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        The header file 'equal.hpp' contains two variants of a the stl algorithm
        equal. The algorithm tests
        to see if two sequences contain equal values;
      
        Before (the proposed) C++14 the algorithm std::equal
        took three iterators and an optional comparison predicate. The first two
        iterators [first1, last1) defined a sequence, and the second one
        first2 defined the start
        of the second sequence. The second sequence was assumed to be the same length
        as the first.
      
        In C++14, two new variants were introduced, taking four iterators and an
        optional comparison predicate. The four iterators define two sequences [first1, last1) and [first2, last2)
        explicitly, rather than defining the second one implicitly. This leads to
        correct answers in more cases (and avoid undefined behavior in others).
      
Consider the two sequences:
auto seq1 = { 0, 1, 2 }; auto seq2 = { 0, 1, 2, 3, 4 }; std::equal ( seq1.begin (), seq1.end (), seq2.begin ()); // true std::equal ( seq2.begin (), seq2.end (), seq1.begin ()); // Undefined behavior std::equal ( seq1.begin (), seq1.end (), seq2.begin (), seq2.end ()); // false
        You can argue that true is the
        correct answer in the first case, even though the sequences are not the same.
        The first N entries in seq2
        are the same as the entries in seq1
        - but that's not all that's in seq2.
        But in the second case, the algorithm will read past the end of seq1, resulting in undefined behavior (large
        earthquake, incorrect results, pregnant cat, etc).
      
However, if the two sequences are specified completely, it's clear that they are not equal.
        The function equal returns
        true if the two sequences compare equal; i.e, if each element in the sequence
        compares equal to the corresponding element in the other sequence. One version
        uses std::equal_to to do the comparison; the other
        lets the caller pass predicate to do the comparisons.
      
template <class InputIterator1, class InputIterator2> bool equal ( InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2 ); template <class InputIterator1, class InputIterator2, class BinaryPredicate> bool equal ( InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, InputIterator2 last2, BinaryPredicate pred );
        Given the container c1 containing
        { 0, 1,
        2, 3, 14, 15 },
        and c2 containing { 1,
        2, 3 }, then
equal ( c1.begin (), c1.end (), c2.begin (), c2.end ()) --> false equal ( c1.begin () + 1, c1.begin () + 3, c2.begin (), c2.end ()) --> true equal ( c1.end (), c1.end (), c2.end (), c2.end ()) --> true // empty sequences are alway equal to each other
        equal works on all iterators
        except output iterators.
      
        Both of the variants of equal
        run in O(N) (linear) time; that is, they compare against
        each element in the list once. If the sequence is found to be not equal at
        any point, the routine will terminate immediately, without examining the
        rest of the elements.
      
        Both of the variants of equal
        take their parameters by value and do not depend upon any global state. Therefore,
        all the routines in this file provide the strong exception guarantee.
      
equal
            is part of the C++14 standard. When C++14 standard library implementations
            become available, the implementation from the standard library should
            be used.
          equal returns true for
            two empty ranges, no matter what predicate is passed to test against.