# Boost.MultiIndex Random access indices reference

Sequenced indices
Boost.MultiIndex reference
Key extraction

## Contents

- Header `"boost/multi_index/random_access_index_fwd.hpp"` synopsis
- Header `"boost/multi_index/random_access_index.hpp"` synopsis
  - `random_access` index specifier
  - Random access indices
    - Complexity signature
    - Instantiation types
    - Constructors, copy and assignment
    - Capacity operations
    - Modifiers
    - List operations
    - Rearrange operations
    - Serialization

## Header `"boost/multi_index/random_access_index_fwd.hpp"` synopsis

>     namespace boost{

>     namespace multi_index{

>     // random_access index specifier

>     template<typename TagList=tag<> > struct random_access;

>     // indices

>     namespace detail{

>     template<implementation defined> class index class name implementation defined;

>     } // namespace boost::multi_index::detail

>     } // namespace boost::multi_index

>     } // namespace boost

`random_access_index_fwd.hpp` provides forward declarations for the `random_access` index specifier and its associated random access index class.

## Header `"boost/multi_index/random_access_index.hpp"` synopsis

>     namespace boost{

>     namespace multi_index{

>     // random_access index specifier

>     template<typename TagList=tag<> > struct random_access;

>     // indices

>     namespace detail{

>     template<implementation defined> class index class name implementation defined;

>     // index comparison:

>     // OP is any of ==,<,!=,>,>=,<=

>     template<arg set 1,arg set 2>
>     bool operator OP(
>       const index class name<arg set 1>& x,const index class name<arg set 2>& y);

>     // index specialized algorithms:

>     template<implementation defined>
>     void swap(index class name& x,index class name& y);

>     } // namespace boost::multi_index::detail

>     } // namespace boost::multi_index

>     } // namespace boost

### `random_access` index specifier

This index specifier allows for insertion of a random access index.

>     template<typename TagList=tag<> > struct random_access;

If provided, `TagList` must be an instantiation of `tag`.

### Random access indices

Random access indices are free-order sequences with constant time positional access and random access iterators. Elements in a random access index are by default sorted according to their order of insertion: this means that new elements inserted through a different index of the `multi_index_container` are appended to the end of the random access index; additionally, facilities are provided for further rearrangement of the elements. The public interface of random access indices includes that of sequenced indices, with differences in the complexity of the operations, plus extra operations for positional access (`operator[]` and `at()`) and for capacity handling. Validity of iterators and references to elements is preserved in all operations, regardless of the capacity status.

As is the case with sequenced indices, random access indices have the following limitations with respect to STL sequence containers:

- Random access indices are not `Assignable` (like any other index.)
- Insertions into a random access index may fail due to clashings with other indices. This alters the semantics of the operations provided with respect to their analogues in STL sequence containers.
- Elements in a random access index are not mutable, and can only be changed by means of `replace` and `modify` member functions.

Having these restrictions into account, random access indices are models of `Random Access Container` and `Back Insertion Sequence`. Although these indices do not model `Front Insertion Sequence`, because front insertion and deletion take linear time, front operations are nonetheless provided to match the interface of sequenced indices. We only describe those types and operations that are either not present in the concepts modeled or do not exactly conform to the requirements for these types of containers.

>     namespace boost{

>     namespace multi_index{

>     namespace detail{

>     template<implementation defined: dependent on types Value, Allocator, TagList>
>     class name is implementation defined
>     {
>     public:
>       // types:

>       typedef typename node_type::value_type             value_type;
>       typedef tuples::null_type                          ctor_args;
>       typedef typename Allocator                         allocator_type;
>       typedef typename allocator_type::reference         reference;
>       typedef typename allocator_type::const_reference   const_reference;
>       typedef implementation defined                     iterator;
>       typedef implementation defined                     const_iterator;
>       typedef std::size_t                                size_type;
>       typedef std::ptrdiff_t                             difference_type;
>       typedef typename allocator_type::pointer           pointer;
>       typedef typename allocator_type::const_pointer     const_pointer;
>       typedef equivalent to
>         std::reverse_iterator<iterator>                  reverse_iterator;
>       typedef equivalent to
>         std::reverse_iterator<const_iterator>            const_reverse_iterator;

>       // construct/copy/destroy:

>       index class name& operator=(const index class name& x);

>       template <class InputIterator>
>       void assign(InputIterator first,InputIterator last);
>       void assign(size_type n,const value_type& value);

>       allocator_type get_allocator()const;

>       // iterators:

>       iterator               begin();
>       const_iterator         begin()const;
>       iterator               end();
>       const_iterator         end()const;
>       reverse_iterator       rbegin();
>       const_reverse_iterator rbegin()const;
>       reverse_iterator       rend();
>       const_reverse_iterator rend()const;

>       // capacity:

>       bool      empty()const;
>       size_type size()const;
>       size_type max_size()const;
>       size_type capacity()const;
>       void      reserve(size_type m);

>       void resize(size_type n,const value_type& x=value_type());

>       // access:

>       const_reference operator[](size_type n)const;
>       const_reference at(size_type n)const;
>       const_reference front()const;
>       const_reference back()const;

>       // modifiers:

>       std::pair<iterator,bool> push_front(const value_type& x);
>       void                     pop_front();
>       std::pair<iterator,bool> push_back(const value_type& x);
>       void                     pop_back();

>       std::pair<iterator,bool> insert(iterator position,const value_type& x);
>       void insert(iterator position,size_type m,const value_type& x);
>       template<typename InputIterator>
>       void insert(iterator position,InputIterator first,InputIterator last);

>       iterator erase(iterator position);
>       iterator erase(iterator first,iterator last);

>       bool replace(iterator position,const value_type& x);
>       template<typename Modifier> bool modify(iterator position,Modifier mod);

>       void swap(index class name& x);

>       void clear();

>       // list operations:

>       void splice(iterator position,index class name& x);
>       void splice(iterator position,index class name& x,iterator i);
>       void splice(
>         iterator position,index class name& x,iterator first,iterator last);

>       void remove(const value_type& value);
>       template<typename Predicate> void remove_if(Predicate pred);

>       void unique();
>       template <class BinaryPredicate>
>       void unique(BinaryPredicate binary_pred);

>       void merge(index class name& x);
>       template <typename Compare> void merge(index class name& x,Compare comp);

>       void sort();
>       template <typename Compare> void sort(Compare comp);

>       void reverse();

>       // rearrange operations:

>       void relocate(iterator position,iterator i);
>       void relocate(iterator position,iterator first,iterator last);
>       template<typename InputIterator> void rearrange(InputIterator first);
>     }

>     // index comparison:

>     template<arg set 1,arg set 2>
>     bool operator==(
>       const index class name<arg set 1>& x,
>       const index class name<arg set 2>& y)
>     {
>       return x.size()==y.size()&&std::equal(x.begin(),x.end(),y.begin());
>     }

>     template<arg set 1,arg set 2>
>     bool operator<(
>       const index class name<arg set 1>& x,
>       const index class name<arg set 2>& y)
>     {
>       return std::lexicographical_compare(x.begin(),x.end(),y.begin(),y.end());
>     }

>     template<arg set 1,arg set 2>
>     bool operator!=(
>       const index class name<arg set 1>& x,
>       const index class name<arg set 2>& y)
>     {
>       return !(x==y);
>     }

>     template<arg set 1,arg set 2>
>     bool operator>(
>       const index class name<arg set 1>& x
>       ,const index class name<arg set 2>& y)
>     {
>       return y<x;
>     }

>     template<arg set 1,arg set 2>
>     bool operator>=(
>       const index class name<arg set 1>& x,
>       const index class name<arg set 2>& y)
>     {
>       return !(x<y);
>     }

>     template<arg set 1,arg set 2>
>     bool operator<=(
>       const index class name<arg set 1>& x,
>       const index class name<arg set 2>& y)
>     {
>       return !(x>y);
>     }

>     // index specialized algorithms:

>     template<implementation defined>
>     void swap(index class name& x,index class name& y);

>     } // namespace boost::multi_index::detail

>     } // namespace boost::multi_index

>     } // namespace boost

#### Complexity signature

Here and in the descriptions of operations of random access indices, we adopt the scheme outlined in the complexity signature section. The complexity signature of random access indices is:

- copying: `c(n)=n*log(n)`,
- insertion: `i(n)=1` (amortized constant),
- hinted insertion: `h(n)=1` (amortized constant),
- deletion: `d(n)=m`, where `m` is the distance from the deleted element to the end of the sequence,
- replacement: `r(n)=1` (constant),
- modifying: `m(n)=1` (constant).

The following expressions are also used as a convenience for writing down some of the complexity formulas:

> `shl(a,b)` = `a+b` if a is nonzero, `0` otherwise.
> `rel(a,b,c)` = if `a<b`, `c-a`, else `a-b`,

(`shl` and `rel` stand for shift left and relocate, respetively.)

#### Instantiation types

Random access indices are instantiated internally to `multi_index_container` and specified by means of `indexed_by` with the `random_access` index specifier. Instantiations are dependent on the following types:

- `Value` from `multi_index_container`,
- `Allocator` from `multi_index_container`,
- `TagList` from the index specifier (if provided).

`TagList` must be an instantiation of `tag`.

#### Constructors, copy and assignment

As explained in the index concepts section, indices do not have public constructors or destructors. Assignment, on the other hand, is provided.

`index class name& operator=(const index class name& x);`

> Effects:

> >     a=b;

> where `a` and `b` are the `multi_index_container` objects to which `*this` and `x` belong, respectively.
> Returns: `*this`.

`template <class InputIterator>
void assign(InputIterator first,InputIterator last);`

> Requires: `InputIterator` is a model of `Input Iterator` over elements of type `value_type` or a type convertible to `value_type`. `first` and `last` are not iterators into any index of the `multi_index_container` to which this index belongs. `last` is reachable from `first`. Effects:

> >     clear();
> >     insert(end(),first,last);

`void assign(size_type n,const value_type& value);`

> Effects:

> >     clear();
> >     for(size_type i=0;i<n;++n)push_back(v);

#### Capacity operations

`size_type capacity()const;`

> Returns: The total number of elements `c` such that, when `size()<c`, back insertions happen in constant time (the general case as described by `i(n)` is amortized constant time.)
> Note: Validity of iterators and references to elements is preserved in all insertions, regardless of the capacity status.

`void reserve(size_type m);`

> Effects: If the previous value of `capacity()` was greater than or equal to `m`, nothing is done; otherwise, the internal capacity is changed so that `capacity()>=m`.
> Complexity: If the capacity is not changed, constant; otherwise `O(n)`.
> Exception safety: If the capacity is not changed, `nothrow`; otherwise, strong.

`void resize(size_type n,const value_type& x=value_type());`

> Effects:

> >     if(n>size())insert(end(),n-size(),x);
> >     else if(n<size())erase(begin()+n,end());

> Note: If an expansion is requested, the size of the index is not guaranteed to be `n` after this operation (other indices may ban insertions.)

#### Modifiers

`std::pair<iterator,bool> push_front(const value_type& x);`

> Effects: Inserts `x` at the beginning of the sequence if no other index of the `multi_index_container` bans the insertion.
> Returns: The return value is a pair `p`. `p.second` is `true` if and only if insertion took place. On successful insertion, `p.first` points to the element inserted; otherwise, `p.first` points to an element that caused the insertion to be banned. Note that more than one element can be causing insertion not to be allowed.
> Complexity: `O(n+I(n))`.
> Exception safety: Strong.

`std::pair<iterator,bool> push_back(const value_type& x);`

> Effects: Inserts `x` at the end of the sequence if no other index of the `multi_index_container` bans the insertion.
> Returns: The return value is a pair `p`. `p.second` is `true` if and only if insertion took place. On successful insertion, `p.first` points to the element inserted; otherwise, `p.first` points to an element that caused the insertion to be banned. Note that more than one element can be causing insertion not to be allowed.
> Complexity: `O(I(n))`.
> Exception safety: Strong.

`std::pair<iterator,bool> insert(iterator position,const value_type& x);`

> Requires: `position` is a valid iterator of the index. Effects: Inserts `x` before `position` if insertion is allowed by all other indices of the `multi_index_container`.
> Returns: The return value is a pair `p`. `p.second` is `true` if and only if insertion took place. On successful insertion, `p.first` points to the element inserted; otherwise, `p.first` points to an element that caused the insertion to be banned. Note that more than one element can be causing insertion not to be allowed.
> Complexity: `O(shl(end()-position,1) + I(n))`.
> Exception safety: Strong.

`void insert(iterator position,size_type m,const value_type& x);`

> Requires: `position` is a valid iterator of the index. Effects:

> >     for(size_type i=0;i<m;++i)insert(position,x);

> Complexity: `O(shl(end()-position,m) + m*I(n+m))`.

`template<typename InputIterator>
void insert(iterator position,InputIterator first,InputIterator last);`

> Requires: `position` is a valid iterator of the index. `InputIterator` is a model of `Input Iterator` over elements of type `value_type` or a type convertible to `value_type`. `first` and `last` are not iterators into any index of the `multi_index_container` to which this index belongs. `last` is reachable from `first`. Effects:

> >     while(first!=last)insert(position,*first++);

> Complexity: `O(shl(end()-position,m) + m*I(n+m))`, where `m` is the number of elements in [`first`,`last`).
> Exception safety: Basic.

`iterator erase(iterator position);`

> Requires: `position` is a valid dereferenceable iterator of the index. Effects: Deletes the element pointed to by `position`.
> Returns: An iterator pointing to the element immediately following the one that was deleted, or `end()` if no such element exists.
> Complexity: `O(D(n))`.
> Exception safety: `nothrow`.

`iterator erase(iterator first,iterator last);`

> Requires: [`first`,`last`) is a valid range of the index.
> Effects: Deletes the elements in [`first`,`last`).
> Returns: `last`.
> Complexity: `O(m*D(n))`, where `m` is the number of elements in [`first`,`last`).
> Exception safety: `nothrow`.

`bool replace(iterator position,const value_type& x);`

> Requires: `position` is a valid dereferenceable iterator of the index. Effects: Assigns the value `x` to the element pointed to by `position` into the `multi_index_container` to which the index belongs if replacing is allowed by all other indices of the `multi_index_container`.
> Postconditions: Validity of `position` is preserved in all cases.
> Returns: `true` if the replacement took place, `false` otherwise.
> Complexity: `O(R(n))`.
> Exception safety: Strong. If an exception is thrown by some user-provided operation the `multi_index_container` to which the index belongs remains in its original state.

`template<typename Modifier> bool modify(iterator position,Modifier mod);`

> Requires: `Modifier` is a model of `Unary Function` accepting arguments of type `value_type&`. `position` is a valid dereferenceable iterator of the index. Effects: Calls `mod(e)` where `e` is the element pointed to by `position` and rearranges `*position` into all the indices of the `multi_index_container`. Rearrangement on rancom access indices does not change the position of the element with respect to the index; rearrangement on other indices may or might not suceed. If the rearrangement fails, the element is erased.
> Postconditions: Validity of `position` is preserved if the operation succeeds.
> Returns: `true` if the operation succeeded, `false` otherwise.
> Complexity: `O(M(n))`.
> Exception safety: Basic. If an exception is thrown by some user-provided operation (except possibly `mod`), then the element pointed to by `position` is erased.

#### List operations

Random access indices replicate the interface of sequenced indices, which in turn includes the list operations provided by `std::list`. The syntax and behavior of these operations exactly matches those of sequenced indices, but the associated complexity bounds differ in general.

`void splice(iterator position,index class name& x);`

> Requires: `position` is a valid iterator of the index. `&x!=this`. Effects: Inserts the contents of `x` before `position`, in the same order as they were in `x`. Those elements succesfully inserted are erased from `x`.
> Complexity: `O(shl(end()-position,x.size()) + x.size()*I(n+x.size()) + x.size()*D(x.size()))`.
> Exception safety: Basic.

`void splice(iterator position,index class name& x,iterator i);`

> Requires: `position` is a valid iterator of the index. `i` is a valid dereferenceable iterator `x`.
> Effects: Inserts the element pointed to by `i` before `position`: if insertion is succesful, the element is erased from `x`. In the special case `&x==this`, no copy or deletion is performed, and the operation is always succesful. If `position==i`, no operation is performed.
> Postconditions: If `&x==this`, no iterator or reference is invalidated.
> Complexity: If `&x==this`, `O(rel(position,i,i+1))`; otherwise `O(shl(end()-position,1) + I(n) + D(n))`.
> Exception safety: If `&x==this`, `nothrow`; otherwise, strong.

`void splice(iterator position,index class name& x,iterator first,iterator last);`

> Requires: `position` is a valid iterator of the index. `first` and `last` are valid iterators of `x`. `last` is reachable from `first`. `position` is not in the range [`first`,`last`).
> Effects: For each element in the range [`first`,`last`), insertion is tried before `position`; if the operation is succesful, the element is erased from `x`. In the special case `&x==this`, no copy or deletion is performed, and insertions are always succesful.
> Postconditions: If `&x==this`, no iterator or reference is invalidated.
> Complexity: If `&x==this`, `O(rel(position,first,last))`; otherwise `O(shl(end()-position,m) + m*I(n+m) + m*D(x.size()))` where `m` is the number of elements in [`first`,`last`).
> Exception safety: If `&x==this`, `nothrow`; otherwise, basic.

`void remove(const value_type& value);`

> Effects: Erases all elements of the index which compare equal to `value`.
> Complexity: `O(n + m*D(n))`, where `m` is the number of elements erased.
> Exception safety: Basic.

`template<typename Predicate> void remove_if(Predicate pred);`

> Effects: Erases all elements `x` of the index for which `pred(x)` holds.
> Complexity: `O(n + m*D(n))`, where `m` is the number of elements erased.
> Exception safety: Basic.

`void unique();`

> Effects: Eliminates all but the first element from every consecutive group of equal elements referred to by the iterator `i` in the range [`first+1`,`last`) for which `*i==*(i-1)`.
> Complexity: `O(n + m*D(n))`, where `m` is the number of elements erased.
> Exception safety: Basic.

`template <class BinaryPredicate> void unique(BinaryPredicate binary_pred);`

> Effects: Eliminates all but the first element from every consecutive group of elements referred to by the iterator `i` in the range [`first+1`,`last`) for which `binary_pred(*i,*(i-1))` holds.
> Complexity: `O(n + m*D(n))`, where `m` is the number of elements erased.
> Exception safety: Basic.

`void merge(index class name& x);`

> Requires: `std::less<value_type>` is a `Strict Weak Ordering` over `value_type`. Both the index and `x` are sorted according to `std::less<value_type>`.
> Effects: Attempts to insert every element of `x` into the corresponding position of the index (according to the order). Elements successfully inserted are erased from `x`. The resulting sequence is stable, i.e. equivalent elements of either container preserve their relative position. In the special case `&x==this`, no operation is performed.
> Postconditions: Elements in the index and remaining elements in `x` are sorted. Validity of iterators to the index and of non-erased elements of `x` references is preserved.
> Complexity: If `&x==this`, constant; otherwise `O(n + x.size()*I(n+x.size()) + x.size()*D(x.size()))`.
> Exception safety: If `&x==this`, `nothrow`; otherwise, basic.

`template <typename Compare> void merge(index class name& x,Compare comp);`

> Requires: `Compare` is a `Strict Weak Ordering` over `value_type`. Both the index and `x` are sorted according to `comp`.
> Effects: Attempts to insert every element of `x` into the corresponding position of the index (according to `comp`). Elements successfully inserted are erased from `x`. The resulting sequence is stable, i.e. equivalent elements of either container preserve their relative position. In the special case `&x==this`, no operation is performed.
> Postconditions: Elements in the index and remaining elements in `x` are sorted according to `comp`. Validity of iterators to the index and of non-erased elements of `x` references is preserved.
> Complexity: If `&x==this`, constant; otherwise `O(n + x.size()*I(n+x.size()) + x.size()*D(x.size()))`.
> Exception safety: If `&x==this`, `nothrow`; otherwise, basic.

`void sort();`

> Requires: `std::less<value_type>` is a `Strict Weak Ordering` over `value_type`.
> Effects: Sorts the index according to `std::less<value_type>`. The sorting is stable, i.e. equivalent elements preserve their relative position.
> Postconditions: Validity of iterators and references is preserved.
> Complexity: `O(n*log(n))`.
> Exception safety: Basic.

`template <typename Compare> void sort(Compare comp);`

> Requires: `Compare` is a `Strict Weak Ordering` over `value_type`.
> Effects: Sorts the index according to `comp`. The sorting is stable, i.e. equivalent elements preserve their relative position.
> Postconditions: Validity of iterators and references is preserved.
> Complexity: `O(n*log(n))`.
> Exception safety: Basic.

`void reverse();`

> Effects: Reverses the order of the elements in the index.
> Postconditions: Validity of iterators and references is preserved.
> Complexity: `O(n)`.
> Exception safety: `nothrow`.

#### Rearrange operations

These operations, without counterpart in STL sequence containers (although `std::list::splice` provides partially overlapping functionality), perform individual and global repositioning of elements inside the index.

`void relocate(iterator position,iterator i);`

> Requires: `position` is a valid iterator of the index. `i` is a valid dereferenceable iterator of the index.
> Effects: Inserts the element pointed to by `i` before `position`. If `position==i`, no operation is performed.
> Postconditions: No iterator or reference is invalidated.
> Complexity: `O(rel(position,i,i+1))`.
> Exception safety: `nothrow`.

`void relocate(iterator position,iterator first,iterator last);`

> Requires: `position` is a valid iterator of the index. `first` and `last` are valid iterators of the index. `last` is reachable from `first`. `position` is not in the range [`first`,`last`).
> Effects: The range of elements [`first`,`last`) is repositioned just before `position`.
> Postconditions: No iterator or reference is invalidated.
> Complexity: `O(rel(position,first,last))`.
> Exception safety: `nothrow`.

`template<typename InputIterator> void rearrange(InputIterator first);`

> Requires: The range [`first`, `std::advance(first,n)`), where `n` is the size of the index, is a free view of the index.
> Effects: The elements are rearranged so as to match the order of the previously described view.
> Postconditions: No iterator or reference is invalidated.
> Complexity: `O(n)`.
> Exception safety: Basic.

#### Serialization

Indices cannot be serialized on their own, but only as part of the `multi_index_container` into which they are embedded. In describing the additional preconditions and guarantees associated to random access indices with respect to serialization of their embedding containers, we use the concepts defined in the `multi_index_container` serialization section.

Operation: saving of a `multi_index_container` `m` to an output archive (XML archive) `ar`.

> Requires: No additional requirements to those imposed by the container.

Operation: loading of a `multi_index_container` `m'` from an input archive (XML archive) `ar`.

> Requires: No additional requirements to those imposed by the container.
> Postconditions: On succesful loading, each of the elements of [`begin()`, `end()`) is a restored copy of the corresponding element in [`m.get<i>().begin()`, `m.get<i>().end()`), where `i` is the position of the random access index in the container.

Operation: saving of an `iterator` or `const_iterator` `it` to an output archive (XML archive) `ar`.

> Requires: `it` is a valid iterator of the index. The associated `multi_index_container` has been previously saved.

Operation: loading of an `iterator` or `const_iterator` `it'` from an input archive (XML archive) `ar`.

> Postconditions: On succesful loading, if `it` was dereferenceable then `*it'` is the restored copy of `*it`, otherwise `it'==end()`.
> Note: It is allowed that `it` be a `const_iterator` and the restored `it'` an `iterator`, or viceversa.

Sequenced indices
Boost.MultiIndex reference
Key extraction

Revised February 6th 2006

© Copyright 2003-2006 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)