Copy assignment help.

[darren78]

Hi,

I'm not sure if the book is incorrect or I simply cannot understand this code yet:

class vector {
	int sz;
	double* elem; // pointer to the elements
	void copy(const vector& arg);
public:
	vector& operator=(const vector&);
};

void vector::copy(const vector& arg)
{
	for (int i = 0; i<arg.sz; ++i) elem[i] = arg.elem[i];
}

vector& vector::operator =(const vector& a)
{
	double* p = new double[a.sz];  // allocate new space
	copy(a);  // copy elements
	delete[] elem;  //deallocate old space
	elem = p;  // reset elem
	sz=a.sz;
	return *this;
}

The part I don't understand is the operator overload function. The new space has been allocated and then elements copied, but where are these elements copied to? According to the copy function, they are copied to elem[i] of the vector being copied to. However, in the = overload, that elem is deleted immediately after the copy function is called and then reset with p. Are the elements copied into p, if so how?

Thanks.

[anon]

Yes, it appears the example is broken.

Rewrite the copy function so it lets you choose, where to copy to or just replace the custom copy with std::copy (#include <algorithm>):

std::copy(a.elem, a.elem + a.sz, p);

(Note, copy to the new buffer that was created!)

[darren78]

Thanks, I did think it was wrong. It's so frustrating when that happens and you are trying to learn from incorrect material.

I will look into it further from your suggestions.

Thanks.