886
STL Containers
Chapter 31
Containers can be categorized like this:
� Sequence containers provide access to (half-open) sequences of elements.
� Associative containers provide associative lookup based on a key.
In addition, the standard library provides types of objects that hold elements while not offering all
of the facilities of sequence containers or associative containers:
� Container adaptors provide specialized access to underlying containers.
� Almost containers are sequences of elements that provide most, but not all, of the facilities
of a container.
The STL containers (the sequence and associative containers) are all resource handles with copy
and move operations (§3.3.1). All operations on containers provide the basic guarantee (§13.2) to
ensure that they interact properly with exception-based error handling.
Sequence Containers
vector<T,A>
list<T,A>
forward_list<T,A>
deque<T,A>
A contiguously allocated sequence of Ts;
the default choice of container
A doubly-linked list of T; use when you need to insert and delete
elements without moving existing elements
A singly-linked list of T; ideal for empty and very short sequences
A double-ended queue of T; a cross between a vector and a list;
slower than one or the other for most uses
The A template argument is the allocator that the container uses to acquire and release memory
(§13.6.1, §34.4). For example:
template<typename T, typename A = allocator<T>>
class vector {
// ...
};
is defaulted to std::allocator<T> (§34.4.1) which uses operator new() and operator delete() when it
needs to acquire or release memory for its elements.
These containers are defined in <vector>, <list>, and <deque>. The sequence containers are contiguously allocated (e.g., vector) or linked lists (e.g., forward_list) of elements of their value_type (T
in the notation used above). A deque (pronounced ‘‘deck’’) is a mixture of linked-list and contiguous allocation.
Unless you have a solid reason not to, use a vector. Note that vector provides operations for
inserting and erasing (removing) elements, allowing a vector to grow and shrink as needed. For
sequences of small elements, a vector can be an excellent representation for a data structure requiring list operations.
When inserting and erasing elements of a vector, elements may be moved. In contrast, elements
of a list or an associative container do not move when new elements are inserted or other elements
are erased.
A forward_list (a singly-linked list) is basically a list optimized for empty and very short lists.
An empty forward_list takes up only one word. There are surprisingly many uses for lists where
most are empty (and the rest are very short).
A
Section 31.2
Container Overview
C
887
Ordered Associative Containers (§iso.23.4.2)
is the type of the comparison; A is the allocator type
map<K,V,C,A>
multimap<K,V,C,A>
set<K,C,A>
multiset<K,C,A>
An ordered map from K to V; a sequence of (K,V) pairs
An ordered map from K to V; duplicate keys allowed
An ordered set of K
An ordered set of K; duplicate keys allowed
These containers are usually implemented as balanced binary trees (usually red-black trees).
The default ordering criterion for a key, K, is std::less<K> (§33.4).
As for sequence containers, the A template argument is the allocator that the container uses to
acquire and release memory (§13.6.1, §34.4). The A template argument is defaulted to std::allocator<std::pair<const K,T>> (§31.4.3) for maps and std::allocator<K> for sets.
H
Unordered Associative Containers (§iso.23.5.2)
is the hash function type; E is the equality test; A is the allocator type
unordered_map<K,V,H,E,A>
unordered_multimap<K,V,H,E,A>
unordered_set<K,H,E,A>
unordered_multiset<K,H,E,A>
An unordered map from K to V
An unordered map from K to V; duplicate keys allowed
An unordered set of K
An unordered set of K; duplicate keys allowed
These containers are implemented as hash tables with linked overflow. The default hash function
type, H, for a type K is std::hash<K> (§31.4.3.2). The default for the equality function type, E, for a
type K is std::equal_to<K> (§33.4); the equality function is used to decide whether two objects with
the same hash code are equal.
The associative containers are linked structures (trees) with nodes of their value_type (in the
notation used above, pair<const K,V> for maps and K for sets). The sequence of a set, map, or multimap is ordered by its key value (K). An unordered container need not have an ordering relation for
its elements (e.g., <) and uses a hash function instead (§31.2.2.1). The sequence of an unordered
container does not have a guaranteed order. A multimap differs from a map in that a key value may
occur many times.
Container adaptors are containers providing specialized interfaces to other containers:
Container Adaptors
is the container type
C
priority_queue<T,C,Cmp>
queue<T,C>
stack<T,C>
Priority queue of Ts; Cmp is the priority function type
Queue of Ts with push() and pop()
Stack of Ts with push() and pop()
The default for a priority_queue’s priority function, Cmp, is std::less<T>. The default for the container type, C, is std::deque<T> for queue and std::vector<T> for stack and priority_queue. See §31.5.
Some data types provide much of what is required of a standard container, but not all. We
sometimes refer to those as ‘‘almost containers.’’ The most interesting of those are: