...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 'boost/algorithm/cxx11/one_of.hpp' contains four variants
of a single algorithm, one_of
.
The algorithm tests the elements of a sequence and returns true if exactly
one of the elements in the sequence has a particular property.
The routine one_of
takes
a sequence and a predicate. It will return true if the predicate returns
true for one element in the sequence.
The routine one_of_equal
takes a sequence and a value. It will return true if one element in the sequence
compares equal to the passed in value.
Both routines come in two forms; the first one takes two iterators to define the range. The second form takes a single range parameter, and uses Boost.Range to traverse it.
The function one_of
returns
true if the predicate returns true for one item in the sequence. There are
two versions; one takes two iterators, and the other takes a range.
namespace boost { namespace algorithm { template<typename InputIterator, typename Predicate> bool one_of ( InputIterator first, InputIterator last, Predicate p ); template<typename Range, typename Predicate> bool one_of ( const Range &r, Predicate p ); }}
The function one_of_equal
is similar to one_of
, but
instead of taking a predicate to test the elements of the sequence, it takes
a value to compare against.
namespace boost { namespace algorithm { template<typename InputIterator, typename V> bool one_of_equal ( InputIterator first, InputIterator last, V const &val ); template<typename Range, typename V> bool one_of_equal ( const Range &r, V const &val ); }}
Given the container c
containing
{ 0, 1,
2, 3, 14, 15 }
,
then
bool isOdd ( int i ) { return i % 2 == 1; } bool lessThan10 ( int i ) { return i < 10; } using boost::algorithm; one_of ( c, isOdd ) --> false one_of ( c.begin (), c.end (), lessThan10 ) --> false one_of ( c.begin () + 3, c.end (), lessThan10 ) --> true one_of ( c.end (), c.end (), isOdd ) --> false // empty range one_of_equal ( c, 3 ) --> true one_of_equal ( c.begin (), c.begin () + 3, 3 ) --> false one_of_equal ( c.begin (), c.begin (), 99 ) --> false // empty range
one_of
and one_of_equal
work on all iterators except
output iterators.
All of the variants of one_of
and one_of_equal
run in
O(N) (linear) time; that is, they compare against each
element in the list once. If more than one of the elements in the sequence
satisfy the condition, then algorithm will return false immediately, without
examining the remaining members of the sequence.
All of the variants of one_of
and one_of_equal
take their
parameters by value or const reference, and do not depend upon any global
state. Therefore, all the routines in this file provide the strong exception
guarantee.
one_of
and one_of_equal
both return false for
empty ranges, no matter what is passed to test against.
one_of_equal
is a template parameter, rather than deduced from the first parameter
(std::iterator_traits<InputIterator>::value_type
) because that allows more
flexibility for callers, and takes advantage of built-in comparisons
for the type that is pointed to by the iterator. The function is defined
to return true if, for one element in the sequence, the expression *iter == val
evaluates to true (where iter
is an iterator to each element in the sequence)