std::flat_set

template<
    class Key,
    class Compare = std::less<Key>,
    class KeyContainer = std::vector<Key>
> class flat_set;

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.

Iterator invalidation

Template parameters

Member types

Member objects

Member functions

Non-member functions

Helper classes

Tags

Deduction guides

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 associative containers 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.

Example

See also