); // call __far version of A::f() for __far instance

}

With inheritance, it no longer suffices to use __far with member functions only. Instead, the whole class should be qualified with the __far modifier. Thus, the compiler is instructed to handle ' this' as a far pointer in all the contexts related to the declaration and definition of that class, for example vptr initialization, assignment operator generation etc. See the following listing.

class B __ far 
{ ... }

class A __far : B

{ ... }

#pragma push

#pragma DATA_SEG __FAR_SEG MY_FAR_RAM

A a_far;

#pragma pop