I am confused on comparing iterators in C++. With the following code:
std::iterator< std::forward_iterator_tag, CP_FileSystemSpec> Iter1;
std::iterator< std::forward_iterator_tag, CP_FileSystemSpec> Iter2;
while( ++Iter1 != Iter2 )
{
}
The error is:
error: no match for 'operator++' in '++Iter1'
I seem to recall that you could not do what the code above is doing. But I dont quite know how to do the comparison.
std::iterator is not an iterator in itself, but a base class other iterators could inherit from to get a few standard typedefs.
template<class Category, class T, class Distance = ptrdiff_t, class Pointer = T*, class Reference = T&>
struct iterator
{
typedef T value_type;
typedef Distance difference_type;
typedef Pointer pointer;
typedef Reference reference;
typedef Category iterator_category;
};
This error has nothing to do with the comparison- it's telling you that that specific iterator does not support incrementing.
You're supposed to derive from std::iterator -- instantiating it directly makes no sense.
To make that sample work, use an iterator that is backed by an sequence, for instance a vector:
std::vector<int> foo(10); // 10 times 0
std::vector<int>::iterator it1 = foo.begin();
std::vector<int>::iterator it2 = foo.end();
while(++it1 != it2) {
// do stuff
}
Note that this is not the canonical way to iterate over a collection. It is also tricky, because it skips the first element of the sequence. Use this:
for(std::vector<int>::iterator it = foo.begin(); it != foo.end(); it++) {
// do stuff
}
