Need of assignment operator in case of copy constructor

alcoholic · #1 01-Jan-2006, 04:38

I always come across this statement...that you should also define assignment operator when you define copy constructor.
I don't think its any rule....can anyone elaborate on this...

Paramesh · #2 01-Jan-2006, 05:09

Happy New Year.. (the second time!)

Quote:

Originally Posted by alcoholic

I always come across this statement...that you should also define assignment operator when you define copy constructor.
I don't think its any rule....can anyone elaborate on this...

Its not a rule. But it results in faulty programs...

And this is applicable only if you have dynamic memories associated with the class.

I Hope you know the difference between assignment and initialization.
Copy constructor initializes the object, whereas, assignment means making one object equal to the other.

Consider this case:

CPP / C++ / C Code:

int a, b=10;
a = b;

So, b is initialized to 10, whereas a is assigned.

This is similar to objects.
Consider that there is a class A, having a character pointer.
(char* is dynamically allocated!! this is the most important thing...)

CPP / C++ / C Code:

class A {
    char *x;
    int length;
public:
    void initialize();
    ~A();
};

Now, consider this in the main function:

CPP / C++ / C Code:

A obj1;
obj1.initialize();

A obj2 = obj1;    // Copy constructor, Initialization

A obj3;
obj3 = obj1;       // Assignment

So, what has happened?
obj2 has called the copy constructor.
If the compiler doesnt find one, it generates the default one-to-one copy constructor for you.

Heck! there lies the error!
Now, both obj2.x and obj1.x points to the same location!!!!

To resolve this, you should explicitly define a copy constructor like this:

CPP / C++ / C Code:

    A( const A &y) {
        length = y.length;
        x = new char[length+1];
        strcpy(x, y.x);
    }

and you should properly allocate the memory.

But what if we perform assignment?
that is the case of the obj3 object.
The compiler automatically generates a default operator=, and again, we get the same error.
So, we have to explicitly define assignment operator like the same way we did for the copy constructor.
Like this:

CPP / C++ / C Code:

void operator=(const A &y) {
        length = y.length;
        x = new char[length+1];
        strcpy(x, y.x);
}

Now, the problem is solved.

Hope This Helps,
Best Regards,

Paramesh.

alcoholic · #3 01-Jan-2006, 07:15

Thanx again...I was thinking in this line but was not sure...you made it clear

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Need of assignment operator in case of copy constructor

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Re: Need of assignment operator in case of copy constructor