views:

138

answers:

3

why if we have pure virtual assignment operator in a base class, then we implement that operator on the derived class, it give linker error on the base class?

currently I only have the following explanation on http://support.microsoft.com/kb/130486 , it said that the behavior is by design since normal inheritance rules does not apply.

it is not clear for me, why is it generate linker error by design? can someone give me more clear explanation about this?

edit: added my simplified code of which the error occured:

class __declspec(dllexport) BaseClass {
public:
    int memberA;
    virtual BaseClass& operator=(const BaseClass& rhs) = 0;
};

class __declspec(dllexport) DerivedClass : public BaseClass {
public:
    int memberB;
    DerivedClass():memberB(0) {}
    virtual BaseClass& operator=(const BaseClass& rhs) {
        this->memberA = rhs.memberA;
        this->memberB = 1;
        return *this;
    }
};

int main(void)
{
    DerivedClass d1;
    DerivedClass d2;

    BaseClass* bd1 = &d1;
    BaseClass* bd2 = &d2;

    *bd1 = *bd2;
}

the code will compile with no errors without __declspec(dllexport) and/or without pure virtual operator= declaration on base class.

without __declspec(dllexport) after assignment of *bd1 = *bd2;, d1::memberB is 1, but with __declspec(dllexport) d1::memberB is left unchanged

with __declspec(dllexport), and without pure virtual declaration, after assignment of *bd1 = *bd2;, d1::memberB is left unchanged

+3  A: 

operator= is not inherited. Your code is meaningless in C++, so compilers are free to issue any error they want for it.

From the KB article you pointed to: http://support.microsoft.com/kb/130486

Since operator= is not inherited, any declaration of operator= in the base class is unused and unnecessary. Do not declare the operator= in the base class.

It's probably just a side-effect of how they compile, and they are just letting you know that they don't consider it a bug, so there is no need to fix it. "By design" doesn't necessarily mean that they specifically decided that this linker error is the right error message to give for this situation -- the code is wrong, you get an error, so from their point of view -- they're done.

Lou Franco
Where is that quote from?
James McNellis
The OP's KB article. I will udpate
Lou Franco
+1 for the quote
bjskishore123
+2  A: 

In the example code:

class A
{
public :
   // To workaround LNK2001, comment the following line.
   virtual const A& operator=( const A& f ) = 0;
};

class B : public A
{
public :
   const A& operator=( const A& g ) {return g;}
};

B aB1, aB2;

int /*void*/ main( void )
{
   aB2 = aB1;
}

the line aB2 = aB1 does not call const A& B::operator=(const A&), but instead calls the automatically supplied B& operator=(const B&); which in turn uses the assignment operator for assigning the base portion of the class. But when it comes to linking, it turns out that that was never implemented.

UncleBens
The assignment operator has been overriden. Like declaring any constructor (non-template) effectively deactivate the implicitly defined default constructor and copy constructor, declaring any `operator=` (non-template) deactivate the implicitly defined assignment operator.
Matthieu M.
@Matthieu: Does that qualify as *copy assignment*? And after all, `struct X { X};` does not prevent me from assigning one X to another which it should, if *any* overload of `operator=` disabled the default one. - Similarly, any constructor does not deactivate the copy constructor, only a valid user-defined copy constructor.
UncleBens
Matthieu M.
+6  A: 
Steve M
I think he's talking about operator=, not the copy constructor. For the copy ctor -- virtual, and pure virtual should be an immediate compiler error as it really makes no sense for them.
Lou Franco
His link is about operator= and that is what my post is about.
Steve M
@Steve From the OP `then we implement that operator on the derived class` I'm going to say that he has an explicit copy assignment operator in the child (not implicit which your answer would cover correctly), and no indication that it is or isn't calling the parent (pure virtual) copy assignment.
Mark B
Steve M