在C ++中复制数组-参考

Question

I have a matrix2d class which consists of a dobule A[2][2]. I am trying to do a constructor which takes obejct of the same type and copies all its values to A[2][2]. I have a problem, here is the class:

class matrix2D {
    public:
    double A[2][2];
    double Z[2][2];

    //Default Constructor.
    matrix2D() {
        A[0][0] = A[1][1] = 1;
        A[0][1] = A[1][0] = 0;
    }

    matrix2D(double x00,double x01, double x10, double x11) {
        A[0][0] = x00;
        A[0][1] = x01;
        A[1][0] = x10;
        A[1][1] = x11;
    }

and now I am to create a constructor which takes matrix2D object and then takes all its values to A.

// Copy Constructor.
    matrix2D(matrix2D& Z) {
        for(int i = 0; i < 2; ++i) {
            for(int j = 0; j < 2; ++j) {
                   A[i][j]=*(Z[i][j]);
            }
        }
    }

It tells my that I try to assign double to matrix2d object. Why does *Z[i][j] does not reference to a double?

SOLVED: I did A[i][j]=ZA[i][j] :)!

Answer 1

The * in that line does not make sense.

Given the data, you don't need a copy constructor at all. However, if you must implement one, it needs to be something along the lines of:

// Use const&, not just &.
// Use a more suitable variable name for the copy
matrix2D(matrix2D const& copy) {
    for(int i = 0; i < 2; ++i) {
        for(int j = 0; j < 2; ++j) {
               A[i][j]= copy.A[i][j];  // Copy A
               Z[i][j]= copy.Z[i][j];  // Copy Z.
        }
    }
}

Answer 2

There are problem in your copy constructor, you are shadowing one of the members (member name Z and parameter name Z ).

I would suggest to not write your own copy constructor and let compiler generate one for you:

matrix2D(const matrix2D & value) = default;

Answer 3

Adding a second answer. And this is the answer I prefer.

Just remove the copy constructor altogether from your class declaration and definition.

Given that your class contains just a pair of fixed sized arrays, you don't need a copy constructor. The compiler will auto generate one for you. A custom copy constructor is typically only needed when your class has dynamically allocated member variables and you need to insure that the pointer values aren't aliased across instances.

Answer 4

You can certainly do the for-loop technique - and I think that would be good to help in your understanding of arrays and pointers. But it's hard to beat the efficiency of memcpy to copy an array of simple types.

   // Copy Constructor.
    matrix2D(const matrix2D& other) {

        memcpy(A, other.A, sizeof(A));
        memcpy(Z, other.Z, sizeof(Z));

    }