# Boost.MultiIndex Tutorial: Container creation

Key extraction
Boost.MultiIndex tutorial
Debugging support

## Contents

- Value semantics
- Use of `ctor_args_list`
- Serialization

## Value semantics

`multi_index_container`s have the usual value semantics associated to copy construction and assignment, i.e. copies of the elements from the source container are created and inserted into the destination container. More interestingly, copying also recreates the original order in which elements are arranged for every index of the container. This implies that equality of all indices is preserved under copying or assignment, for those index types where equality is defined. This behavior can be regarded as a natural extension to the general rule on copy semantics stating that if `y` is a copy of `x`, then `y==x`.

## Use of `ctor_args_list`

Although in most cases `multi_index_container`s will be default constructed (or copied from a preexisting `multi_index_container`), sometimes it is necessary to specify particular values for the internal objects used (key extractors, comparison predicates, allocator), for instance if some of these objects do not have a default constructor. The same situation can arise with standard STL containers, which allow for the optional specification of such objects:

>     // example of non-default constructed std::set
>     template<typename IntegralType>
>     struct modulo_less
>     {
>       modulo_less(IntegralType m):modulo(m){}

>       bool operator()(IntegralType x,IntegralType y)const
>       {
>         return (x%modulo)<(y%modulo);
>       }

>     private:
>       IntegralType modulo;
>     };

>     typedef std::set<unsigned int,modulo_less<unsigned int> > modulo_set;

>     modulo_set m(modulo_less<unsigned int>(10));

`multi_index_container` does also provide this functionality, though in a considerably more complex fashion, due to the fact that the constructor of a `multi_index_container` has to accept values for all the internal objects of its indices. The full form of `multi_index_container` constructor is

>     explicit multi_index_container(
>         const ctor_args_list& args_list=ctor_args_list(),
>         const allocator_type& al=allocator_type());

The specification of the allocator object poses no particular problems; as for the `ctor_args_list`, this object is designed so as to hold the necessary construction values for every index in the `multi_index_container`. From the point of view of the user, `ctor_args_list` is equivalent to the type

>     boost::tuple<C0,...,CI-1>

where `I` is the number of indices, and `Ci` is

>     nth_index<i>::type::ctor_args

that is, the nested type `ctor_args` of the `i`-th index. Each `ctor_args` type is in turn a tuple holding values for constructor arguments of the associated index: so, ordered indices demand a key extractor object and a comparison predicate, hashed indices take an initial number of buckets, a key extractor, a hash function and an equality predicate; while sequenced and random access indices do not need any construction argument. For instance, given the definition

>     typedef multi_index_container<
>       unsigned int,
>       indexed_by<
>         hashed_unique<identity<unsigned int> >,
>         ordered_non_unique<identity<unsigned int>, modulo_less<unsigned int> >,
>         sequenced<>,
>         random_access<>
>       >
>     > modulo_indexed_set;

the corresponding `ctor_args_list` type is equivalent to

>     boost::tuple<
>       // ctr_args of index #0
>       boost::tuple<
>         std::size_t, // initial number of buckets; 0 if unspecified
>         identity<unsigned int>,
>         boost::hash<unsigned int>,
>         std::equal_to<unsigned int> >,

>       // ctr_args of index #1
>       boost::tuple<
>         identity<unsigned int>,
>         modulo_less<unsigned int> >,

>       // sequenced indices do not have any construction argument
>       boost::tuple<>,

>       // neither do random access indices
>       boost::tuple<>
>     >

Such a `modulo_indexed_set` cannot be default constructed, because `modulo_less` does not provide a default constructor. The following shows how the construction can be done:

>     modulo_indexed_set::ctor_args_list args_list=
>       boost::make_tuple(
>         // ctor_args for index #0 is default constructible
>         modulo_indexed_set::nth_index<0>::type::ctor_args(),

>         boost::make_tuple(identity<unsigned int>(),modulo_less<unsigned int>(10)),

>         // these are also default constructible (actually, empty tuples)
>         modulo_indexed_set::nth_index<2>::type::ctor_args(),
>         modulo_indexed_set::nth_index<3>::type::ctor_args()
>       );

>     modulo_indexed_set m(args_list);

A program is provided in the examples section that puts in practise these concepts.

## Serialization

`multi_index_container`s can be archived and retrieved by means of the Boost Serialization Library. Both regular and XML archives are supported. The usage is straightforward and does not differ from that of any other serializable type. For instance:

>     #include <boost/archive/text_oarchive.hpp>
>     #include <boost/archive/text_iarchive.hpp>
>     #include <fstream>

>     ...

>     void save(const employee_set& es)
>     {
>       std::ofstream ofs("data");
>       boost::archive::text_oarchive oa(ofs);
>       oa<<es;
>     }

>     void load(employee_set& es)
>     {
>       std::ifstream ifs("data");
>       boost::archive::text_iarchive ia(ifs);
>       ia>>es;
>     }

>     ...

>     employee_set es;
>     ... // fill it with data
>     save(es);

>     ...

>     employee_set restored_es;
>     load(restored_es);

Serialization capabilities are automatically provided by just linking with the appropriate Boost.Serialization library module: it is not necessary to explicitly include any header from Boost.Serialization, apart from those declaring the type of archive used in the process. If not used, however, serialization support can be disabled by globally defining the macro `BOOST_MULTI_INDEX_DISABLE_SERIALIZATION`. Disabling serialization for Boost.MultiIndex can yield a small improvement in build times, and may be necessary in those defective compilers that fail to correctly process Boost.Serialization headers.

In accordance with Boost.MultiIndex value semantics, retrieving an archived `multi_index_container` restores not only the elements, but also the order they were arranged into for every index of the container. There is an exception to this rule, though: for hashed indices, no guarantee is made about the order in which elements will be iterated in the restored container; in general, it is unwise to rely on the ordering of elements of a hashed index, since it can change in arbitrary ways during insertion or rehashing --this is precisely the reason why hashed indices and TR1 unordered associative containers do not define an equality operator.

Iterators to indices of a `multi_index_container` can also be serialized. Serialization of iterators must be done only after serializing their corresponding container.

Example 9 in the examples section shows the serialization capabilities of Boost.MultiIndex.

Key extraction
Boost.MultiIndex tutorial
Debugging support

Revised February 27th 2007

© Copyright 2003-2007 Joaquín M López Muñoz. 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)