std::flat_set
<flat_set>
class Key,
class Compare = std::less <Key>,
class KeyContainer = std::vector <Key>
The flat set is a container adaptor that gives the functionality of an associative container that stores a sorted set of unique objects of type Key. Sorting is done using the key comparison function Compare.
The class template flat_set acts as a wrapper to the underlying sorted container passed as object of type KeyContainer.
Everywhere the standard library uses the Compare requirements, uniqueness is determined by using the equivalence relation. Informally, two objects a and b are considered equivalent if neither compares less than the other: !comp(a, b) && !comp(b, a).
std::flat_set meets the requirements of Container, ReversibleContainer, optional container requirements, and all requirements of AssociativeContainer (including logarithmic search complexity), except that:
- requirements related to nodes are not applicable,
- iterator invalidation requirements differ,
- the complexity of insertion and erasure operations is linear.
A flat set supports most AssociativeContainer's operations that use unique keys.
std::flat_set are constexpr: it is possible to create and use std::flat_set objects in the evaluation of a constant expression.However, std::flat_set objects generally cannot be constexpr, because any dynamically allocated storage must be released in the same evaluation of constant expression.
Contents
[edit] Iterator invalidation
[edit] Template parameters
Key is not the same type as KeyContainer::value_type.
random_access_iterator.
The standard containers std::vector and std::deque satisfy these requirements.
[edit] Member types
iterator
implementation-defined LegacyRandomAccessIterator , ConstexprIterator (since C++26) and random_access_iterator to value_type[edit]
const_iterator
implementation-defined LegacyRandomAccessIterator , ConstexprIterator (since C++26) and random_access_iterator to const value_type[edit]
[edit] Member objects
container_type c (private)
the adapted container(exposition-only member object*)
key_compare compare (private)
the comparison function object(exposition-only member object*)
[edit] Member functions
(public member function)
Iterators
Capacity
Modifiers
Lookup
(public member function) [edit]
Observers
value_type (public member function) [edit]
[edit] Non-member functions
[edit] Helper classes
[edit] Tags
[edit] Deduction guides
[edit] 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.
Some advantages of flat set over other standard container adaptors are:
- Potentially faster lookup (even though search operations have logarithmic complexity).
- Much faster iteration: random access iterators instead of bidirectional iterators.
- Less memory consumption for small objects (and for big objects if KeyContainer::shrink_to_fit() is available).
- Better cache performance (depending on
KeyContainer, keys are stored in a contiguous block(s) of memory).
Some disadvantages of flat set are:
- Non-stable iterators (iterators are invalidated when inserting and erasing elements).
- Non-copyable and non-movable type values can not be stored.
- Weaker exception safety (copy/move constructors can throw when shifting values in erasures and insertions).
- Slower (i.e., linear) insertion and erasure, especially for non-movable types.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_flat_set |
202207L |
(C++23) | std::flat_set and std::flat_multiset
|
__cpp_lib_constexpr_flat_set |
202502L |
(C++26) | constexpr std::flat_set
|
[edit] Example
Reason: no example