std::set

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C++
 
Containers library
 
std::set
Member functions
Non-member functions
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides(C++17)
 
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.

Template parameters

This section is incomplete
Reason: Add descriptions of the template parameters.

Member types

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 value_type&
const_reference const value_type&
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 to value_type
const_iterator LegacyBidirectionalIterator to const value_type
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

 constructs the set
(public member function)
destructs the set
(public member function)
assigns values to the container
(public member function)
returns the associated allocator
(public member function)
Iterators
(C++11)
 returns an iterator to the beginning
(public member function)
(C++11)
 returns an iterator to the end
(public member function)
(C++11)
 returns a reverse iterator to the beginning
(public member function)
(C++11)
 returns a reverse iterator to the end
(public member function)
Capacity
checks whether the container is empty
(public member function)
returns the number of elements
(public member function)
returns the maximum possible number of elements
(public member function)
Modifiers
clears the contents
(public member function)
inserts elements or nodes(since C++17)
(public member function)
(C++23)
 inserts a range of elements
(public member function)
(C++11)
 constructs element in-place
(public member function)
(C++11)
 constructs elements in-place using a hint
(public member function)
erases elements
(public member function)
swaps the contents
(public member function)
(C++17)
 extracts nodes from the container
(public member function)
(C++17)
 splices nodes from another container
(public member function)
Lookup
returns the number of elements matching specific key
(public member function)
finds element with specific key
(public member function)
(C++20)
 checks if the container contains element with specific key
(public member function)
returns range of elements matching a specific key
(public member function)
returns an iterator to the first element not less than the given key
(public member function)
returns an iterator to the first element greater than the given key
(public member function)
Observers
returns the function that compares keys
(public member function)
returns the function that compares keys in objects of type value_type
(public member function)

Non-member functions

(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
 lexicographically compares the values of two sets
(function template)
specializes the std::swap algorithm
(function template)
(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. This means defining a pair of function overloads using the two types as parameter types may violate the One Definition Rule. Since iterator is convertible to const_iterator, a single function with a const_iterator as parameter type will work instead.

Feature-test macro Value Std Feature
__cpp_lib_containers_ranges 202202L (C++23) Ranges construction and insertion for containers

Example

Run this code
#include <algorithm>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <set>
#include <string_view>
 
template<typename T>
std::ostream& operator<<(std::ostream& out, const std::set<T>& set)
{
    if (set.empty())
        return out << "{}";
    out << "{ " << *set.begin();
    std::for_each(std::next(set.begin()), set.end(), [&out](const T& element)
    {
        out << ", " << element;
    });
    return out << " }";
}
 
int main()
{
    std::set<int> set{1, 5, 3};
    std::cout << set << '\n';
 
    set.insert(2);
    std::cout << set << '\n';
 
    set.erase(1);
    std::cout << set << "\n\n";
 
    std::set<int> keys{3, 4};
    for (int key : keys)
    {
        if (set.contains(key))
            std::cout << set << " does contain " << key << '\n';
        else
            std::cout << set << " doesn't contain " << key << '\n';
    }
    std::cout << '\n';
 
    std::string_view word = "element";
    std::set<char> characters(word.begin(), word.end());
    std::cout << "There are " << characters.size() << " unique characters in "
              << std::quoted(word) << ":\n" << characters << '\n';
}

Output: 

{ 1, 3, 5 }
{ 1, 2, 3, 5 }
{ 2, 3, 5 }
 
{ 2, 3, 5 } does contain 3
{ 2, 3, 5 } doesn't contain 4
 
There are 5 unique characters in "element":
{ e, l, m, n, t }

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
LWG 230 C++98 Key was not required to be CopyConstructible
(a key of type Key might not be able to be constructed) 		Key is also required to
be CopyConstructible

See also

 collection of keys, sorted by keys
(class template)
(C++11)
 collection of unique keys, hashed by keys
(class template)
(C++23)
 adapts a container to provide a collection of unique keys, sorted by keys
(class template)
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