c++ - Why is comparing against "end()" iterator legal?

Question

According to C++ standard (3.7.3.2/4) using (not only dereferencing, but also copying, casting, whatever else) an invalid pointer is undefined behavior (in case of doubt also see this question). Now the typical code to traverse an STL containter looks like this:

std::vector<int> toTraverse;
//populate the vector
for( std::vector<int>::iterator it = toTraverse.begin(); it != toTraverse.end(); ++it ) {
    //process( *it );
}

std::vector::end() is an iterator onto the hypothetic element beyond the last element of the containter. There's no element there, therefore using a pointer through that iterator is undefined behavior.

Now how does the != end() work then? I mean in order to do the comparison an iterator needs to be constructed wrapping an invalid address and then that invalid address will have to be used in a comparison which again is undefined behavior. Is such comparison legal and why?

Answer 1

The only requirement for end() is that ++(--end()) == end(). The end() could simply be a special state the iterator is in. There is no reason the end() iterator has to correspond to a pointer of any kind.

Besides, even if it were a pointer, comparing two pointers doesn't require any sort of dereference anyway. Consider the following:

char[5] a = {'a', 'b', 'c', 'd', 'e'};
char* end = a+5;
for (char* it = a; it != a+5; ++it);

That code will work just fine, and it mirrors your vector code.