unordered/doc/comparison.qbk

[/ Copyright 2006-2011 Daniel James.
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) ]

[section:comparison Comparison with Associative Containers]

[table:interface_differences Interface differences.
    [[Associative Containers] [Unordered Associative Containers]]

    [
        [Parameterized by an ordering relation `Compare`]
        [Parameterized by a function object `Hash` and an equivalence relation
            `Pred`]
    ]
    [
        [Keys can be compared using `key_compare` which is accessed by member function `key_comp()`,
         values can be compared using `value_compare` which is accessed by member function `value_comp()`.]
        [Keys can be hashed using `hasher` which is accessed by member function `hash_function()`,
         and checked for equality using `key_equal` which is accessed by member function `key_eq()`.
         There is no function object for compared or hashing values.]
    ]
    [
        [Constructors have optional extra parameters for the comparison object.]
        [Constructors have optional extra parameters for the initial minimum
            number of buckets, a hash function and an equality object.]
    ]

    [
        [Keys `k1`, `k2` are considered equivalent if
            `!Compare(k1, k2) && !Compare(k2, k1)`]
        [Keys `k1`, `k2` are considered equivalent if `Pred(k1, k2)`]
    ]
    [
        [Member function `lower_bound(k)` and `upper_bound(k)`]
        [No equivalent. Since the elements aren't ordered `lower_bound` and
        `upper_bound` would be meaningless.]
    ]
    [
        [`equal_range(k)` returns an empty range at the position that k
            would be inserted if k isn't present in the container.]
        [`equal_range(k)` returns a range at the end of the container if
            k isn't present in the container. It can't return a positioned
            range as k could be inserted into multiple place. To find out the
            bucket that k would be inserted into use `bucket(k)`. But remember
            that an insert can cause the container to rehash - meaning that the
            element can be inserted into a different bucket.]
    ]
    [
        [`iterator`, `const_iterator` are of the bidirectional category.]
        [`iterator`, `const_iterator` are of at least the forward category.]
    ]
    [
        [Iterators, pointers and references to the container's elements are
            never invalidated.]
        [[link unordered.buckets.iterator_invalidation Iterators can
            be invalidated by calls to insert or rehash]. Pointers and
            references to the container's elements are never invalidated.]
    ]
    [
        [Iterators iterate through the container in the order defined by
            the comparison object.]
        [Iterators iterate through the container in an arbitrary order, that
            can change as elements are inserted, although equivalent elements
            are always adjacent.]
    ]
    [
        [No equivalent]
        [Local iterators can be used to iterate through individual buckets.
            (The order of local iterators and iterators aren't
            required to have any correspondence.)]
    ]
    [
        [Can be compared using the `==`, `!=`, `<`, `<=`, `>`, `>=` operators.]
        [Can be compared using the `==` and `!=` operators.]
    ]
    [
        []
        [When inserting with a hint, implementations are permitted to ignore
            the hint.]
    ]
    [
        [`erase` never throws an exception]
        [The containers' hash or predicate function can throw exceptions
            from `erase`]
    ]
]

[table:complexity_guarantees Complexity Guarantees
    [[Operation] [Associative Containers] [Unordered Associative Containers]]
    [
        [Construction of empty container]
        [constant]
        [O(/n/) where /n/ is the minimum number of buckets.]
    ]
    [
        [Construction of container from a range of /N/ elements]
        [O(/N/ log /N/), O(/N/) if the range is sorted with `value_comp()`]
        [Average case O(/N/), worst case
            O(/N/'''<superscript>2</superscript>''')]
    ]
    [
        [Insert a single element]
        [logarithmic]
        [Average case constant, worst case linear]
    ]
    [
        [Insert a single element with a hint]
        [Amortized constant if t elements inserted right after hint,
            logarithmic otherwise]
        [Average case constant, worst case linear (ie. the same as
            a normal insert).]
    ]
    [
        [Inserting a range of /N/ elements]
        [ /N/ log(`size()`+/N/) ]
        [Average case O(/N/), worst case O(/N/ * `size()`)]
    ]
    [
        [Erase by key, `k`]
        [O(log(`size()`) + `count(k)`)]
        [Average case: O(`count(k)`), Worst case: O(`size()`)]
    ]
    [
        [Erase a single element by iterator]
        [Amortized constant]
        [Average case: O(1), Worst case: O(`size()`)]
    ]
    [
        [Erase a range of /N/ elements]
        [O(log(`size()`) + /N/)]
        [Average case: O(/N/), Worst case: O(`size()`)]
    ]
    [
        [Clearing the container]
        [O(`size()`)]
        [O(`size()`)]
    ]
    [
        [Find]
        [logarithmic]
        [Average case: O(1), Worst case: O(`size()`)]
    ]
    [/ TODO: Average case is probably wrong. ]
    [
        [Count]
        [O(log(`size()`) + `count(k)`)]
        [Average case: O(1), Worst case: O(`size()`)]
    ]
    [
        [`equal_range(k)`]
        [logarithmic]
        [Average case: O(`count(k)`), Worst case: O(`size()`)]
    ]
    [
        [`lower_bound`,`upper_bound`]
        [logarithmic]
        [n/a]
    ]
]

[endsect]