Programming Forums > Application Development > C++

Lakrids · Jan 18th, 2008, 12:22 PM

Hello,

I have a weird problem with an inherited operator=. Here's a simplified code of the situation :

 cplusplus Syntax (Toggle Plain Text) 
class A {
  public:
      A& operator=(const A&);
};
 
class B : public A {
  public:
      B& operator=(const B&);
      B& operator=(const A&);
};
class A {
  public:
      A& operator=(const A&);
};

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

Now, this works FINE:

 cplusplus Syntax (Toggle Plain Text) 
B& B::operator=(const A& a) {
    this->A::operator=(a);
    return (*this);
}
B& B::operator=(const A& a) {
    this->A::operator=(a);
    return (*this);
}

gives the accurate results. However, this doesn't :

 cplusplus Syntax (Toggle Plain Text) 
B& B::operator=(const B& b) {
    this->A::operator=(b);
    return (*this);
}
B& B::operator=(const B& b) {
    this->A::operator=(b);
    return (*this);
}

Any idea why? It compiles fine, but I get very inaccurate results. I had several ideas, but none of them solved the problem :

-putting 'this->' before every private variable of class A
-converting b into an A using every _cast possible

Any ideas? Thank you

peaceofpi · Jan 18th, 2008, 12:42 PM

Quote:

Originally Posted by Lakrids

 cplusplus Syntax (Toggle Plain Text) 
B& B::operator=(const B& b) {
    this->A::operator=(b);
    return (*this);
}
B& B::operator=(const B& b) {
    this->A::operator=(b);
    return (*this);
}

I'm guessing (literally, not sarcastically) that the problem is the second line: class A doesn't have an overloaded operator= for a type B.

Lakrids · Jan 18th, 2008, 1:11 PM

Yes, but if you pass an object B to it, it will automatically be converted into an object A. So no need for such an operator! Also, I am not allowed to add such a method into A (project restrictions).

What do you think would be a good solution?

grumpy · Jan 18th, 2008, 4:18 PM

Depends on your notion of "best".

The reason for the "inaccurate results" in the second B::operator=() is probably that you're slicing the objects. You need to do something with the B part of the left hand side.

It is also possible you have no need to implement B's assignment operators at all. If you don't do it the compiler will supply defaults (which essentially do assignments of individual members of B after calling the assignment operator for the base class). If that is sufficient, then you don't need to implement B's assignment operator at all.

From a design perspective, it is generally considered a bad idea for a base class to be non-abstract. If you follow that guideline, you will usually have no need for B &operator=(const A &); in class B anyway.

Lakrids · Jan 19th, 2008, 5:32 AM

Quote:

It is also possible you have no need to implement B's assignment operators at all. If you don't do it the compiler will supply defaults (which essentially do assignments of individual members of B after calling the assignment operator for the base class). If that is sufficient, then you don't need to implement B's assignment operator at all.

Yeah, your right as usual Grumpy. Now it works. Thanks!

Jan 18th, 2008, 12:22 PM	#1
Lakrids Programmer Join Date: Dec 2007 Posts: 41 Rep Power: 0	Inherited operator= malfunctioning Hello, I have a weird problem with an inherited operator=. Here's a simplified code of the situation : cplusplus Syntax (Toggle Plain Text) class A { public: A& operator=(const A&); }; class B : public A { public: B& operator=(const B&); B& operator=(const A&); }; class A { public: A& operator=(const A&); }; class B : public A { public: B& operator=(const B&); B& operator=(const A&); }; Now, this works FINE: cplusplus Syntax (Toggle Plain Text) B& B::operator=(const A& a) { this->A::operator=(a); return (this); } B& B::operator=(const A& a) { this->A::operator=(a); return (this); } gives the accurate results. However, this doesn't : cplusplus Syntax (Toggle Plain Text) B& B::operator=(const B& b) { this->A::operator=(b); return (this); } B& B::operator=(const B& b) { this->A::operator=(b); return (this); } Any idea why? It compiles fine, but I get very inaccurate results. I had several ideas, but none of them solved the problem : -putting 'this->' before every private variable of class A -converting b into an A using every _cast possible Any ideas? Thank you

Jan 18th, 2008, 1:11 PM	#3
Lakrids Programmer Join Date: Dec 2007 Posts: 41 Rep Power: 0	Re: Inherited operator= malfunctioning Yes, but if you pass an object B to it, it will automatically be converted into an object A. So no need for such an operator! Also, I am not allowed to add such a method into A (project restrictions). What do you think would be a good solution?

Jan 18th, 2008, 4:18 PM	#4
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,318 Rep Power: 5	Re: Inherited operator= malfunctioning Depends on your notion of "best". The reason for the "inaccurate results" in the second B::operator=() is probably that you're slicing the objects. You need to do something with the B part of the left hand side. It is also possible you have no need to implement B's assignment operators at all. If you don't do it the compiler will supply defaults (which essentially do assignments of individual members of B after calling the assignment operator for the base class). If that is sufficient, then you don't need to implement B's assignment operator at all. From a design perspective, it is generally considered a bad idea for a base class to be non-abstract. If you follow that guideline, you will usually have no need for B &operator=(const A &); in class B anyway.

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Programming Forums > Application Development > C++

Inherited operator= malfunctioning