Re: how to use functions of base class

From:
Paavo Helde <myfirstname@osa.pri.ee>
Newsgroups:
comp.lang.c++
Date:
Sat, 19 Mar 2011 02:29:36 -0500
Message-ID:
<Xns9EAD609297471myfirstnameosapriee@216.196.109.131>
m0shbear <andrey.vul@gmail.com> wrote in news:4b81ed97-3a18-4c34-b77c-
dbb3f9061ec9@a26g2000vbo.googlegroups.com:

Given
struct A {
  int x;
  int y;
  bool operator==(const A& a) const {
    return (x==a.x)&&(y==a.y);
  }
};
struct B : public A {
  int s;
  int t;
  B(int u = 0, int v = 0, int w = 0, int z = 0) : A(u,v), s(w), t(z)
{ }
  bool operator==(const B& b) const {
    return (dynamic_cast<A&>(*this)).operator==(dynamic_cast<const
A&>(b)) &&
            (s==b.s)&&(t==b.t);
  }
};


This does not compile for several reasons. There are no virtual functions
around, so dynamic_cast does not work, and is not needed at all. One
could write it a bit cleaner as:

bool operator==(const B& b) const {
     return A::operator==(b) && s==b.s && t==b.t;
}

If you are just inheriting implementation, then you should use private
derivation, and then I guess this design would work more or less. On the
other hand, if you use public inheritance as here, you have a danger to
compare objects of different types. e.g.

A a(1,2);
B b(1,2,3,4);

if (a==b) {//...

This comparison works, compares a to the A part of b and returns true,
what is probably not intended. To support operator== properly in publicly
derived class hierarchies one should use a private virtual function for
actual comparison and call this from the base class operator==, after
ensuring that the operands are of the same type (via typeid() or
dynamic_cast<>). But from your description it looks like that private
inheritance would suit you better, it would refuse comparison of objects
of different types automatically.

hth
Paavo

Will B::operator==(const B&) behave as intended, according to the
spec?
I'm using composition inheritance to remove duplicate code and want to
know if it's for naught.

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