Wednesday, November 9, 2011

stl containers

containers in c++ / stl containers examples

Container classes, or containers for short, manage a collection of elements. To meet different needs, the STL provides different kinds of containers, as shown in Figure below

There are two general kinds of containers:
  1. Sequence containers are ordered collections in which every element has a certain position. This position depends on the time and place of the insertion, but it is independent of the value of the element. For example, if you put six elements into a collection by appending each element at the end of the actual collection, these elements are in the exact order in which you put them. The STL contains three predefined sequence container classes: vector, deque, and list. 
  2. Associative containers are sorted collections in which the actual position of an element depends on its value due to a certain sorting criterion. If you put six elements into a collection, their order depends only on their value. The order of insertion doesn't matter. The STL contains four predefined associative container classes: set, multiset, map, and multimap.
An associative container can be considered a special kind of sequence container because sorted collections are ordered according to a sorting criterion. You might expect this especially if you have used other libraries of collection classes like those in Smalltalk or the NIHCL, in which sorted collections are derived from ordered collections. However, note that the STL collection types are completely distinct from each other. They have different implementations that are not derived from each other.

The automatic sorting of elements in associative containers does not mean that those containers are especially designed for sorting elements. You can also sort the elements of a sequence
container. The key advantage of automatic sorting is better performance when you search elements. In particular, you can always use a binary search, which results in logarithmic complexity rather than linear complexity. For example, this means that for a search in a collection of 1,000 elements you need, on average, only 10 instead of 500 comparisons. Thus, automatic sorting is only a (useful) "side effect" of the implementation of an associative container, designed to enable better performance.

The following subsections discuss the different container classes in detail. Among other aspects, they describe how containers are typically implemented. Strictly speaking, the particular implementation of any container is not defined inside the C++ standard library. However, the behavior and complexity specified by the standard do not leave much room for variation. So, in practice, the implementations differ only in minor details. Later in posts I will covers the exact behavior of the container classes. It describes their common and individual abilities, and member functions in detail.

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