Re: Polymorphic abstract base class with operator=3D=3D or e= quivalen= t functionality; design considerations

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Tue, 14 Sep 2010 08:49:13 CST
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On 14 Sep., 00:55, BanMido <> wrote:

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Here is the scenario:
I am attempting to design a polymorphic abstract base class (i.e. an
interface), which will be extended, and its pure virtual functions
implemented, by a set of concrete classes.
I want all the concrete classes to support equality testing, so I
thought of adding a pure virtual 'operator==' method to the abs=


base class which will be implemented by any concrete class
implementing the interface. But this approach poses some problems as
the signature of base class's 'operator==' method and derived c=


'operator==' method do not match.

class IString
    virtual ~IString() {}

    virtual unsigned int length() const = 0;
    virtual bool operator==(const IString&) = 0;
    virtual bool operator!=(const IString&) = 0;

To begin with, I consider it as a very *bad* idea,
to provide two virtual functions that are mutually
dependent: I have never seen a pair of
operator== and operator!= where the second
behaves different from the negation of the first.
If I would see something like this, this is probably
a design error. Allowing derived classes to
implement them differently is the best way
to allow for inconsistent classes.

Second, I have only seen *rare* cases, where
a virtual operator==/!= makes sense and I
would always ensure that these are const

So, if your really want this, I suggest to
provide a single virtual function like this:

virtual bool is_equal_to(const IString&) const = 0;

and *possibly* (but not necessarily) the following
two non-member functions:


inline bool operator==(const IString& lhs, const IString& rhs) {
   return lhs.is_equal_to(rhs);

inline bool operator!=(const IString& lhs, const IString& rhs) {
   return !(lhs == rhs);

class CString : public IString
    CString :
    ~CString() {}

    virtual unsigned int length() const
    { return strLen; }

    virtual bool operator==(const CString&)
    { return true; } // dummy implementation

    virtual bool operator!=(const CString&)
    { return true; } // dummy implementation

Btw.: Even though this is just a "dummy"
implementation, you have already provided
an example where == and != are defined

HTH & Greetings from Bremen,

Daniel Kr=FCgler

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