creating a copy constructor

[yotama9]

Hi guys.

I have created a class which is basically an array that allows me to do vector algebra on the array. I have overloaded the copy constructor ass follows:

void myVec::operator=(myVec other){
	coor = new double[length];
	for (int i = 0; i<length; i++){
		double temp = other[i];
		coor[i] = temp;
	}
}

where length defines the length of the vector (usually 3) and coor is an array that holds the coordinates of the vector (x,y,z). I used the temp part to make sure that the vector I copy to doesn't get the pointer of the vector I'm copying from.

Nevertheless, when I change a value in the vector I copied to I can see that I have also changed the vector I copied from, what can I do to fix this?

Thanks.

Yotam.

[filipe]

myVec(const myVec& vec);

A copy constructor is a constructor which takes a const reference to an object of the same type as its one argument, not an overloaded operator.

By the way, why are you using an array when you could use a (typical) struct?

struct Vector3 {
	int x, y, z;
};

[yotama9]

Thanks.

I'm not sure of how to implement your line. Should it be something like that:?

void myVec::operator=(const myVec& vec)

or should I put in my code:

myVec::myVec(const myVec& vec)

I'm using the array because I want to be able to change easily the size of the vector, the way I'm doing it now, all I need to change is a definition in the header and the only damaged function will be the scalar product calculation

[R0mai]

That's not the copy constructor, that is the assignment operator.

Assuming your class looks something like this:

class myVec {
    double *coor;
    unsigned length;
};

An assingment operator and a copy constructor would look like this:
(Note: these functions assume that the length of the vector is never 0)

myVec::matrix(const myVec& o) : length(o.length) {    
    coor = new double[length];
    std::copy( o.coor, o.coor + length, coor );
}


myVec& myVec::operator=(const myVec& o) {
    if ( this == &o ) {
        return *this; //Self assignment : nothing to do
    }

    delete[] coor;

    length = o.length;
    coor = new double[length];
    std::copy( o.coor, o.coor + length, coor );
	
    return *this;
}

EDIT : code corrected

[filipe]

How often will you need different sizes of vectors? If it's not that often, I'd still go with structs. Otherwise, a std::vector is definitely preferable over an array.

Anyway, myVec(const myVec& vec); is the function signature for a copy constructor for your class. You're confusing it with the assignment operator.

[yotama9]

filipe:

I don't think I'll need to change the vector size anytime soon. However, the whole class is written with the thought of using an array. I can change the class to a vector instead of array, but I can't see how I will be able to do so easily with the construct.

R0mai:

I guess I'll use the operator=. I still can't understand one part of your code though. What does the "data_" line means, shouldn't it be coor?


Thanks.

[R0mai]

I'm sorry, data_ was a leftover from the code I copied from. I edited it.