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-   -   operator= for derived class (http://www.velocityreviews.com/forums/t288336-operator-for-derived-class.html)

 MojoRison 01-21-2005 09:27 PM

operator= for derived class

struct A{
A& operator=( const A& a ){ i = a.i; return *this; }

private:
int i;
};

struct B : public A{
B& operator=( const B& b ){ j = b.j; return *this; } // how to copy i?

private:
int j;
};

In the above code, what would be the standard way of coping A::i in
B::operator= ?

 Victor Bazarov 01-21-2005 09:36 PM

Re: operator= for derived class

MojoRison wrote:
> struct A{
> A& operator=( const A& a ){ i = a.i; return *this; }
>
> private:
> int i;
> };
>
> struct B : public A{
> B& operator=( const B& b ){ j = b.j; return *this; } // how to copy i?
>
> private:
> int j;
> };
>
> In the above code, what would be the standard way of coping A::i in
> B::operator= ?

For the classes in their presented form, there is no need for operator=.
But you probably already knew that.

The simple way is to call the A::operator= directly inside B::operator= :

B& operator=(const B& b) { A::operator=(b); j = b.j; return *this }

Victor

 Andrey Tarasevich 01-21-2005 09:41 PM

Re: operator= for derived class

MojoRison wrote:

> struct A{
> A& operator=( const A& a ){ i = a.i; return *this; }
>
> private:
> int i;
> };
>
> struct B : public A{
> B& operator=( const B& b ){ j = b.j; return *this; } // how to copy i?
>
> private:
> int j;
> };
>
> In the above code, what would be the standard way of coping A::i in
> B::operator= ?

Firstly, for such classes you don't need to implement the copy
assignment operator explicitly. The one automatically provided by the
compiler will do exactly what you want.

If you still want to implement these operators manually, then the proper
way to do it would be calling the inherited copy assignment operator
('A::operator=') from B's implementation

B& operator=( const B& b ) {
A::operator=(b);
j = b.j;
return *this;
}

--
Best regards,
Andrey Tarasevich

 Thomas Matthews 01-21-2005 09:56 PM

Re: operator= for derived class

MojoRison wrote:
> struct A{
> A& operator=( const A& a ){ i = a.i; return *this; }
>
> private:
> int i;
> };
>
> struct B : public A{
> B& operator=( const B& b ){ j = b.j; return *this; } // how to copy i?
>
> private:
> int j;
> };
>
> In the above code, what would be the standard way of coping A::i in
> B::operator= ?
>
>

I would make the operator= in the parent class
protected. This would prevent "slicing", where only the
parent portion is copied. See [1] below.

One method of implementing assignment operator in child
classes is:
Child&
Child ::
operator=(const Child& ch)
{
if (this != &child) /* guard against self assignment */
{
(Parent&)(*this) = (Parent&)(ch);
/* assignment of child members */
}
return *this;
}
The above fragment comes from Effective C++ or More Effective C++
by Scott Meyers.

[1] When parents allow public assignment, one can assign the
parent portion of child A to child B, using pointers to
the parent class.
struct Parent
{
Parent& operator=(const Parent& p);
};

struct Son : Parent
{
Son& operator=(const Son& s);
};

struct Daughter : Parent
{
Daughter& operator=(const Daughter& d);
};

/* ... */

Son bill;
Son jack;
Daughter amy;
Parent * p_male = &bill;
Parent * p_female = &amy;
Parent * p_brother = & jack;
*p_brother = *p_male; /* Are the child members copied? */
/* One _might_ think so... */
*p_male = *p_female; /* copies only parent portion */
/* Do you want this allowed? */

This has cost me many hours of debugging, so I've learned
to make assignment operators of base classes as protected.
to prevent the above scenario from occurring.

--
Thomas Matthews

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