Re: pure operator=

James Kanze <>
Sun, 13 Jan 2008 02:01:09 -0800 (PST)
On Jan 13, 2:31 am, Mike -- Email Ignored <>

Pure operator= thus:

class A // abstract
   virtual A& operator=(const A&); // has a purpose

class B : public A // never instantiated alone
   virtual B& operator=(const B&) = 0; // nothing needed, so pure

So why provide it?


class C : public B; // instantiated
   virtual C& operator=(const C&); // has a purpose

Won't compile as long as I leave the assignment operator for
class B pure. Why not? Chapter & verse?

Anytime you declare a function pure virtual, you must declare it
in the derived class. I don't see any B& operator=( B const& )
in C, so C remains abstract.

Note that virtuality and assignment don't work very well
together anyway, and usually, if a class is polymorphic, you
need to inhibit assignment---just declaring the assignment
operator private in the base class is enough. The one
exception, of course, is the letter/envelope idiom, and there,
the only assignment operator is in the base class---you never
assign derived class instances either.

Think of it for a moment. What should happen in the following

    Base* p1 = new Derived1 ;
    Base* p2 = new Derived2 ;
    *p1 = *p2 ;

What should the type of *p1 be after this operation?

James Kanze (GABI Software)
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