What do the std::equal overloads do?

Question

I'm looking at the recent c++14 overloads for std::equal, and I can't figure out just what they do and are used for...

The two overloads are:

template< class InputIt1, class InputIt2 >
bool equal( InputIt1 first1, InputIt1 last1, 
            InputIt2 first2, InputIt2 last2 );

template< class InputIt1, class InputIt2, class BinaryPredicate >
bool equal( InputIt1 first1, InputIt1 last1, 
            InputIt2 first2, InputIt2 last2,
            BinaryPredicate p );

I fully understand the traditional std::equal that uses just one InputIt2, but the second InputIt2 last2 is twisting my brain. Could someone explain and give an example of this?

Answer 1

The new overloads are actually pretty great. You pass in two full ranges, beginning and end, and rather than run off the end of the shorter one and invoke undefined behaviour, the algorithm stops.

Such improvements were also added to std::mismatch and std::is_permutation. You can read more about this in the proposal

For std::equal, the algorithm will simply return false if the lengths are not equal.
For std::mismatch, if the algorithm hits the end of one range, it will return that iterator and the corresponding iterator from the other range.
For std::is_permutation, the algorithm will also simply return false if the ranges are not equal in length.

For reasoning about why, consider that the programmer checking the length is not necessarily possible or cheap. A range obtained from a std::list without the original list would need to be traversed through to get the size. A range that uses an InputIterator, such as for reading from standard input, is potentially infinite until it hits an end, and it is only allowed to be traversed through once, so the algorithm could no longer use it after you do that. Thank Benjamin Lindley below for that last example.