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downloads/boost_1_33_1/libs/mpl/doc/src/refmanual/AssociativeSequence.rst
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Associative Sequence
Description
An |Associative Sequence| is a |Forward Sequence| that allows efficient retrieval of elements based on keys. Unlike associative containers in the C++ Standard Library, MPL associative sequences have no associated ordering relation. Instead, type identity is used to impose an equivalence relation on keys, and the order in which sequence elements are traversed during iteration is left unspecified.
Definitions
- A key is a part of the element type used to identify and retrieve the element within the sequence.
- A value is a part of the element type retrievied from the sequence by its key.
Expression requirements
|In the following table...| s is an |Associative Sequence|, x is a sequence element, and k and def are arbitrary types.
In addition to the requirements defined in |Forward Sequence|, the following must be met:
| Expression | Type | Complexity |
|---|---|---|
| has_key<s,k>::type | Boolean |Integral Constant| | Amortized constant time |
| count<s,k>::type | |Integral Constant| | Amortized constant time |
| order<s,k>::type | |Integral Constant| or void_ | Amortized constant time |
| at<s,k>::type | Any type | Amortized constant time |
| at<s,k,def>::type | Any type | Amortized constant time |
| key_type<s,x>::type | Any type | Amortized constant time |
| value_type<s,x>::type | Any type | Amortized constant time |
Expression semantics
|Semantics disclaimer...| |Forward Sequence|.
| Expression | Semantics |
|---|---|
| has_key<s,k>::type | |true if and only if| there is one or more elements with the key k in s; see |has_key|. |
| count<s,k>::type | The number of elements with the key k in s; see |count|. |
| order<s,k>::type | A unique unsigned |Integral Constant| associated with the key k in the sequence s; see |order|. |
at<s,k>::type at<s,k,def>::type |
The first element associated with the key k in the sequence s; see |at|. |
| key_type<s,x>::type | The key part of the element x that would be used to identify x in s; see |key_type|. |
| value_type<s,x>::type | The value part of the element x that would be used for x in s; see |value_type|. |
See also
|Sequences|, |Extensible Associative Sequence|, |has_key|, |count|, |order|, |at|, |key_type|, |value_type|
<reference anonymous="1" name="key">
key
.. |key| replace:: `key`__
<reference anonymous="1" name="value">
value
.. |value| replace:: `value`__
