c++ - How to correctly specify the copy constructor -

assume have class matrix, constructor follows:

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

suppose have 'copy' constructor 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 suppose have following lines in main function:

int main()     {         matrix a(2,2);         matrix b(2,2);         = matrix(b); //call overloaded assignment operator, , copy ctor/      } 

here '=' operator overloaded assign elements in b a. issue once call copy constructor made, matrix object new object.

is there better way of writing copy constructor if matrix exists calling = matrix(b) results in error?

instead of using dynamically allocated arrays, recommend using std::vector

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

then constructor can be

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

along all of other benefits of using std::vector on dynamically allocated arrays, compiler-generated copy constructor, don't need write one.

if don't want class copyable, delete copy constructor , 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; }