Reference, Copy C'tor etc.

[chameleon]

matrix_columns MUST point on the same vector as matrix_rows.

Copy constructor copy the vector: unacceptable.
Move constructor destroy vector of matrix_rows: unacceptable.
memcpy in code below, does the right thing.

Is there a better approach? buffer of chars is a little bit crapy coding.

struct Matrix : public vector<Vector> { bool vertical; };
int main()
{
	Matrix matrix_rows;
	matrix_rows.vertical = false;

	char buffer[sizeof(Matrix)];
	memcpy(buffer, &matrix_rows, sizeof(Matrix));

	Matrix &matrix_columns = reinterpret_cast<Matrix&>(*buffer);
	matrix_columns.vertical = true;

	// do things.

	return 0;
}

[webJose]

Your code is incorrect if Matrix is not a POD struct. Only POD's can be cloned using memcpy().

[chameleon]

I think, you are wrong.

Your problem exists only in virtual classes or classes with multiple inheritance:
- With multiple inheritance reinterpret_cast blows the universe.
- If you upcast with single inheritance, reinterpret_cast keeps wrong virtual table.

I don't use virtual class.
I don't use multiple inheritance.
I don't use upcasting.

So, I believe I am ok.
Wrong?

[webJose]

I have never delved that deeply into the issue so I cannot really assure you it is wrong even under special circumstances. What I CAN say is that bitwise cloning is for POD's only; non-POD's must use memberwise cloning. This is my instruction talking and I have never read about any exceptions anywhere. I'm not the theorist guy that can prove or disprove your theory, so hopefully someone else can stop by and collaborate.

[coder777]

Wrong?

Yes (sorry couldn't resist)

the problem with the virtual table is that it is a pointer. A non POD type like vector has likely more pointer. If you copy those pointers the memory pointed to is probably invalid. It would rather be a surprise if not.

Like webJose said don't copy non POD types. Instead overload the operator=(). And you shouldn't use a STL container as a base class.

reinterpret_cast is horrible don't use that. Only dynamic_cast is safe.

[chameleon]

Ooops!!
I forget to say, that since I add needed elements to vector, class stay const (no changes at all, until destruction). So then I create a reference to that vector (only difference is orientation of vectors - the member vertical).

All of these, you say, are correct.
If you copy the 3 pointers of vector, and you remove an element from that vector, these 3 pointers become invalid. But if you don't change vector, the 3 pointers remain valid.

[Athar]

It's still undefined behavior and therefore incorrect. Even if it seems to work now, it doesn't mean it will work tomorrow or with a different compiler or with different compiler settings or when it starts raining outside.

If you need different views on the same object, then create a MatrixView class that contains some sort of pointer to the real Matrix. And if the original object is constant anyway, then how exactly is copying the vector "unacceptable"?