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

You’re right that an invalid pointer can’t be used, but you’re wrong that a pointer to an element one past the last element in an array is an invalid pointer – it’s valid.

The C standard, section 6.5.6.8 says that it’s well defined and valid:

…if the expression P points to the
last element of an array object, the
expression (P)+1 points one past the
last element of the array object…

but cannot be dereferenced:

…if the result points one past the
last element of the array object, it
shall not be used as the operand of a
unary * operator that is evaluated…