Function returning classes

[panecasareccio]

Dear all,

I have a problem with a function that returns a class defined by myself. The code is compiled without problems, but program crashes, for a reason I cannot understand.

I report the code below. The function that is troubling me is named "catV". Could anyone help me in understanding what is wrong?

  #include <iostream>
using namespace std;

class mtx
{
public:
	int nr, nc;
	double* data;
	mtx();							//default constructor
	mtx(int, int);					//constructor
	mtx(mtx&);						//copy constructor
	~mtx() {delete [] data;}		//destructor
	void display();
};

mtx::mtx()
{
	nr = 0;
	nc = 0;
	data = NULL;
}
	
mtx::mtx(int n_rows, int n_cols)
{
	nr = n_rows;
	nc = n_cols;
	data = new double [nr*nc];
	for (int i=0; i<nr*nc; i++)
		data[i] = 0;
}

mtx::mtx(mtx& M)
{
	nr = M.nr;
	nc = M.nc;
	for (int i=0; i<nr*nc; i++)
		data[i] = M.data[i];
}


void mtx::display()
{
	cout << endl;
	int j=0;
	for (int i=0; i<nr; i++)
	{
		for (int k=0; k<nc; k++)
			cout << data[j++] << '\t';
		cout << endl;
	}
	cout << endl;
	return;
}

	
mtx catV(mtx A, mtx B)
{
	if (A.nc == B.nc)
	{
		mtx C(A.nr+B.nr, A.nc);
		/*int QA = A.nr*A.nc;
		int QB = B.nr*B.nc;
		for (int i=0; i<QA; i++) C.data[i] = A.data[i];
		for (int i=0; i<QB; i++) C.data[i+QA] = B.data[i];*/
		return C;
	}
	else
	{
		mtx C;
		return C;
	}
}	

int main()
{
    mtx A(4,3), B;
	A.display();
	B = A;
	for (int i=0; i < 12; i++) B.data[i] = 12;
	B.display();
	
	mtx C;
	C = catV(A,B);
	//C.display();
    return 0;
}
    

I thank you in advance.
Panecasareccio.

[vlad from moscow]

You did not allocate memory for data member data in your copy constructor

mtx::mtx(mtx& M)
{
	nr = M.nr;
	nc = M.nc;
	for (int i=0; i<nr*nc; i++)
		data[i] = M.data[i];
}

The value of data is undefined.

[jlb]

You're not always allocating memory for your pointer. Check your copy constructor to insure it properly re-allocates the proper amount of memory for the copy.

[panecasareccio]

Thank you all.

you were right, I had not allocated memory doe the array data in my copy constructor. Now I have changed the code accordingly, it runs well, and I get the desired output.

However, something even more weird happens: I run the executable from a command line (windows 8), the program runs correctly. Then, if I run it again, it gets stuck, like it was in an infinite loop. I cannot understand why.

The new code is reported below. could anyone please take a look at that?


My felt thnks.

#include <iostream>
using namespace std;

class mtx
{
public:
	int nr, nc;
	double* data;
	mtx();							//default constructor
	mtx(int, int);					//constructor
	mtx(mtx&);						//copy constructor
	~mtx() {delete [] data;}		//destructor
	void display();
};

//default constructor
mtx::mtx()
{
	nr = 0;
	nc = 0;
	data = NULL;
}

//constructor: accepts size, sets data to zeros	
mtx::mtx(int n_rows, int n_cols)
{
	nr = n_rows;
	nc = n_cols;
	data = new double [nr*nc];
	for (int i=0; i<nr*nc; i++)
		data[i] = 0;
}

//copy constructor
mtx::mtx(mtx& M)
{
	nr = M.nr;
	nc = M.nc;
	data = new double [nr*nc];
	for (int i=0; i<nr*nc; i++)
		data[i] = M.data[i];
}

//diaplay a matrix on the screen
void mtx::display()
{
	cout << endl;
	int j=0;
	for (int i=0; i<nr; i++)
	{
		for (int k=0; k<nc; k++)
			cout << data[j++] << '\t';
		cout << endl;
	}
	cout << endl;
	return;
}

	
mtx catV(mtx A, mtx B)
{
	if (A.nc == B.nc)
	{
		mtx C(A.nr+B.nr, A.nc);
		int QA = A.nr*A.nc;
		int QB = B.nr*B.nc;
		for (int i=0; i<QA; i++) C.data[i] = A.data[i];
		for (int i=0; i<QB; i++) C.data[i+QA] = B.data[i];
		return C;
	}
	else
	{
		mtx C;
		return C;
	}
}	

int main()
{
    mtx A(4,3), B(4,3);
	A.display();
	
	for (int i=0; i < 12; i++) B.data[i] = 12;
	B.display();
	
	mtx C;
	C = catV(A,B);
	C.display();
    return 0;
}

[vlad from moscow]

Here you are using the copy assignment operator

C = catV(A,B);

But you shall explicitly define this operator the same way as the copy constructor because you need to manipulate allocated memory.