Re: Virtual overloading of operators

From:

Victor Bazarov <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Mon, 26 Apr 2010 10:47:14 -0400

Message-ID:

<hr491k$eth$1@news.datemas.de>

J?rg Rolef wrote:

I'd like to do a virtual overload of the operator<. If I do it like in
the example listed below, then I get an error message for accessing op.b
in SpecialContents::operator<(): "Member element Contents::op.b is
protected."
But I do not get an error for access of op.a in Contents::operator<().

First: Do I have to use public member functions to access the values of
the second operand? (First I thought that declaring the members as
protected would do me the job, but of course this seems only to make the
object's own elements acessible to descendant classes.)

Second: Why does the access of op.a in Contents::operator<() not lead to
an error message then? At least, op is a foreign object, so this access
should also lead to an error - am I wrong?

Greetings and thank you,
J?rg

class Contents
{
protected:
        int a;
        char b;

public:
    // ...
    virtual bool operator<(Contents& op);
};

bool Contents::operator<(Contents& op)
{
    return (a < op.a);
           // Returns no error for the access of op.a
}

class SpecialContents : public Contents
{
    bool operator<(Contents& op);
};

bool SpecialContents::operator<(Contents& op)
{
    return (b < op.b);
           // Returns error: "Contents::op.b is protected"
}

Protected data members of the object not of your type (but of the type
in your hierarchy) are not accessible.

If you define another class, like so

class OtherContents : public Contents {};

and then compare your 'SpecialContents' object with an instance of it,
then your 'SpecialContents' object is trying to access the protected
area of the object not related to it directly. Only access up and down
the hierarchy is allowed. IOW, protected data members are allowed to be
accessed only through 'this->'. IIRC, anyway.

You need to implement your 'b' comparison as a protected function of the
base class and call it from 'SpecialContents::operator<'.

V
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