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# C++ concepts: Compare

From cppreference.com
< cpp‎ | concept

The concept `Compare` is a set of requirements expected by some of the standard library facilities from the user-provided function object types.

The return value of the function call operation applied to an object of type `Compare`, when contextually converted to bool, yields true if the first argument of the call appears before the second in the strict weak ordering relation induced by this `Compare` type, and false otherwise.

As with any `BinaryPredicate`, evaluation of that expression is not allowed to call non-const member functions of the dereferenced iterators.

### [edit] Requirements

The type `T` satisfies `Compare` if

- The type `T` satisfies `BinaryPredicate`, and

Given

- `comp`, an object of type `Compare`
- `equiv(a, b)`, an expression equivalent to `!comp(a, b) && !comp(b, a)`

The following expressions must be valid and have their specified effects

Expression | Return type | Requirements
--- | --- | ---
comp(a, b) | implicitly convertible to bool | Establishes strict weak ordering relation with the following properties For all `a`, comp(a,a)==false If comp(a,b)==true then comp(b,a)==false if comp(a,b)==true and comp(b,c)==true then comp(a,c)==true
equiv(a, b) | bool | Establishes equivalence relationship with the following properties For all `a`, equiv(a,a)==true. If equiv(a,b)==true, then equiv(b,a)==true If equiv(a,b)==true and equiv(b,c)==true, then equiv(a,c)==true

Note: `comp` induces a strict total ordering on the equivalence classes determined by `equiv`

### [edit] Standard library

The following standard library facilities expect a `Compare` type.

set | collection of unique keys, sorted by keys (class template) [edit]
map | collection of key-value pairs, sorted by keys, keys are unique (class template) [edit]
multiset | collection of keys, sorted by keys (class template) [edit]
multimap | collection of key-value pairs, sorted by keys (class template) [edit]
priority_queue | adapts a container to provide priority queue (class template) [edit]
sort | sorts a range into ascending order (function template) [edit]
sort | sorts the elements (public member function of `std::forward_list`) [edit]
sort | sorts the elements (public member function of `std::list`) [edit]
stable_sort | sorts a range of elements while preserving order between equal elements (function template) [edit]
partial_sort | sorts the first N elements of a range (function template) [edit]
partial_sort_copy | copies and partially sorts a range of elements (function template) [edit]
is_sorted (C++11) | checks whether a range is sorted into ascending order (function template) [edit]
is_sorted_until (C++11) | finds the largest sorted subrange (function template) [edit]
nth_element | partially sorts the given range making sure that it is partitioned by the given element (function template) [edit]
lower_bound | returns an iterator to the first element not less than the given value (function template) [edit]
upper_bound | returns an iterator to the first element greater than a certain value (function template) [edit]
binary_search | determines if an element exists in a certain range (function template) [edit]
equal_range | returns range of elements matching a specific key (function template) [edit]
merge | merges two sorted ranges (function template) [edit]
merge | merges two sorted lists (public member function of `std::forward_list`) [edit]
merge | merges two sorted lists (public member function of `std::list`) [edit]
inplace_merge | merges two ordered ranges in-place (function template) [edit]
includes | returns true if one set is a subset of another (function template) [edit]
set_difference | computes the difference between two sets (function template) [edit]
set_intersection | computes the intersection of two sets (function template) [edit]
set_symmetric_difference | computes the symmetric difference between two sets (function template) [edit]
set_union | computes the union of two sets (function template) [edit]
push_heap | adds an element to a max heap (function template) [edit]
pop_heap | removes the largest element from a max heap (function template) [edit]
make_heap | creates a max heap out of a range of elements (function template) [edit]
sort_heap | turns a max heap into a range of elements sorted in ascending order (function template) [edit]
is_heap (C++11) | checks if the given range is a max heap (function template) [edit]
is_heap_until (C++11) | finds the largest subrange that is a max heap (function template) [edit]
max | returns the larger of two elements (function template) [edit]
max_element | returns the largest element in a range (function template) [edit]
min | returns the smaller of two elements (function template) [edit]
min_element | returns the smallest element in a range (function template) [edit]
minmax (C++11) | returns the larger and the smaller of two elements (function template) [edit]
minmax_element (C++11) | returns the smallest and the largest element in a range (function template) [edit]
lexicographical_compare | returns true if one range is lexicographically less than another (function template) [edit]
next_permutation | generates the next greater lexicographic permutation of a range of elements (function template) [edit]
prev_permutation | generates the next smaller lexicographic permutation of a range of elements (function template) [edit]

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