Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for an old version of Boost. Click here to view this page for the latest version.

C++14 Algorithms


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

Iterator Requirements

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.

Exception Safety

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.

  • The four iterator version of the routine 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.