# std::set

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< cpp‎ | container

Defined in header `<set>` |  |
template< class Key, class Compare = std::less <Key>, class Allocator = std::allocator <Key> > class set; | (1) |
namespace pmr { template <class Key, class Compare = std::less <Key>> using set = std::set<Key, Compare, std::pmr::polymorphic_allocator <Key>>; } | (2) | (since C++17)

`std::set` is an associative container that contains a sorted set of unique objects of type `Key`. Sorting is done using the key comparison function Compare. Search, removal, and insertion operations have logarithmic complexity. Sets are usually implemented as red-black trees.

Everywhere the standard library uses the Compare requirements, uniqueness is determined by using the equivalence relation. In imprecise terms, two objects `a` and `b` are considered equivalent if neither compares less than the other: `!comp(a, b) && !comp(b, a)`.

`std::set` meets the requirements of Container, AllocatorAwareContainer, AssociativeContainer and ReversibleContainer.

### Member types

Member type | Definition
`key_type` | `Key`
`value_type` | `Key`
`size_type` | Unsigned integer type (usually std::size_t)
`difference_type` | Signed integer type (usually std::ptrdiff_t)
`key_compare` | `Compare`
`value_compare` | `Compare`
`allocator_type` | `Allocator`

`reference`

`Allocator::reference` | (until C++11)
`value_type&` | (since C++11)

`const_reference`

`Allocator::const_reference` | (until C++11)
`const value_type&` | (since C++11)

`pointer`

`Allocator::pointer` | (until C++11)
std::allocator_traits <Allocator>::pointer | (since C++11)

`const_pointer`

`Allocator::const_pointer` | (until C++11)
std::allocator_traits <Allocator>::const_pointer | (since C++11)

`iterator` | Constant LegacyBidirectionalIterator
`const_iterator` | Constant LegacyBidirectionalIterator
`reverse_iterator` | std::reverse_iterator <iterator>
`const_reverse_iterator` | std::reverse_iterator <const_iterator>
`node_type`(since C++17) | a specialization of node handle representing a container node

`insert_return_type`(since C++17)

type describing the result of inserting a `node_type`, a specialization of

    template <class Iter, class NodeType> struct /*unspecified*/ {
        Iter     position;
        bool     inserted;
        NodeType node;
    };

instantiated with template arguments `iterator` and `node_type`.

### Member functions

(constructor) | constructs the `set` (public member function)
(destructor) | destructs the `set` (public member function)
operator= | assigns values to the container (public member function)
get_allocator | returns the associated allocator (public member function)

##### Iterators

begin cbegin | returns an iterator to the beginning (public member function)
end cend | returns an iterator to the end (public member function)
rbegin crbegin | returns a reverse iterator to the beginning (public member function)
rend crend | returns a reverse iterator to the end (public member function)

##### Capacity

empty | checks whether the container is empty (public member function)
size | returns the number of elements (public member function)
max_size | returns the maximum possible number of elements (public member function)

##### Modifiers

clear | clears the contents (public member function)
insert | inserts elements or nodes (since C++17) (public member function)
emplace (C++11) | constructs element in-place (public member function)
emplace_hint (C++11) | constructs elements in-place using a hint (public member function)
erase | erases elements (public member function)
swap | swaps the contents (public member function)
extract (C++17) | extracts nodes from the container (public member function)
merge (C++17) | splices nodes from another container (public member function)

##### Lookup

count | returns the number of elements matching specific key (public member function)
find | finds element with specific key (public member function)
contains (C++20) | checks if the container contains element with specific key (public member function)
equal_range | returns range of elements matching a specific key (public member function)
lower_bound | returns an iterator to the first element not less than the given key (public member function)
upper_bound | returns an iterator to the first element greater than the given key (public member function)

##### Observers

key_comp | returns the function that compares keys (public member function)
value_comp | returns the function that compares keys in objects of type value_type (public member function)

### Non-member functions

operator==operator!=operator<operator<=operator>operator>= | lexicographically compares the values in the set (function template)
std::swap(std::set) | specializes the std::swap algorithm (function template)
erase_if(std::set) (C++20) | Erases all elements satisfying specific criteria (function template)

### Deduction guides (since C++17)

### Notes

The member types `iterator` and `const_iterator` may be aliases to the same type. Since `iterator` is convertible to `const_iterator`, `const_iterator` should be used in function parameter lists to avoid violations of the One Definition Rule.

### Defect Reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 103 | C++98 | iterator allows modification of keys | iterator made constant

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