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In computer science, a container is a class, a data structure, or an abstract data type (ADT) whose instances are collections of other objects. They are used to store objects in an organized way following specific access rules.

Generally, container classes are expected to implement methods to do the following:

• create a new empty container (constructor),
• report the number of objects it stores (size),
• delete all the objects in the container (clear),
• insert new objects into the container,
• remove objects from it,
• provide access to the stored objects.

There are two types of containers: value containers and reference containers.

Value based containers store copies of the objects. Accessing an object also returns a copy of it. Modifying an external object after it has been inserted in the container will not affect the content of the container.

Reference based containers only store pointers or references to the objects. Accessing an object returns a reference to it. Modifying an external object after it has been inserted in the container could result in modifying the content of the container (or more precisely, the object stored in the container).

Containers are sometimes implemented in conjunction with iterators.

Examples of container classes include

• array
• sets
• lists
• stacks
• queues
• table
• maps
• dictionary
• tree
• graph

See also: Collection class

In object-oriented programming, a collection class is any class that is capable of storing other objects. Collection classes usually implement some kind of data structure, such as a list, map, set, array, or tree. A collection class is usually able to store an arbitrary number of data items, i.e. the size of the collection is adjusted automatically.

Widget toolkits use special widgets also called Containers to group the other widgets together (windows, panels, ...). Apart from their graphical properties, they have the same type of behavior as container classes, as they keep a list of their child widgets, and allow to add, remove, or retrieve widgets amongst their children.

Please help improve this article or section by expanding it. Further information might be found on the talk page or at requests for expansion. (May 2007)

The Java Collections Framework, as well as the standard C++ library's Standard Template Library provides APIs and implementations for many kinds of container classes.

• Standard Template Library#Containers
• C++ Open Containers Library