reverse_iterator

Hosam Aly Mahmoud

I have an STL list, and I want to move on it from back to front. I am trying to use the reverse_iterator, but I am getting a compilation error. Here is my code:

typedef std::list<RECT> RectList;
RectList myList; // The list is created and initialised elsewhere

for ( RectList::reverse_iterator ri = myList.rbegin();
ri != RectList.rend(); ++ri )
{
*ri = *(ri + 1);
}

I get the following errors in the last line: error C2784: 'class std::reverse_iterator<_RI,_Ty,_Rt,_Pt,_D> __cdecl std::operator +(_D,const class std::reverse_iterator<_RI,_Ty,_Rt,_Pt,_D> &)' : could not deduce template argument for '' from 'class std::reverse_bidirectional_iterator<class std::list<struct tagRECT,class std::allocator<struct tagRECT> >::iterator,struct tagRECT,struct tagRECT &,struct tagRECT *,int>' error C2676: binary '+' : 'class std::reverse_bidirectional_iterator<class std::list<struct tagRECT,class std::allocator<struct tagRECT> >::iterator,struct tagRECT,struct tagRECT &,struct tagRECT *,int>' does not define this operator or a conversion to a type acceptable to the predefined operator

Hosam Aly Mahmoud

Taka Muraoka

You can only increment or decrement iterators, not add a numeric offset to them.

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jhwurmbach

But he can dereference them and use operators on the referenced object. Provided that the operators are defined for the object. Was it that what he tried: *r+1 ?

Who is 'General Failure'? And why is he reading my harddisk?!?

Taka Muraoka

His code is doing this: *ri = *(ri + 1); i.e. he is trying to add one to the iterator, *then* dereferencing it.

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jhwurmbach

You get an error here, because you can not use '+' with an iterator. If you want to assign the value of the next object to the current one, I would do:

{
RectList::reverse_iterator next = ri++;
*ri = *next;
}

If you want to access an iterator that is more than one step away, you can use std::advance() to step the iterator many steps. If you intended to add a value to the value referenced by ri, you need to write:

*ri = *ri + value;

Who is 'General Failure'? And why is he reading my harddisk?!?

Hosam Aly Mahmoud

Thanks for your reply. But why can't I add to the iterator? According to MSDN[^], the reverse iterator contains operator+. What I don't know is what this operator takes as input. Anyway, I changed my code to this and it compiled:

for ( RectList::reverse_iterator ri = myList.rbegin(), ri2 = myList.rbegin();
ri2 != myList.rend(); ++ri )
*ri = *(++ri2);

Thank you for your help.

Hosam Aly Mahmoud

Hosam Aly Mahmoud

Thanks for your reply. Your code does the opposite of what I wanted. I would change it to:

{
RectList::reverse_iterator next = ri;
*ri = *(++next);
}

Thanks anyway.