...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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 (), seq1.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.