Defined in header
template<classInputIt1,classInputIt2,classOutputIt>OutputItset_difference(InputIt1first1,InputIt1last1,InputIt2first2,InputIt2last2,OutputItd_first); (1)(constexpr since C++20)template<classExecutionPolicy,classForwardIt1,classForwardIt2,classForwardIt3>ForwardIt3set_difference(ExecutionPolicy&&policy,ForwardIt1first1,ForwardIt1last1,ForwardIt2first2,ForwardIt2last2,ForwardIt3d_first); (2) (since C++17)template<classInputIt1,classInputIt2,classOutputIt,classCompare>OutputItset_difference(InputIt1first1,InputIt1last1,InputIt2first2,InputIt2last2,OutputItd_first,Comparecomp); (3)(constexpr since C++20)template<classExecutionPolicy,classForwardIt1,classForwardIt2,classForwardIt3,classCompare>ForwardIt3set_difference(ExecutionPolicy&&policy,ForwardIt1first1,ForwardIt1last1,ForwardIt2first2,ForwardIt2last2,ForwardIt3d_first,Comparecomp); (4) (since C++17)Copies the elements from the sorted range [first1, last1) which are not found in the sorted range [first2, last2) to the range beginning at d_first. The output range is also sorted.
If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the final std::max(m-n,0) elements will be copied from [first1, last1) to the output range, preserving order.
1) If [first1, last1) or [first2, last2) is not
with respect to operator<(until C++20)std::less{}(since C++20), the behavior is undefined.
3) If [first1, last1) or [first2, last2) is not sorted with respect to comp, the behavior is undefined.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if the value of the following expression is true:
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(until C++20)std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(since C++20)If the output range overlaps with [first1, last1) or [first2, last2), the behavior is undefined.
Parameters
first1, last1 - the pair of iterators defining the first input sorted
of elements to examine first2, last2 - the pair of iterators defining the second input sorted
of elements to search for d_first - the beginning of the output range policy - the
to use comp - comparison function object (i.e. an object that satisfies the requirements of
) which returns true if the first argument is less than (i.e. is ordered before) the second. The signature of the comparison function should be equivalent to the following:
boolcmp(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 types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to both Type1 and Type2.
Type requirements -InputIt1, InputIt2 must meet the requirements of
. -OutputIt must meet the requirements of
. -ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of
. -Compare must meet the requirements of
. Return value
Iterator past the end of the constructed range.
Complexity
Given N1 as std::distance(first1,last1) and N2 as std::distance(first2,last2):
1,2) At most 2⋅(N1+N2)-1 comparisons using operator<(until C++20)std::less{}(since C++20).
3,4) At most 2⋅(N1+N2)-1 applications of the comparison function comp.
Exceptions
The overloads with a template parameter named ExecutionPolicy report errors as follows:
If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the
,
is called. For any other ExecutionPolicy, the behavior is implementation-defined.
If the algorithm fails to allocate memory,
is thrown.
Possible implementation
template<classInputIt1,classInputIt2,classOutputIt>OutputItset_difference(InputIt1first1,InputIt1last1,InputIt2first2,InputIt2last2,OutputItd_first){while(first1!=last1){if(first2==last2)returnstd::copy(first1,last1,d_first);if(*first1<*first2)*d_first++=*first1++;else{if(!(*first2<*first1))++first1;++first2;}}returnd_first;}
template<classInputIt1,classInputIt2,classOutputIt,classCompare>OutputItset_difference(InputIt1first1,InputIt1last1,InputIt2first2,InputIt2last2,OutputItd_first,Comparecomp){while(first1!=last1){if(first2==last2)returnstd::copy(first1,last1,d_first);if(comp(*first1,*first2))*d_first++=*first1++;else{if(!comp(*first2,*first1))++first1;++first2;}}returnd_first;}Example
Run this code
#include<algorithm>#include<iostream>#include<iterator>#include<vector>template<typenameT>std::ostream&operator<<(std::ostream&os,conststd::vector<T>&v){os<<'{';for(auton{v.size()};constauto&e:v)os<<e<<(--n?", ":"");returnos<<'}';}structOrder// a struct with very interesting data{intorder_id{};friendstd::ostream&operator<<(std::ostream&os,constOrder&ord){returnos<<ord.order_id;}};intmain(){conststd::vector<int>v1{1,2,5,5,5,9};conststd::vector<int>v2{2,5,7};std::vector<int>diff;std::set_difference(v1.begin(),v1.end(),v2.begin(),v2.end(),std::inserter(diff,diff.begin()));std::cout<<v1<<" ∖ "<<v2<<" == "<<diff<<"\n\n";// we want to know which orders "cut" between old and new states:std::vector<Order>old_orders{{1},{2},{5},{9}};std::vector<Order>new_orders{{2},{5},{7}};std::vector<Order>cut_orders;std::set_difference(old_orders.begin(),old_orders.end(),new_orders.begin(),new_orders.end(),std::back_inserter(cut_orders),[](auto&a,auto&b){returna.order_id<b.order_id;});std::cout<<"old orders: "<<old_orders<<'\n'<<"new orders: "<<new_orders<<'\n'<<"cut orders: "<<cut_orders<<'\n';}Output:
{1, 2, 5, 5, 5, 9} ∖ {2, 5, 7} == {1, 5, 5, 9} old orders: {1, 2, 5, 9} new orders: {2, 5, 7} cut orders: {1, 9} 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 it was unspecified how to handle equivalent elements in the input ranges specified See also