Dynamic 2D array

[vmohanaraj]

Hi,

I have implemented a template class which encaptulates a 2d dynamic array. Basically the class looks like this:


template <class T>
class Dynamic2dArray {

private:
std::vector<std::vector<T> > data;

public:
Dynamic2dArray();

Dynamic2dArray(int rows);
Dynamic2dArray(int row, int col);
..............................
................................

std::vector<std::vector<T> >::const_iterator begin() const;
};

template <class T>
std::vector<std::vector<T> >::const_iterator Dynamic2dArray<T>::begin() const{
return data.begin();
}

I have implemented everything except this last method: begin() which is supposed to return the iterator pointing the first element in the vector in the same manner as used in std::vector.

I am getting the following compilation error pointing to the declaration of this end method: error: expected `;' before "begin".

I also noticed that when I try to declare a variable itRow (shown below) in another instance the same error is raised.

std::vector<std::vector<T> >::const_iterator itRow;


Does any one know how to solve this problem? I would really appreciate your prompt response.

Thanks and Regards,
Mohan.

[Banfa]

Must be something you've not posted, the code you have posted compiles for me without error. Perhaps you have a mismatched { or }

BTW a common mistake that is made is that the internal data in a class has to match the external interface. By this I mean that you are implementing a 2D dynamic array so you have declared your class internally with a 2D array using vectors. Now sometimes this may be the sensible way of doing it (I can not judge for your case) but it can also make sense to have a 1D array of data internally and calculate your offsets into it.

[vmohanaraj]

Hi Banfa,

Thanks for your reply. I just can not believe the behaviour of this code that even when I compile only the part of the code that I sent you(with empty implementation of the constructors added), I get the same error. Could you explain me what you mean by "mismatched {or}". BTW I am using Code::Block IDE with compiler GNU GCC version 3.4.4.

Yes. I agree with your suggestion. I really thought about implementing 1D vector inside. But in my case, the number of rows for each column varies immensely in size. For example, the first row might have 1 element while the second has 500 elements. So in order to save the space, I decided in this way.
In the code, I have made sure that only the required amount of space is allocated, not more. I thought that If we want to have different number of rows as in my case, the offset technique won't work. Am I correct in that?

Thanks and Regards,
Mohan.

[vmohanaraj]

It just clicked into my mind what you mean by { or }. But they are not mismatched. I checked them. Still I am getting the same error.

[Banfa]

I have to say I am a little bit stumped by this one, I wonder if it's a sneaky ANSI compliance thing, I can't switch on ANSI compilance beacuse I am use MSVC++ and they have not written their C++ headers so they compile under ANSI compliance.

[Prasannaa]

Hi,
you have to prefix std::vector<std::vector<T> >::const_iterator with
typename as

typename   std::vector<std::vector<T> >::const_iterator 

on both your declaration and definition.

What would be more elegant is to typedef the const_iterator as

 typedef typename std::vector<std::vector<T> >::const_iterator  const_iterator; 

and then start using them in places.This also allows the clinets to write code
like Dynamic2dArray::const_iterator itr = darray.begin();

Regards
Prasannaa