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STL Containers

The ISO Standard Template Library (STL) provides containers for storing collections of related objects. The containers are template classes that enable specification of objects that are allowed in the containers.

Note Note

This article does not apply to the STL/CLR. For more information, see STL/CLR Library Reference.

Containers in the STL can be divided into three categories: sequence containers, associative containers, and container adapters.

Sequence containers maintain the ordering of inserted elements that you specify.

A vector container behaves like an array, but can automatically grow as required. It is random access and contiguously stored, and length is highly flexible. For these reasons and more, vector is the preferred sequence container for most applications. For more information, see vector Class.

An array container has some of the strengths of vector, but the length is not as flexible. For more information, see array Class (STL).

A deque (double-ended queue) container allows for fast insertions and deletions at the beginning and end of the container. It shares the random-access and flexible-length advantages of vector, but is not contiguous. For more information, see deque Class.

A list container is a doubly linked list that enables bidirectional access, fast insertions, and fast deletions anywhere in the container, but you cannot randomly access an element in the container. For more information, see list Class.

A forward_list container is a singly linked list—the forward-access version of list. For more information, see forward_list Class.

In associative containers, elements are inserted in a pre-defined order—for example, as sorted ascending. Unordered associative containers are also available. The associative containers can be grouped into two subsets: maps and sets.

A map, sometimes referred to as a dictionary, consists of a key/value pair. The key is used to order the sequence, and the value is associated with that key. For example, a map might contain keys that represent every unique word in a text and corresponding values that represent the number of times that each word appears in the text. The unordered version of map is unordered_map. For more information, see map Class and unordered_map Class.

A set is just an ascending container of unique elements—the value is also the key. The unordered version of set is unordered_set. For more information, see set Class and unordered_set Class.

Both map and set only allow one instance of a key or element to be inserted into the container. If multiple instances of elements are required, use multimap or multiset. The unordered versions are unordered_multimap and unordered_multiset. For more information, see multimap Class, unordered_multimap Class, multiset Class, and unordered_multiset Class.

Ordered maps and sets support bi-directional iterators, and their unordered counterparts support forward iterators. For more information, see Iterators.

A container adapter is a variation of a sequence or associative container that restricts the interface for simplicity and clarity. Container adapters do not support iterators.

A queue container follows FIFO (first in, first out) semantics. The first element pushed—that is, inserted into the queue—is the first to be popped—that is, removed from the queue. For more information, see queue Class.

A priority_queue container is organized such that the element that has the highest value is always first in the queue. For more information, see priority_queue Class.

A stack container follows LIFO (last in, first out) semantics. The last element pushed on the stack is the first element popped. For more information, see stack Class.

Because container adapters do not support iterators, they cannot be used with the STL algorithms. For more information, see Algorithms.

In general, elements inserted into an STL container can be of just about any object type if they are copyable. Movable-only elements—for example, those such as vector<unique_ptr<T>> that are created by using unique_ptr<> will work as long as you don't call member functions that attempt to copy them.

The destructor is not permitted to throw an exception.

Ordered associative containers—described earlier in this article—must have a public comparison operator defined. (By default, the operator is operator<, but even types that don't work with operator< are supported.

Some operations on containers might also require a public default constructor and a public equivalence operator. For example, the unordered associative containers require support for equality and hashing.

The elements of containers are accessed by using iterators. For more information, see Iterators.

Note Note

You can also use range-based for loops to iterate over STL collections.

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