Thread: Vector iterators incompatible

Hello,

First, let me state that I have solved the issue that was giving me the VC++ debug assertion: "Expression: vector iterators incompatible". It's just that I'm not exactly how my change made a difference.

I have a 2-d array template class which as its only member has a vector-of-vectors. An abbreviated version is shown below:

Code:

template <class T>
class vector2
{
protected:
    vector<vector<T> > elements;

public:
    vector2();	
    vector2(unsigned rows, unsigned columns);
    vector2(unsigned rows, unsigned columns, const T& value);
    vector2(const vector2<T>& other);

    //Overloaded operators
    vector2<T>& operator=(const vector2<T>& other);
    vector<T>&  operator[] (unsigned index);
    vector<T>   operator[] (unsigned index) const;

    // ...other member methods...
};

The problem occurred in the implementation of the copy constructor body:

Code:

template <class T>
vector2<T>::vector2(const vector2<T>& other) :
elements(other.elements)
{
}

...which usually worked, but I subsequently changed it to:

Code:

template <class T>
vector2<T>::vector2(const vector2<T>& other)
{
	elements = other.elements;
}

...and VC++ no longer asserted. Thus, it appears the incompatible vector iterators has something to do copy-constructing a std::vector-of-std::vectors, rather than using the assignment operator. Can anyone explain why exactly this occurs? I'm not a newbie but I'm also not an expert, so this might be a little beyond my current understanding. Normally, using the copy constructor works just fine - each vector-of-vector elements are always copied as I expect. It's just this specific assertion that I've never run into before.

Thanks in advance!

Judging by similar posts elsewhere (none exactly similar to my situation), it appears to be an issue with iterator stuff specifically used in DEBUG mode. If one were to ask Microsoft, I'm sure they'd tell you it was "by design." Not that I would argue - I'd just like to know what's going on behind the scenes, and why my first version would cause an assertion. If nobody knows, that's fine, I'm just curious.

Can you make a compilable example that shows the error (other than the entire source)? My instinct says "difference in const" but it's hard to say.

Originally Posted by brewbuck

Why are you writing a copy constructor and assignment operator in the first place? The default ones already do what you want.

Good point. I should note that originally, this class did not use std::vector internally, and those are relics from the old code I should have just eliminated. Had I done that in the first place, I'd never have run into this issue.