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## Chapter 37. Boost.Unordered

### Daniel James

Copyright © 2003, 2004 Jeremy B. Maitin-Shepard

Copyright © 2005-2008 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)

## Introduction

For accessing data based on key lookup, the C++ standard library offers `std::set`, `std::map`, `std::multiset` and `std::multimap`. These are generally implemented using balanced binary trees so that lookup time has logarithmic complexity. That is generally okay, but in many cases a hash table can perform better, as accessing data has constant complexity, on average. The worst case complexity is linear, but that occurs rarely and with some care, can be avoided.

Also, the existing containers require a 'less than' comparison object to order their elements. For some data types this is impossible to implement or isn't practical. In contrast, a hash table only needs an equality function and a hash function for the key.

With this in mind, unordered associative containers were added to the C++ standard. This is an implementation of the containers described in C++11, with some deviations from the standard in order to work with non-C++11 compilers and libraries.

`unordered_set` and `unordered_multiset` are defined in the header <`boost/unordered_set.hpp`>

    namespace boost {
        template <
            class Key,
            class Hash = boost::hash<Key>,
            class Pred = std::equal_to<Key>,
            class Alloc = std::allocator<Key> >
        class unordered_set;

        template<
            class Key,
            class Hash = boost::hash<Key>,
            class Pred = std::equal_to<Key>,
            class Alloc = std::allocator<Key> >
        class unordered_multiset;
    }

`unordered_map` and `unordered_multimap` are defined in the header <`boost/unordered_map.hpp`>

    namespace boost {
        template <
            class Key, class Mapped,
            class Hash = boost::hash<Key>,
            class Pred = std::equal_to<Key>,
            class Alloc = std::allocator<std::pair<Key const, Mapped> > >
        class unordered_map;

        template<
            class Key, class Mapped,
            class Hash = boost::hash<Key>,
            class Pred = std::equal_to<Key>,
            class Alloc = std::allocator<std::pair<Key const, Mapped> > >
        class unordered_multimap;
    }

When using Boost.TR1, these classes are included from `<unordered_set>` and `<unordered_map>`, with the classes added to the `std::tr1` namespace.

The containers are used in a similar manner to the normal associative containers:

    typedef boost::unordered_map<std::string, int> map;
    map x;
    x["one"] = 1;
    x["two"] = 2;
    x["three"] = 3;

    assert(x.at("one") == 1);
    assert(x.find("missing") == x.end());

But since the elements aren't ordered, the output of:

    BOOST_FOREACH(map::value_type i, x) {
        std::cout<<i.first<<","<<i.second<<"\n";
    }

can be in any order. For example, it might be:

    two,2
    one,1
    three,3

To store an object in an unordered associative container requires both an key equality function and a hash function. The default function objects in the standard containers support a few basic types including integer types, floating point types, pointer types, and the standard strings. Since Boost.Unordered uses `boost::hash` it also supports some other types, including standard containers. To use any types not supported by these methods you have to extend Boost.Hash to support the type or use your own custom equality predicates and hash functions. See the Equality Predicates and Hash Functions section for more details.

There are other differences, which are listed in the Comparison with Associative Containers section.

Last revised: October 30, 2014 at 10:20:57 GMT |