Boost.MultiIndex Hashed indices reference

Header "boost/multi_index/hashed_index_fwd.hpp" synopsis
Header "boost/multi_index/hashed_index.hpp" synopsis
- Index specifiers hashed_unique and hashed_non_unique
- Hashed indices

Header `"boost/multi_index/hashed_index_fwd.hpp"` synopsis

namespace boost{

namespace multi_index{

// index specifiers hashed_unique and hashed_non_unique

template<consult hashed_unique reference for arguments>
struct hashed_unique;
template<consult hashed_non_unique reference for arguments>
struct hashed_non_unique;

// indices

namespace detail{

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

} // namespace boost::multi_index::detail

} // namespace boost::multi_index 

} // namespace boost

hashed_index_fwd.hpp provides forward declarations for index specifiers hashed_unique and hashed_non_unique and their associated hashed index classes.

Header `"boost/multi_index/hashed_index.hpp"` synopsis

#include <initializer_list>

namespace boost{

namespace multi_index{

// index specifiers hashed_unique and hashed_non_unique

template<consult hashed_unique reference for arguments>
struct hashed_unique;
template<consult hashed_non_unique reference for arguments>
struct hashed_non_unique;

// indices

namespace detail{

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

// index comparison:

// OP is any of ==,!=

template<implementation defined>
bool operator OP(const index class name& x,const index class name& 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

Index specifiers `hashed_unique` and `hashed_non_unique`

These index specifiers allow for insertion of hashed indices without and with allowance of duplicate elements, respectively. The syntax of hashed_unique and hashed_non_unique coincide, thus we describe them in a grouped manner. hashed_unique and hashed_non_unique can be instantiated in two different forms, according to whether a tag list for the index is provided or not:

template<
  typename KeyFromValue,
  typename Hash=boost::hash<KeyFromValue::result_type>,
  typename Pred=std::equal_to<KeyFromValue::result_type>
>
struct (hashed_unique | hashed_non_unique);

template<
  typename TagList,
  typename KeyFromValue,
  typename Hash=boost::hash<KeyFromValue::result_type>,
  typename Pred=std::equal_to<KeyFromValue::result_type>
>
struct (hashed_unique | hashed_non_unique);

If provided, TagList must be an instantiation of the class template tag. The template arguments are used by the corresponding index implementation, refer to the hashed indices reference section for further explanations on their acceptable type values.

Hashed indices

A hashed index provides fast retrieval of elements of a multi_index_container through hashing techniques. A hashed index is particularized according to a given Key Extractor that retrieves keys from elements of multi_index_container, a Hash function object which returns hash values for the keys and a binary predicate Pred acting as an equivalence relation on values of Key.

There are two variants of hashed indices: unique, which do not allow duplicate elements (with respect to its associated equality predicate) and non-unique, which accept those duplicates. The interface of these two variants is the same, so they are documented together, with minor differences explicitly stated when they exist.

Except where noted or if the corresponding interface does not exist, hashed indices (both unique and non-unique) satisfy the C++ requirements for unordered associative containers at [unord.req] (supporting unique and equivalent keys, respectively.) Iterators (including to the end of the index) and pointers and references to an element remain valid during the lifetime of the associated container (which can change upon swapping), or until the referred-to element is erased or extracted; pointers and references to an extracted element, but not so for iterators, become valid again once the element is re-inserted. Occasionally, the exception safety guarantees provided are actually stronger than required by the standard. We only provide descriptions of those types and operations that do not exactly conform to or are not mandated by the standard requirements.

namespace boost{

namespace multi_index{

namespace detail{

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

  typedef typename KeyFromValue::result_type         key_type;
  typedef Value                                      value_type;
  typedef KeyFromValue                               key_from_value;
  typedef Hash                                       hasher;
  typedef Pred                                       key_equal;
  typedef boost::tuple<
    size_type,key_from_value,hasher,key_equal>       ctor_args;
  typedef TagList                                    tag_list;
  typedef Allocator                                  allocator_type;
  typedef typename Allocator::pointer                pointer;
  typedef typename Allocator::const_pointer          const_pointer;
  typedef typename Allocator::reference              reference;
  typedef typename Allocator::const_reference        const_reference;
  typedef implementation defined                     size_type;      
  typedef implementation defined                     difference_type;
  typedef implementation defined                     iterator;
  typedef implementation defined                     const_iterator;
  typedef implementation defined                     local_iterator;
  typedef implementation defined                     const_local_iterator;
  typedef same as owning container                   node_type;
  typedef following [container.insert.return] spec   insert_return_type;

  // construct/destroy/copy:

  index class name& operator=(const index class name& x);
  index class name& operator=(std::initializer_list<value_type> list);

  allocator_type get_allocator()const noexcept;

  // size and capacity:

  bool      empty()const noexcept;
  size_type size()const noexcept;
  size_type max_size()const noexcept;

  // iterators:

  iterator       begin()noexcept;
  const_iterator begin()const noexcept;
  iterator       end()noexcept;
  const_iterator end()const noexcept;
  const_iterator cbegin()const;
  const_iterator cend()const;
 
  iterator       iterator_to(const value_type& x);
  const_iterator iterator_to(const value_type& x)const;

  // modifiers:


  template<typename... Args>
  std::pair<iterator,bool> emplace(Args&&... args);
  template<typename... Args>
  iterator emplace_hint(iterator position,Args&&... args);
  std::pair<iterator,bool> insert(const value_type& x);
  std::pair<iterator,bool> insert(value_type&& x);
  iterator insert(iterator position,const value_type& x);
  iterator insert(iterator position,value_type&& x);
  template<typename InputIterator>
  void insert(InputIterator first,InputIterator last);
  void insert(std::initializer_list<value_type> list);
  insert_return_type insert(node_type&& nh);
  iterator insert(const_iterator position,node_type&& nh);

  node_type extract(const_iterator position);
  node_type extract(const key_type& x);

  iterator  erase(iterator position);
  size_type erase(const key_type& x);
  iterator  erase(iterator first,iterator last);

  bool replace(iterator position,const value_type& x);
  bool replace(iterator position,value_type&& x);
  template<typename Modifier> bool modify(iterator position,Modifier mod);
  template<typename Modifier,typename Rollback>
  bool modify(iterator position,Modifier mod,Rollback back);
  template<typename Modifier> bool modify_key(iterator position,Modifier mod);
  template<typename Modifier,typename Rollback>
  bool modify_key(iterator position,Modifier mod,Rollback back);
  
  void clear()noexcept;
  void swap(index class name& x);

  template<typename Index> void merge(Index&& x);
  template<typename Index>
  std::pair<iterator,bool> merge(
    Index&& x,typename std::remove_reference_t<Index>::const_iterator i);
  template<typename Index>
  void merge(
    Index&& x,
    typename std::remove_reference_t<Index>::const_iterator first,
    typename std::remove_reference_t<Index>::const_iterator last);

  // observers:

  key_from_value key_extractor()const;
  hasher         hash_function()const;
  key_equal      key_eq()const;

  // lookup:

  template<typename CompatibleKey>
  iterator find(const CompatibleKey& x)const;
  template<
    typename CompatibleKey,typename CompatibleHash, typename CompatiblePred
  >
  iterator find(
    const CompatibleKey& x,
    const CompatibleHash& hash,const CompatiblePred& eq)const; 

  template<typename CompatibleKey>
  size_type count(const CompatibleKey& x)const;
  template<
    typename CompatibleKey,typename CompatibleHash, typename CompatiblePred
  >
  size_type count(
    const CompatibleKey& x,
    const CompatibleHash& hash,const CompatiblePred& eq)const;

  template<typename CompatibleKey>
  bool contains(const CompatibleKey& x)const;
  template<
    typename CompatibleKey,typename CompatibleHash, typename CompatiblePred
  >
  bool contains(
    const CompatibleKey& x,
    const CompatibleHash& hash,const CompatiblePred& eq)const;

  template<typename CompatibleKey>
  std::pair<iterator,iterator> equal_range(const CompatibleKey& x)const;
  template<
    typename CompatibleKey,typename CompatibleHash, typename CompatiblePred
  >
  std::pair<iterator,iterator> equal_range(
    const CompatibleKey& x,
    const CompatibleHash& hash,const CompatiblePred& eq)const;

  // bucket interface:

  size_type bucket_count()const noexcept;
  size_type max_bucket_count()const noexcept;
  size_type bucket_size(size_type n)const;
  size_type bucket(const key_type& k)const;

  local_iterator       begin(size_type n);
  const_local_iterator begin(size_type n)const;
  local_iterator       end(size_type n);
  const_local_iterator end(size_type n)const;
  const_local_iterator cbegin(size_type n)const;
  const_local_iterator cend(size_type n)const;

  local_iterator       local_iterator_to(const value_type& x);
  const_local_iterator local_iterator_to(const value_type& x)const;

  // hash policy:

  float load_factor()const noexcept;
  float max_load_factor()const noexcept;
  void  max_load_factor(float z);
  void  rehash(size_type n);
  void  reserve(size_type n);
};

// index comparison:

template<implementation defined>
bool operator==(const index class name& x,const index class name& y);

template<implementation defined>
bool operator!=(const index class name& x,const index class name& 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 hashed indices, we adopt the scheme outlined in the complexity signature section. The complexity signature of hashed indices is:

copying: c(n)=n*log(n),
insertion: average case i(n)=1 (amortized constant), worst case i(n)=n_dist,
hinted insertion: average case h(n)=1 (amortized constant), worst case h(n)=n_dist,
deletion: d(n)=1 (constant),
replacement:
- if the new element key is equivalent to the original, r(n)=1 (constant),
- otherwise, average case r(n)=1 (constant), worst case r(n)=n_dist,
modifying: average case m(n)=1 (constant), worst case m(n)=n_dist,

where n_dist is the number of non-equivalent elements out of the total n.

Instantiation types

Hashed indices are instantiated internally to multi_index_container and specified by means of indexed_by with index specifiers hashed_unique and hashed_non_unique. Instantiations are dependent on the following types:

Value from multi_index_container,
Allocator from multi_index_container,
TagList from the index specifier (if provided, otherwise tag<> is assumed),
KeyFromValue from the index specifier,
Hash from the index specifier,
Pred from the index specifier.

TagList must be an instantiation of tag. The type KeyFromValue, which determines the mechanism for extracting a key from Value, must be a model of Key Extractor from Value. Hash is a CopyConstructibleunary function object taking a single argument of type KeyFromValue::result_type and returning a value of type std::size_t in the range [0, std::numeric_limits<std::size_t>::max()). Pred is a CopyConstructible binary predicate inducing an equivalence relation on elements of KeyFromValue::result_type. It is required that the Hash object return the same value for keys equivalent under Pred.

Nested types

ctor_args

The first element of this tuple indicates the minimum number of buckets set up by the index on construction time. If the default value 0 is used, an implementation defined number is used instead.

iterator

const_iterator

local_iterator

const_local_iterator

These types are forward iterators. They depend only on node_type, the position of the index in the multi_index_container, and whether the index is unique or not (this implies that, for instance, iterators to elements transferred from a unique index to a non-unique one will become invalid).

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. Upon construction, max_load_factor() is 1.0.

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.

index class name& operator=(std::initializer_list<value_type> list);

Effects:
a=list;
where a is the multi_index_container object to which *this belongs.
Returns: *this.

Iterators

iterator       iterator_to(const value_type& x);

const_iterator iterator_to(const value_type& x)const;

Requires: x is a reference to an element of the container.
Returns: An iterator to x.
Complexity: Constant.
Exception safety: nothrow.

Modifiers

template<typename... Args>

std::pair<iterator,bool> emplace(Args&&... args);

Requires: value_type is EmplaceConstructible into multi_index_container from args.
Effects: Inserts a value_type object constructed with std::forward<Args>(args)... into the multi_index_container to which the index belongs if

the index is non-unique OR no other element exists with equivalent key,

AND 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(I(n)).
Exception safety: Strong, except that rehashing may occur even if the operation fails.

template<typename... Args>

iterator emplace_hint(iterator position, Args&&... args);

Requires: value_type is EmplaceConstructible into multi_index_container from args. position is a valid iterator of the index.
Effects: Inserts a value_type object constructed with std::forward<Args>(args)... into the multi_index_container to which the index belongs if

the index is non-unique OR no other element exists with equivalent key,

AND insertion is allowed by all other indices of the multi_index_container.

position is used as a hint to improve the efficiency of the operation.
Returns: On successful insertion, an iterator to the newly inserted element. Otherwise, an iterator 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(H(n)).
Exception safety: Strong, except that rehashing may occur even if the operation fails.

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

Requires (first version): value_type is CopyInsertable into multi_index_container.
Requires (second version): value_type is MoveInsertable into multi_index_container.
Effects: Inserts x into the multi_index_container to which the index belongs if

the index is non-unique OR no other element exists with equivalent key,

AND 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(I(n)).
Exception safety: Strong, except that rehashing may occur even if the operation fails.

iterator insert(iterator position,const value_type& x);
iterator insert(iterator position,value_type&& x);

Requires (first version): value_type is CopyInsertable into multi_index_container. position is a valid iterator of the index.
Requires (second version): value_type is MoveInsertable into multi_index_container. position is a valid iterator of the index.
Effects: Inserts x into the multi_index_container to which the index belongs if

the index is non-unique OR no other element exists with equivalent key,

AND insertion is allowed by all other indices of the multi_index_container.

position is used as a hint to improve the efficiency of the operation.
Returns: On successful insertion, an iterator to the newly inserted element. Otherwise, an iterator 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(H(n)).
Exception safety: Strong, except that rehashing may occur even if the operation fails.

template<typename InputIterator>

void insert(InputIterator first,InputIterator last);

Requires: InputIterator is an input iterator. value_type is EmplaceConstructible into multi_index_container from *first. 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: For each element of [first, last), in this order, inserts it into the multi_index_container to which this index belongs if

the index is non-unique OR no other element exists with equivalent key,

AND insertion is allowed by all other indices of the multi_index_container.

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

void insert(std::initializer_list<value_type> list);

Effects:

insert(list.begin(),list.end());

insert_return_type insert(node_type&& nh);

Requires: nh.empty() || get_allocator()==nh.get_allocator().
Effects: Does nothing if nh is empty; otherwise, inserts the node owned by nh into the multi_index_container to which the index belongs if

the index is non-unique OR no other element exists with equivalent key,

AND insertion is allowed by all other indices of the multi_index_container.

Postconditions: nh is empty.
Returns: A value p of type insert_return_type. If nh is empty, p.position is end(), p.inserted is false and p.node is empty; on successful insertion, p.position points to the element inserted, p.inserted is true and p.node is empty; if the insertion failed, p.position points to an element that caused the insertion to be banned, p.inserted is false and p.node owns the original node. Note that more than one element can be causing insertion not to be allowed.
Complexity: O(I(n)).
Exception safety: Strong, except that rehashing may occur even if the operation fails. If an exception is thrown, nh is not changed.

iterator insert(const_iterator position,node_type&& nh);

Requires: nh.empty() || get_allocator()==nh.get_allocator(). position is a valid iterator of the index.
Effects: Does nothing if nh is empty; otherwise, inserts the node owned by nh into the multi_index_container to which the index belongs if

the index is non-unique OR no other element exists with equivalent key,

AND insertion is allowed by all other indices of the multi_index_container.

position is used as a hint to improve the efficiency of the operation.
Postconditions: nh is empty if insertion succeeds, and is not changed otherwise.
Returns: end() if nh is empty. On successful insertion, an iterator to the newly inserted element; otherwise, an iterator 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(H(n)).
Exception safety: Strong, except that rehashing may occur even if the operation fails. If an exception is thrown, nh is not changed.

node_type extract(const_iterator position);

Requires: position is a valid dereferenceable iterator of the index.
Effects: Extracts the node of the element pointed to by position.
Returns: A node handle owning the extracted node.
Complexity: O(D(n)).
Exception safety: nothrow.

node_type extract(const key_type& x);

Effects: Extracts the node of the first element with key equivalent to x, if there is any.
Returns: A node handle owning the extracted node, or empty otherwise.
Complexity: Average case O(1 + D(n)), worst case O(n_dist + D(n)).
Exception safety: Strong.

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.

size_type erase(const key_type& x);

Effects: Deletes the elements with key equivalent to x.
Returns: Number of elements deleted.
Complexity: Average case O(1 + m*D(n)), worst case O(n_dist + m*D(n)), where m is the number of elements deleted.
Exception safety: Basic.

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

Requires (first version): value_type is CopyAssignable. position is a valid dereferenceable iterator of the index.
Requires (second version): value_type is MoveAssignable. 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, for the value x

the index is non-unique OR no other element exists (except possibly *position) with equivalent key,

AND replacing is allowed by all other indices of the multi_index_container.

Postconditions: Validity of position is preserved in all cases. If the key of the new value is equivalent to that of the replaced value, the position of the element does not change.
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: mod is a unary function object accepting arguments of type value_type&. position is a valid dereferenceable iterator of the index. The execution of mod(e), where e is the element pointed to by position, does not invoke any operation of the multi_index_container after e is directly modified or, before modification, if the operation would invalidate position.
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 is successful if

the index is non-unique OR no other element exists with equivalent key,

AND rearrangement is allowed by all other indices of the multi_index_container.

If the rearrangement fails, the element is erased.
Postconditions: Validity of position is preserved if the operation succeeds. If the key of the modified value is equivalent to that of the original value, the position of the element does not change.
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 (including mod), then the element pointed to by position is erased.

template<typename Modifier,typename Rollback>

bool modify(iterator position,Modifier mod,Rollback back);

Requires: mod and back are unary function objects accepting arguments of type value_type&. position is a valid dereferenceable iterator of the index. The execution of mod(e), where e is the element pointed to by position, does not invoke any operation of the multi_index_container after e is directly modified or, before modification, if the operation would invalidate position. back(e) does not invoke any operation of the multi_index_container.
Effects: Calls mod(e) where e is the element pointed to by position and tries to rearrange *position into all the indices of the multi_index_container. Rearrangement is successful if

the index is non-unique OR no other element exists with equivalent key,

AND rearrangement is allowed by all other indices of the multi_index_container.

If the rearrangement fails, back(e) is invoked: if the resulting value of e is consistent with its original position and constraints in all indices, the element is kept, otherwise it is erased.
Postconditions: Validity of position is preserved except if the element is erased under the conditions described below. If the key of the modified value is equivalent to that of the original value, the position of the element does not change.
Returns: true if the operation succeeded, false otherwise.
Complexity: O(M(n)).
Exception safety: Strong, except if mod or back throw an exception or back(e) fails to properly restore the element or there is a throwing user-provided operation after invoking back(e), in which cases the modified element is erased. If back throws inside the handling code executing after some other user-provided operation has thrown, it is the exception generated by back that is rethrown.

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

Requires: key_from_value is a read/write Key Extractor from value_type. mod is a unary function object accepting arguments of type key_type&. position is a valid dereferenceable iterator of the index. The execution of mod(k), where k is the key of the element pointed to by position, does not invoke any operation of the multi_index_container after k is directly modified or, before modification, if the operation would invalidate position.
Effects: Equivalent to modify(position,mod'), with mod' defined in such a way that mod'(x) is the same as mod(key(x)), where key is the internal KeyFromValue object of the index.

template<typename Modifier,typename Rollback>

bool modify_key(iterator position,Modifier mod,Rollback back);

Requires: key_from_value is a read/write Key Extractor from value_type. mod and back are unary function objects accepting arguments of type key_type&. position is a valid dereferenceable iterator of the index. The execution of mod(k), where k is the key of the element pointed to by position, does not invoke any operation of the multi_index_container after k is directly modified or, before modification, if the operation would invalidate position. back(k) does not invoke any operation of the multi_index_container.
Effects: Equivalent to modify(position,mod',back'), with mod' and back defined in such a way that mod'(x) is the same as mod(key(x)) and back'(x) is the same as back(key(x)), where key is the internal KeyFromValue object of the index.

template<typename Index> void merge(Index&& x);

Requires: x is a non-const reference to an index of a node-compatible multi_index_container. get_allocator()==x.get_allocator().
Effects:
merge(x,x.begin(),x.end());


template<typename Index> std::pair<iterator,bool> merge(

  Index&& x,typename std::remove_reference_t<Index>::const_iterator i);

Requires: x is a non-const reference to an index of a node-compatible multi_index_container. get_allocator()==x.get_allocator(). i is a valid dereferenceable iterator of x.
Effects: Does nothing if the source and destination containers are the same; otherwise, transfers the node of the element referred to by i into the multi_index_container to which the destination index belongs if

the index is non-unique OR no other element exists with equivalent key,

AND insertion is allowed by all other indices of the multi_index_container.

Note that no element is copied or destroyed in the process.
Postconditions: If transfer succeeds, for any index in the source container having the same iterator/const_iterator types as the corresponding index in the destination container, iterators referring to *i remain valid and behave as iterators of the destination index.
Returns: The return value is a pair p. p.second is true if and only if transfer took place or the source and destination containers are the same. If p.second is true, p.first points to *i; 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: If the source and destination containers are the same, constant; otherwise, O(I(n)+D(x.size())).
Exception safety: If the source and destination containers are the same, nothrow; otherwise strong.


template<typename Index> void merge(

  Index&& x,

  typename std::remove_reference_t<Index>::const_iterator first,

  typename std::remove_reference_t<Index>::const_iterator last);

Requires: x is a non-const reference to an index of a node-compatible multi_index_container. get_allocator()==x.get_allocator(). [first,last) is a valid range of x.
Effects: Does nothing if the source and destination containers are the same; otherwise, for each node in [first,last), in this order, the node is transferred to the multi_index_container to which the destination index belongs if

the index is non-unique OR no other element exists with equivalent key,

AND insertion is allowed by all other indices of the multi_index_container.

Note that no element is copied or destroyed in the process.
Postconditions: For any index in the source container having the same iterator/const_iterator types as the corresponding index in the destination container, iterators referring to the transferred elements remain valid and behave as iterators of the destination index.
Complexity: If the source and destination containers are the same, constant; otherwise, O(m*(I(n+m)+D(x.size()))), where m is the number of elements in [first, last).
Exception safety: If the source and destination containers are the same, nothrow; otherwise basic.

Observers

Apart from standard hash_function and key_eq, hashed indices have a member function for retrieving the internal key extractor used.

key_from_value key_extractor()const;

Returns a copy of the key_from_value object used to construct the index.
Complexity: Constant.

Lookup

Hashed indices provide the full lookup functionality required by [unord.req], namely find, count, and equal_range. Additionally, these member functions are templatized to allow for non-standard arguments, so extending the types of search operations allowed. The kind of arguments permissible when invoking the lookup member functions is defined by the following concept.

Consider a pair (Hash, Pred) where Hash is a hash functor over values of type Key and Pred is a binary predicate inducing an equivalence relation on Key, with the additional constraint that equivalent keys have the same hash value. A triplet of types (CompatibleKey, CompatibleHash, CompatiblePred) is said to be a compatible extension of (Hash, Pred) if

CompatibleHash is a hash functor on values of type CompatibleKey,
CompatiblePred is a binary predicate over (Key, CompatibleKey),
CompatiblePred is a binary predicate over (CompatibleKey, Key),
if c_eq(ck,k1) then c_eq(k1,ck),
if c_eq(ck,k1) and eq(k1,k2) then c_eq(ck,k2),
if c_eq(ck,k1) and c_eq(ck,k2) then eq(k1,k2),
if c_eq(ck,k1) then c_hash(ck)==hash(k1),

for every c_hash of type CompatibleHash, c_eq of type CompatiblePred, hash of type Hash, eq of type Pred, ck of type CompatibleKey and k1, k2 of type Key.

Additionally, a type CompatibleKey is said to be a compatible key of (Hash, Pred) if (CompatibleKey, Hash, Pred) is a compatible extension of (Hash, Pred). This implies that Hash and Pred accept arguments of type CompatibleKey, which usually means they have several overloads of their corresponding operator() member functions.

In the context of a compatible extension or a compatible key, the expression "equivalent key" takes on its obvious interpretation.

template<typename CompatibleKey> iterator find(const CompatibleKey& x)const;

Requires: CompatibleKey is a compatible key of (hasher, key_equal).
Effects: Returns a pointer to an element whose key is equivalent to x, or end() if such an element does not exist.
Complexity: Average case O(1) (constant), worst case O(n_dist).

template<

  typename CompatibleKey,typename CompatibleHash, typename CompatiblePred

>

iterator find(

  const CompatibleKey& x,

  const CompatibleHash& hash,const CompatiblePred& eq)const;

Requires: (CompatibleKey, CompatibleHash, CompatiblePred) is a compatible extension of (hasher, key_equal).
Effects: Returns a pointer to an element whose key is equivalent to x, or end() if such an element does not exist.
Complexity: Average case O(1) (constant), worst case O(n_dist).

template<typename CompatibleKey>

size_type count(const CompatibleKey& x)const;

Requires: CompatibleKey is a compatible key of (hasher, key_equal).
Effects: Returns the number of elements with key equivalent to x.
Complexity: Average case O(count(x)), worst case O(count(x)+n_dist).

template<

  typename CompatibleKey,typename CompatibleHash, typename CompatiblePred

>

size_type count(

  const CompatibleKey& x,

  const CompatibleHash& hash,const CompatiblePred& eq)const;

Requires: (CompatibleKey, CompatibleHash, CompatiblePred) is a compatible extension of (hasher, key_equal).
Effects: Returns the number of elements with key equivalent to x.
Complexity: Average case O(count(x,hash,eq)), worst case O(count(x,hash,eq)+n_dist).

template<typename CompatibleKey>

bool contains(const CompatibleKey& x)const;

Requires: CompatibleKey is a compatible key of (hasher, key_equal).
Effects: Returns true iff there is some element with key equivalent to x.
Complexity: Average case O(1) (constant), worst case O(n_dist).

template<

  typename CompatibleKey,typename CompatibleHash, typename CompatiblePred

>

bool contains(

  const CompatibleKey& x,

  const CompatibleHash& hash,const CompatiblePred& eq)const;

Requires: (CompatibleKey, CompatibleHash, CompatiblePred) is a compatible extension of (hasher, key_equal).
Effects: Returns true iff there is some element with key equivalent to x.
Complexity: Average case O(1) (constant), worst case O(n_dist).

template<typename CompatibleKey>

std::pair<iterator,iterator> equal_range(const CompatibleKey& x)const;

Requires: CompatibleKey is a compatible key of (hasher, key_equal).
Effects: Returns a range containing all elements with keys equivalent to x (and only those), or (end(),end()) if no such elements exist.
Complexity: Average case O(1) (constant), worst case O(n_dist).

template<

  typename CompatibleKey,typename CompatibleHash, typename CompatiblePred

>

std::pair<iterator,iterator> equal_range(

  const CompatibleKey& x,

  const CompatibleHash& hash,const CompatiblePred& eq)const;

Requires: (CompatibleKey, CompatibleHash, CompatiblePred) is a compatible extension of (hasher, key_equal).
Effects: Returns a range containing all elements with keys equivalent to x (and only those), or (end(),end()) if no such elements exist.
Complexity: Average case O(1) (constant), worst case O(n_dist).

Bucket interface

local_iterator       local_iterator_to(const value_type& x);

const_local_iterator local_iterator_to(const value_type& x)const;

Requires: x is a reference to an element of the container.
Returns: An iterator to x.
Complexity: Constant.
Exception safety: nothrow.

Hash policy

void rehash(size_type n);

Effects: Increases if necessary the number of internal buckets so that size()/bucket_count() does not exceed the maximum load factor, and bucket_count()>=n.
Postconditions: Validity of iterators and references to the elements contained is preserved.
Complexity: O(m), where m is the number of non-equivalent elements in the index.
Exception safety: Strong.

void reserve(size_type n);

Effects:

rehash(std::ceil(n/max_load_factor()));

Comparison

template<implementation defined>

bool operator==(const index class name& x,const index class name& y);

Requires: x.key_extractor(), x.hash_function() and x.key_eq() have the same behavior as the corresponding objects in y. For any two elements e1, e2 in x or y, if e1==e2 then their keys are equivalent.
Returns: true iff x and y have the same size and for each key k present in x the range x.equal_range(k) is equal (considering the == operator of value_type) to y.equal_range(k) under permutations of the elements.
Complexity: Let k₁,...,k_m be the different keys present in x:

If, for each k_i, x.equal_range(k_i) is arranged in the same order as y.equal_range(k_i), average case is O(x.size()), worst case O(x.size()²).

Otherwise, average case is O(Σ(x.count(k_i)²)), worst case O(x.size()²).

(For unique indices, the formulas above reduce to average case O(x.size()), worst case O(x.size()²).)

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 hashed 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: Additionally to the general requirements, key_eq() must be serialization-compatible with m.get().key_eq(), where i is the position of the hashed index in the container.
Postconditions: On successful loading, the range [begin(), end()) contains restored copies of every element in [m.get().begin(), m.get().end()), though not necessarily in the same order.

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 successful 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 vice versa.

Operation: saving of a local_iterator or const_local_iterator it to an output archive (XML archive) ar.

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

Operation: loading of a local_iterator or const_local_iterator it' from an input archive (XML archive) ar.

Postconditions: On successful loading, if it was dereferenceable then *it' is the restored copy of *it; if it was m.get().end(n) for some n, then it'==m'.get().end(n) (where m is the original multi_index_container, m' its restored copy and i is the ordinal of the index.)
Note: It is allowed that it be a const_local_iterator and the restored it' a local_iterator, or vice versa.

Ranked indices

Boost.MultiIndex reference

Sequenced indices

Revised August 30th 2021

Boost.MultiIndex Hashed indices reference

Contents

Header "boost/multi_index/hashed_index_fwd.hpp" synopsis

Header "boost/multi_index/hashed_index.hpp" synopsis

Header `"boost/multi_index/hashed_index_fwd.hpp"` synopsis

Header `"boost/multi_index/hashed_index.hpp"` synopsis