std::binary_search
Defined in header <algorithm>


template< class ForwardIt, class T > bool binary_search( ForwardIt first, ForwardIt last, const T& value ); 
(1)  
template< class ForwardIt, class T, class Compare > bool binary_search( ForwardIt first, ForwardIt last, const T& value, Compare comp ); 
(2)  
Checks if an element equivalent to value
appears within the range [first, last)
.
For std::binary_search
to succeed, the range [first, last)
must be at least partially ordered, i.e. it must satisfy all of the following requirements:
 partitioned with respect to element < value or comp(element, value)
 partitioned with respect to !(value < element) or !comp(value, element)
 for all elements, if element < value or comp(element, value) is true then !(value < element) or !comp(value, element) is also true
A fullysorted range meets these criteria, as does a range resulting from a call to std::partition.
The first version uses operator< to compare the elements, the second version uses the given comparison function comp
.
Contents 
Parameters
first, last    the range of elements to examine  
value    value to compare the elements to  
comp    comparison function object (i.e. an object that satisfies the requirements of Compare ) which returns true if the first argument is less than the second. The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The signature does not need to have const &, but the function object must not modify the objects passed to it.
 
Type requirements  
ForwardIt must meet the requirements of ForwardIterator .

Return value
true if an element equal to value
is found, false otherwise.
Complexity
The number of comparisons performed is logarithmic in the distance between first
and last
Possible implementation
First version 

template<class ForwardIt, class T> bool binary_search(ForwardIt first, ForwardIt last, const T& value) { first = std::lower_bound(first, last, value); return (!(first == last) && !(value < *first)); } 
Second version 
template<class ForwardIt, class T, class Compare> bool binary_search(ForwardIt first, ForwardIt last, const T& value, Compare comp) { first = std::lower_bound(first, last, value, comp); return (!(first == last) && !(comp(value, *first)); } 
Example
#include <iostream> #include <algorithm> #include <vector> int main() { std::vector<int> haystack {1, 3, 4, 5, 9}; std::vector<int> needles {1, 2, 3}; for (auto needle : needles) { std::cout << "Searching for " << needle << '\n'; if (std::binary_search(haystack.begin(), haystack.end(), needle)) { std::cout << "Found " << needle << '\n'; } else { std::cout << "no dice!\n"; } } }
Output:
Searching for 1 Found 1 Searching for 2 no dice! Searching for 3 Found 3
See also
returns range of elements matching a specific key (function template) 