C++ for Scientists - Technische Universität Dresden

194 CHAPTER 6. INHERITANCE
Our compilers are so sophisticated, they certainly handle de Morgan’s law perfectly. Negating
the equality operator is something we can do on every type that has an equality operator. We
could copy-and-past this code snippet and just replace the type of the argument.
Alternatively, we can write a class like this:
template
struct unequality
{
bool operator!=(const T& that) const { return !(static cast(∗this) == that); }
};
and derive from it:
class point : public unequality { ... };
This mutual dependency:
• One class is derived from the other and
• The latter takes the derived class’ type as template argument
is somewhat confusing at the first view.
Essential for this to work is that the code of a template class member is only generated when
the class is instantiated and the function is actually called. At the time the template class
‘unequality is parsed, the compiler checks only the correctness of the syntax.
When we write
int main (int argc, char∗ argv[])
{
point p1(3, 4), p2(3, 5);
std::cout ≪ ”p1 != p2 is ” ≪ (p1 != p2 ? ”true” : ”false”) ≪ ’\n’;
}
return 0 ;
fter the definition of unequality and point both types are completely known to the compiler.
What happens when we call p1 != p2?
1. The compiler searches for operator!= in class point → without success.
2. The compiler looks for operator!= in the base class unequality → with success.
3. The this pointer of unequality refers a component of point’s this pointer.
4. Both types are completely known and we can statically down-cast the this pointer to point.
5. Since we know that the this pointer of unequality is an up-casted this pointer to
point 3 we are save to down-cast it to its original type.
6. The equality operator for point is called. Its implementation is already known at this point
because the code of unequality’s operator!= is not generated before the instantiation
of point.
3 Unless the first argument is really of type unequality. There are also ways to impede this, e.g. http:
//en.wikipedia.org/wiki/Barton-Nackman_trick but we used this unary operator notation for the sake of
simplicity.