Given a function such as the following:
template<typename ForwardIterator>
void MyFunc(ForwardIterator first, ForwardIterator last) {
...
}
Given that first and last are in the same data structure, how can I check that first is in fact before last in this data structure?
Instead of solving this problem, just do what the standard library does: Assert the precondition that last must be reachable from first and be done (note that it may be reachable by identity).
If you don't make that precondition you have to also be given a reference/pointer to the container, or the begin/end of the container in order to do the validation checks.
just use std::distance with someContainer.begin() as argument:
auto d1 = std::distance(someContainer.begin() , iterator1);
auto d2 = std::distance(someContainer.begin() , iterator2);
if (d1>d2){..
The short answer is "in general, you can't".
The longer answer is: You can increment first over and over again, and if you get to last, then you know that first was "before" last. If you don't get to last, well, maybe you haven't gone far enough - there's no way to tell.
As Mark B says in his answer, the way the standard library does it is to tell the user "It's your responsibility to make sure that last is reachable from first".
