Checking if an iterator is valid

I assume you mean "is an iterator valid," that it hasn't been invalidated due to changes to the container (e.g., inserting/erasing to/from a vector). In that case, no, you cannot determine if an iterator is (safely) dereferencable.

As jdehaan said, if the iterator wasn't invalidated and points into a container, you can check by comparing it to container.end().

Note, however, that if the iterator is singular -- because it wasn't initialized or it became invalid after a mutating operation on the container (vector's iterators are invalidated when you increase the vector's capacity, for example) -- the only operation that you are allowed to perform on it is assignment. In other words, you can't check whether an iterator is singular or not.

std::vector<int>::iterator iter = vec.begin();
vec.resize(vec.capacity() + 1);
// iter is now singular, you may only perform assignment on it,
// there is no way in general to determine whether it is singular or not

Non-portable answer: Yes - in Visual Studio

Visual Studio's STL iterators have a "debugging" mode which do exactly this. You wouldn't want to enable this in ship builds (there is overhead) but useful in checked builds.

Read about it on VC10 here (this system can and in fact does change every release, so find the docs specific to your version).

Edit Also, I should add: debug iterators in visual studio are designed to immediately explode when you use them (instead undefined behavior); not to allow "querying" of their state.