How to correctly specify the copy constructor

Question

Assume I have a class Matrix, with a constructor as follows:

Matrix::Matrix(int rows, int cols)
{
    nrows = a; //here nrows is the number of rows for each matrix 
    ncols = b; //here ncols is the number of cols for each matrix
    p = new double [rows*cols];
    for(int i=0;i<rows*cols;i++) 
        { 
            *p++ = 0.0;
        }
}

Suppose I also have a 'copy' constructor as follows:

Matrix::Matrix(const Matrix& mat)
{   p = new double[mat.nrows*mat.ncols];
    for(int i=0;i<mat.nrows*mat.ncols;i++)
        { 
             p[i] = mat.p[i];

        }
}

Now also suppose I have the following lines in my main function:

int main()
    {
        Matrix A(2,2);
        Matrix B(2,2);
        A = Matrix(B); //call overloaded assignment operator, and copy ctor/
     }

Here the '=' operator is overloaded to assign all the elements in B to A. My issue is that once the call to the copy constructor is made, the Matrix A object is a completely new object.

Is there a better way of writing the copy constructor so that if Matrix A already exists then calling A = Matrix(B) results in an error?

What do you mean by "the Matrix A object is a completely new object"? — Pradhan, Aug 19 '15 at 15:30
You can't assign to an object that doesn't already exist. If you want to disallow assignment, ask about that. (Remember that `Matrix A = B;` isn't assignment but initialisation.) — molbdnilo, Aug 19 '15 at 15:31
@NathanOliver I guess I was wondering in general, if there was something I should be doing in order to prevent this happening. What I have presented above is an example, but was curious if in general I should be coding in a way that prevents this type of issue.... — Don Shanil, Aug 19 '15 at 15:48
@molbdnilo Yes you are right. I have amended the code I was missing a statement here. — Don Shanil, Aug 19 '15 at 16:04
`p = new double[mat.nrows * ncols]` should be `p = new double[mat.nrows * mat.ncols]`. (i.e. you forgot a `mat.`) — Aaron McDaid, Aug 19 '15 at 16:04

score 8 · Accepted Answer · answered Aug 19 '15 at 15:22

Instead of using dynamically allocated arrays, I would recommend using std::vector

class Matrix
{
public:
    Matrix(long rows, long cols);
private:
    long nrows;
    long ncols;
    std::vector<double> p;
}

Then your constructor can be

Matrix::Matrix(long rows, long cols)
: nrows(rows),
  ncols(cols),
  p(rows * cols)
{ }

Along with all of the other benefits of using std::vector over dynamically allocated arrays, you now get a compiler-generated copy constructor, so you don't need to write one.

If you don't want your class to be copyable, delete the copy constructor and copy assignment operator.

class Matrix
{
public:
    Matrix(long rows, long cols);
    Matrix(const Matrix& mat) = delete;
    Matrix& operator=(const Matrix& mat) = delete;
private:
    long nrows;
    long ncols;
    std::vector<double> p;
}

score 3 · Answer 2 · answered Aug 19 '15 at 15:24

3

With the statement

A = Matrix(B);

first a temporary object is create using the copy-constructor. This temporary object is then used in the assignment.

So it's true that the copy-constructor is used, but not as part of the assignment.

answered Aug 19 '15 at 15:24

Some programmer dude

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score 2 · Answer 3 · answered Aug 19 '15 at 15:24

2

It would be far better to delete the assignment operator:

Matrix& operator=(const Matrix&) = delete;

Then use of A = Matrix(B) would emit a compile time error. You'd then be forced to use the constructor and your specific issue would cease to be relevant.

answered Aug 19 '15 at 15:24

Bathsheba

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Thanks for this...not entirely sure though how the '=delete' works. Does this mean that any special member function/operator defined this way, cannot be called? so in effect we don't have a assignment operator (including default assignment operator) and thus we use the copy constructor for assignments as well? Thanks again! – Don Shanil Aug 20 '15 at 06:46
Google "C++ member deletion". – Bathsheba Aug 20 '15 at 06:49

score 0 · Answer 4 · answered Aug 19 '15 at 15:22

0

No. Because then it wouldn't be a copy constructor. A constructor constructs.

answered Aug 19 '15 at 15:22

lordbah

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How to correctly specify the copy constructor

4 Answers4