Defined in header
Call signature
template<std::input_iteratorI1,std::sentinel_for<I1>S1,std::input_iteratorI2,std::sentinel_for<I2>S2,classPred=ranges::equal_to,classProj1=std::identity,classProj2=std::identity>requiresstd::indirectly_comparable<I1,I2,Pred,Proj1,Proj2>constexprboolequal(I1first1,S1last1,I2first2,S2last2,Predpred={},Proj1proj1={},Proj2proj2={}); (1) (since C++20)template<ranges::input_rangeR1,ranges::input_rangeR2,classPred=ranges::equal_to,classProj1=std::identity,classProj2=std::identity>requiresstd::indirectly_comparable<ranges::iterator_t<R1>,ranges::iterator_t<R2>,Pred,Proj1,Proj2>constexprboolequal(R1&&r1,R2&&r2,Predpred={},Proj1proj1={},Proj2proj2={}); (2) (since C++20)template</*execution-policy*/Ep,std::random_access_iteratorI1,std::sized_sentinel_for<I1>S1,std::random_access_iteratorI2,std::sized_sentinel_for<I2>S2,classPred=ranges::equal_to,classProj1=std::identity,classProj2=std::identity>requiresstd::indirectly_comparable<I1,I2,Pred,Proj1,Proj2>boolequal(Ep&&policy,I1first1,S1last1,I2first2,S2last2,Predpred={},Proj1proj1={},Proj2proj2={}); (3) (since C++26)template</*execution-policy*/Ep,/*sized-random-access-range*/R1,/*sized-random-access-range*/R2,classPred=ranges::equal_to,classProj1=std::identity,classProj2=std::identity>requiresstd::indirectly_comparable<ranges::iterator_t<R1>,ranges::iterator_t<R2>,Pred,Proj1,Proj2>boolequal(Ep&&policy,R1&&r1,R2&&r2,Predpred={},Proj1proj1={},Proj2proj2={}); (4) (since C++26)For the definition of /*execution-policy*/, see
; for the definition of /*sized-random-access-range*/, see
.
Checks whether two target ranges are equal. The elements (projected by proj1 and proj2 respectively) are compared using the binary predicate pred.
1) The target ranges are [first1, last1) and [first2, last2).
2) The target ranges are r1 and r2.
3,4) Same as (1,2), but executed according to policy.
The function-like entities described on this page are
(informally known as niebloids), that is:
Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to
.
When any of them are found by
as the name to the left of the function-call operator,
is inhibited.
Parameters
first1, last1 - the iterator-sentinel pair defining the first target
r1 - the first target range first2, last2 - the iterator-sentinel pair defining the second target
r2 - the second target range pred - the predicate to be applied to the (projected) elements proj1 - the projection to be applied to the elements in the first target range proj2 - the projection to be applied to the elements in the second target range Return value
If the two target ranges have the same size, and each corresponding (projected) elements in the two ranges are equal, returns true. Otherwise returns false.
Complexity
Given
N1 as ranges::distance(first1,last1) or ranges::distance(r1), and
N2 as ranges::distance(first2,last2) or ranges::distance(r2):
1,2) At most min(N1,N2) applications of pred, proj1 and proj2.
3,4)𝓞(min(N1,N2)) applications of pred, proj1 and proj2.
If I1, S1, I2 and S2 pairwise model
(or both R1 and R2 model
), and N1≠N2, then no comparison will be made.
Exceptions
3,4) During the execution process:
If the temporary memory resources required for parallelization are not available,
is thrown.
If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for
,
is invoked).
Notes
ranges::equal should not be used to compare the ranges formed by the iterators from
,
,
, or
because the order in which the elements are stored in those containers may be different even if the two containers store the same elements.
When comparing entire containers or string views for equality, operator== for the corresponding type are usually preferred.
ranges::equal is not guaranteed to be short-circuit. E.g. if the first pair elements of both ranges do not compare equal, the rest of elements may also be compared. Non-short-circuit comparison may happen when the ranges are compared with
or implementation-specific vectorized algorithms.
Possible implementation
structequal_fn{template<std::input_iteratorI1,std::sentinel_for<I1>S1,std::input_iteratorI2,std::sentinel_for<I2>S2,classPred=ranges::equal_to,classProj1=std::identity,classProj2=std::identity>requiresstd::indirectly_comparable<I1,I2,Pred,Proj1,Proj2>constexprbooloperator()(I1first1,S1last1,I2first2,S2last2,Predpred={},Proj1proj1={},Proj2proj2={})const{ifconstexpr(std::sized_sentinel_for<S1,I1>&&std::sized_sentinel_for<S2,I2>)if(ranges::distance(first1,last1)!=ranges::distance(first2,last2))returnfalse;for(;first1!=last1;++first1,(void)++first2)if(!std::invoke(pred,std::invoke(proj1,*first1),std::invoke(proj2,*first2)))returnfalse;returntrue;}template<ranges::input_rangeR>constexprautoget_end(R&&r){returnranges::end(r);}template<ranges::forward_rangeR>constexprautoget_end(R&&r){returnranges::next(ranges::begin(r),ranges::end(r));}template<ranges::input_rangeR1,ranges::input_rangeR2,classPred=ranges::equal_to,classProj1=std::identity,classProj2=std::identity>requiresstd::indirectly_comparable<ranges::iterator_t<R1>,ranges::iterator_t<R2>,Pred,Proj1,Proj2>constexprbooloperator()(R1&&r1,R2&&r2,Predpred={},Proj1proj1={},Proj2proj2={})const{return(*this)(ranges::begin(r1),get_end(r1),ranges::begin(r2),get_end(r2),std::ref(pred),std::ref(proj1),std::ref(proj2));}};inlineconstexprequal_fnequal;Example
The following code uses
to test if a string is a palindrome.
Run this code
#include<algorithm>#include<iomanip>#include<iostream>#include<ranges>#include<string_view>constexprboolis_palindrome(conststd::string_views){namespaceviews=std::views;autoforward=s|views::take(s.size()/2);autobackward=s|views::reverse|views::take(s.size()/2);returnstd::ranges::equal(forward,backward);}voidtest(conststd::string_views){std::cout<<std::quoted(s)<<" is "<<(is_palindrome(s)?"":"not ")<<"a palindrome\n";}intmain(){test("radar");test("hello");static_assert(is_palindrome("ABBA")andnotis_palindrome("AC/DC"));}Output:
"radar" is a palindrome "hello" is not a palindrome See also
determines if two sets of elements are the same
(function template)
ranges::findranges::find_ifranges::find_if_not
(C++20)(C++20)(C++20)
finds the first element satisfying specific criteria
(algorithm function object)
ranges::lexicographical_compare
(C++20)
compares two ranges lexicographically
(algorithm function object)
(C++20)
finds the first position where two ranges differ
(algorithm function object)
(C++20)
searches for the first occurrence of a range of elements
(algorithm function object)
(C++20)
finds the range of elements matching the given value using binary search
(algorithm function object)
function object implementing x==y
(class template)