From cppreference.com
Defined in header
template<classForwardIt,classT>boolbinary_search(ForwardItfirst,ForwardItlast,constT&value); (1)(constexpr since C++20)
(until C++26)template<classForwardIt,classT=typenamestd::iterator_traits<ForwardIt>::value_type>constexprboolbinary_search(ForwardItfirst,ForwardItlast,constT&value);(since C++26)template<classForwardIt,classT,classCompare>boolbinary_search(ForwardItfirst,ForwardItlast,constT&value,Comparecomp); (2)(constexpr since C++20)
(until C++26)template<classForwardIt,classT=typenamestd::iterator_traits<ForwardIt>::value_type,classCompare>constexprboolbinary_search(ForwardItfirst,ForwardItlast,constT&value,Comparecomp);(since C++26)Checks if an element equivalent to value exists in the partitioned source range [first, last). An element is considered equivalent to value if it neither orders before nor orders after value.
1) The order is determined by operator<(until C++20)std::less{}(since C++20).
Given two expressions bool(e<value) and !bool(value<e)(until C++20)bool(std::less{}(e,value)) and !bool(std::less{}(value,e))(since C++20). If any of the following conditions is satisfied, the behavior is undefined:
The elements e of the source range are not
with respect to these two expressions at the same time.
There exists an element e in the source range such that these two expressions yield different values.
2) The order is determined by comp.
Given two expressions bool(comp(e,value)) and !bool(comp(value,e)). If any of the following conditions is satisfied, the behavior is undefined:
The elements e of the source range are not partitioned with respect to these two expressions at the same time.
There exists an element e in the source range such that these two expressions yield different values.
Parameters
first, last - the pair of iterators defining the source
value - value to compare the elements to comp - binary predicate which returns true if the first argument is ordered before the second. The signature of the predicate function should be equivalent to the following:
boolpred(constType1&a,constType2&b);
While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of
(thus, Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)).
The type Type1 must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to Type1. The type Type2 must be such that an object of type T can be implicitly converted to Type2.
Type requirements -ForwardIt must meet the requirements of
. -Compare must meet the requirements of
. It is not required to satisfy
. Return value
true if an element equivalent to value exists, false otherwise.
Complexity
Given N as std::distance(first,last):
1) At most log2(N)+𝓞(1) comparisons with value using operator<(until C++20)std::less{}(since C++20).
2) At most log2(N)+𝓞(1) applications of the comparator comp.
Notes
Although std::binary_search only requires the source range to be partitioned, this algorithm is usually used in the case where the source range is sorted, so that the binary search is valid for any value.
std::binary_search only checks whether an equivalent element exists. To obtain an iterator to that element (if exists),
should be used instead.
If ForwardIt is not a
, the number of iterator increments is linear in std::distance(first,last).
macroValueStdFeature
__cpp_lib_algorithm_default_value_type
(C++26)
for algorithms (
)Possible implementation
See also the implementations in
and
.
template<classForwardIt,classT=typenamestd::iterator_traits<ForwardIt>::value_type>boolbinary_search(ForwardItfirst,ForwardItlast,constT&value){returnstd::binary_search(first,last,value,std::less{});}
template<classForwardIt,classT=typenamestd::iterator_traits<ForwardIt>::value_type,classCompare>boolbinary_search(ForwardItfirst,ForwardItlast,constT&value,Comparecomp){first=std::lower_bound(first,last,value,comp);return(!(first==last)and!(comp(value,*first)));}Example
Run this code
#include<algorithm>#include<cassert>#include<complex>#include<iostream>#include<vector>intmain(){constautohaystack={1,3,4,5,9};for(constautoneedle:{1,2,3}){std::cout<<"Searching for "<<needle<<'\n';if(std::binary_search(haystack.begin(),haystack.end(),needle))std::cout<<"Found "<<needle<<'\n';elsestd::cout<<"Not found!\n";}usingCD=std::complex<double>;std::vector<CD>nums{{1,1},{2,3},{4,2},{4,3}};autocmpz=[](CDx,CDy){returnabs(x)<abs(y);};#ifdef __cpp_lib_algorithm_default_value_typeassert(std::binary_search(nums.cbegin(),nums.cend(),{4,2},cmpz));#elseassert(std::binary_search(nums.cbegin(),nums.cend(),CD{4,2},cmpz));#endif}Output:
Searching for 1 Found 1 Searching for 2 Not found! Searching for 3 Found 3 Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++98 Compare was required to satisfy
and T was required
to be
(strict weak ordering required) only a partitioning is required;
heterogeneous comparisons permitted
C++98 at most log2(N)+2 comparisons were allowed corrected to 2log2(N)+𝓞(1)See also
(C++20)
determines if an element exists in a range using binary search
(algorithm function object)
finds the range of elements matching the given value using binary search
(function template & algorithm function object)
(C++20)
finds the first element not less than the given value using binary search
(function template & algorithm function object)
(C++20)
finds the first element greater than the given value using binary search
(function template & algorithm function object)
(C++20)