## Containers

- Summary
- Data Type and Member Function Indexes
- Description
- Container Requirements
- Reversible Containers
- Sequences
- Associative Containers
- See Also

### Summary

A standard template library (STL) collection.

### Data Type and Member Function Indexes
(exclusive of constructors and destructors)

None

### Description

Within the standard template library, collection classes are often described as containers. A container stores a collection of other objects and includes basic functions that support the use of generic algorithms. Containers come in two types: sequences and associative containers. They are further distinguished by the type of iterator they support.

A sequence supports a linear arrangement of single elements. vector, list, deque, bitset, and string fall into this category. Associative containers map values onto keys, which allows for retrieval of the values based on the keys. The STL includes the map, multimap, set, and multiset associative containers. map and multimap store the value and the key separately, and allow for fast retrieval of the value, based upon fast retrieval of the key. set and multiset store only keys allowing fast retrieval of the key itself.

### Container Requirements

Containers within the STL must meet the following requirements. Sequences and associative containers must also meet their own separate sets of requirements. The requirements for containers are:

- A container allocates all storage for the objects it holds.

- A container `X` of objects of type `T` includes the following types:

`X::value_type` | a `T`
`X::reference` | lvalue of `T`
`X::const_reference` | const lvalue of `T`
`X::iterator` | an iterator type pointing to `T`. `X::iterator` cannot be an output iterator
`X::const_iterator` | an iterator type pointing to const `T`. `X::iterator` cannot be an output iterator
`X::difference_type` | a signed integral type (must be the same as the distance type for `X::iterator` and `X::const_iterator`)
`X::size_type` | an unsigned integral type representing any non-negative value of `difference_type`
`X::allocator_type` | type of allocator used to obtain storage for elements stored in the container

- A container includes a default constructor, a copy constructor, an assignment operator, and a full complement of comparison operators (==, !=, <, >, <=, >=).

- A container includes the following member functions:

`begin()` | Returns an `iterator` or a `const_iterator` pointing to the first element in the collection
`end()` | Returns an iterator or a const_iterator pointing just beyond the last element in the collection
`swap(container)` | Swaps elements between this container and the swap's argument
`clear()` | Deletes all the elements in the container
`size()` | Returns the number of elements in the collection as a `size_type`
`max_size()` | Returns the largest possible number of elements for this type of container as a `size_type`
`empty()` | Returns `true` if the container is empty, `false` otherwise
`get_allocator()` | Returns the allocator used by this container

### Reversible Containers

A container may be reversible. Essentially, a reversible container includes a reverse iterator that allows traversal of the collection in a direction opposite that of the default iterator. A reversible container must meet the following requirements in addition to those listed above:

- A reversible container includes the following types:

`X::reverse_iterator` | An iterator type pointing to `T`
`X::const_reverse_iterator` | An iterator type pointing to `const T`

- A reversible container includes the following member functions:

`rbegin()` | Returns a `reverse_iterator` or a `const_reverse_iterator` pointing past the end of the collection
`rend()` | Returns a `reverse_iterator` or a `const_reverse_iterator` pointing to the first element in the collection

### Sequences

In addition to the requirements for containers, the following requirements hold for sequences:

- `iterator` and `const_iterator` must be forward iterators, bidirectional iterators or random access iterators.

- A sequence includes the following constructors:

`X(n, t)` | Constructs a container with `n` copies of `t`
`X(i, j)` | Constructs a container with elements from the range `[i,j)`

- A sequence includes the following member functions:

`insert(p,t)` | Inserts the element `t` in front of the position identified by the iterator `p`
`insert(p,n,t)` | Inserts `n` copies of `t` in front of the position identified by the iterator `p`
`insert(p,i,j)` | Inserts elements from the range `[i,j)` in front of the position identified by the iterator `p`
`erase(q)` | Erases the element pointed to by the iterator `q`
`erase(q1,q2)` | Erases the elements in the range `[q1,q2)`

- A sequence may also include the following member functions if they can be implemented with constant time complexity.

`front()` | Returns the element pointed to by `begin()`
`back()` | Returns the element pointed to by `end() - 1`
`push_front(x)` | Inserts the element `x` at `begin()`
`push_back(x)` | Inserts the element `x` at `end()`
`pop_front()` | Erases the element at `begin()`
`pop_back()` | Erases the element at `end() - 1`
`operator[](n)` | Returns the element at `a.begin() + n`
`at(n)` | Returns the element at `a.begin() + n`; throws out_of_range if `n` is invalid

### Associative Containers

In addition to the requirements for a container, the following requirements hold for associative containers:

- For an associative container `iterator` and `const_iterator` must be bidirectional iterators. Associative containers are inherently sorted. Their iterators proceed through the container in the non-descending order of keys (where non-descending order is defined by the comparison object that was used to construct the container).

- An associative container includes the following types:

`X::key_type` | the type of the `Key`
`X::key_compare` | the type of the comparison to use to put the keys in order
`X::value_compare` | the type of the comparison used on values

- The default constructor and copy constructor for associative containers use the template parameter comparison class.

- An associative container includes the following additional constructors:

`X(c)` | Constructs an empty container using `c` as the comparison object
`X(i,j,c)` | Constructs a container with elements from the range `[i,j)` and the comparison object `c`
`X(i, j)` | Constructs a container with elements from the range [i,j) using the template parameter comparison object

- An associative container includes the following member functions:

`key_comp()` | Returns the comparison object used in constructing the associative container
`value_comp()` | Returns the value comparison object used in constructing the associative container
`insert(t)` | If the container does NOT support redundant key values, then this function only inserts `t` if there is no key present that is equal to the key of `t`. If the container DOES support redundant keys, then this function always inserts the element `t`. Returns a `pair<iterator,bool>`. The `bool` component of the returned pair indicates the success or failure of the operation and the `iterator` component points to the element with key equal to key of `t`.
`insert(p,t)` | If the container does NOT support redundant key values, then this function only inserts `t` if there is no key present that is equal to the key of `t`. If the container DOES support redundant keys, then this function always inserts the element `t`. The iterator `p` serves as a hint of where to start searching, allowing for some optimization of the insertion. It does not restrict the algorithm from inserting ahead of that location if necessary.
`insert(i,j)` | Inserts elements from the range `[i,j).` A prerequisite is that `i` and `j` cannot be iterators into the container.
`erase(k)` | Erases all elements with key equal to `k`. Returns number of erased elements.
`erase(q)` | Erases the element pointed to by `q`
`erase(q1,q2)` | Erases the elements in the range `[q1,q2)`
`find(k)` | Returns an iterator pointing to an element with key equal to `k` or `end()`, if such an element is not found
`count(k)` | Returns the number of elements with key equal to `k`
`lower_bound(k)` | Returns an iterator pointing to the first element with a key greater than or equal to `k`
`upper_bound(k)` | Returns an iterator pointing to the first element with a key greater than `k`
`equal_range(k)` | Returns a pair of iterators such that the first element of the pair is equivalent to `lower_bound(k)` and the second element equivalent to `upper_bound(k)`

### See Also

bitset, deque, list, map, multimap, multiset, priority_queue, queue, set, stack, vector

Copyright (c) 1998, Rogue Wave Software, Inc.
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OEM Release, June 1998