concurrent_unordered_map and concurrent_unordered_multimap Template Classes

Summary

Template classes for associative containers that supports concurrent insertion and traversal.

Syntax

template <typename Key,
             typename Element,
             typename Hasher = tbb_hash<Key>,
             typename Equality = std::equal_to<Key>,
             typename Allocator = tbb::tbb_allocator<std::pair<const Key, Element> > >
class concurrent_unordered_map;
template <typename Key,
             typename Element,
             typename Hasher = tbb_hash<Key>,
             typename Equality = std::equal_to<Key>,
             typename Allocator = tbb::tbb_allocator<std::pair<const Key, Element> > >
class concurrent_unordered_multimap;

Header

#include "tbb/concurrent_unordered_map.h"

Description

concurrent_unordered_map and concurrent_unordered_multimap support concurrent insertion and traversal, but not concurrent erasure. The interfaces have no visible locking. They may hold locks internally, but never while calling user-defined code. They have semantics similar to the C++11 std::unordered_map and std::unordered_multimap respectively, except as follows:

  • Some methods requiring C++11 language features (e.g. rvalue references) are omitted.

  • The erase methods are prefixed with unsafe_, to indicate that they are not concurrency safe.

  • Bucket methods are prefixed with unsafe_ as a reminder that they are not concurrency safe with respect to insertion.

  • The insert methods may create a temporary pair that is destroyed if another thread inserts the same key concurrently.

  • Like std::list, insertion of new items does not invalidate any iterators, nor change the order of items already in the map. Insertion and traversal may be concurrent.

  • The iterator types iterator and const_iterator are of the forward iterator category.

  • Insertion does not invalidate or update the iterators returned by equal_range, so insertion may cause non-equal items to be inserted at the end of the range. However, the first iterator will nonethless point to the equal item even after an insertion operation.

The following table lists the key differences between classes concurrent_unordered_map and concurrent_hash_map and concurrent_unordered_multimap.

Class

Key Difference

concurrent_hash_map

Permits concurrent erasure, and has built-in locking

concurrent_unordered_map

  • Permits concurrent traversal and insertion, no visible locking, closely resembles the C++11 unordered_map.

  • Has the [] and at accessors.

concurrent_unordered_multimap

  • More than one <key,value> pair with the same key value may be inserted in concurrent_unordered_multimap.

  • find returns the first <key,value> pair with a matching key at the point the search is made, though concurrent accesses to the container may insert other pairs with the same key before the one returned.

Caution

As with any form of hash table, keys that are equal must have the same hash code, and the ideal hash function distributes keys uniformly across the hash code space.

Members of both concurrent_unordered_map and concurrent_unordered_multimap

In the following synopsis, methods shown in bold font may be concurrently invoked. For example, three different threads can concurrently call methods insert, begin, and size. Their results might be non-deterministic. For example, the result from size might correspond to before or after the insertion.

public:
    // types
    typedef Key key_type;
    typedef std::pair<const Key, T> value_type;
    typedef Element mapped_type;
    typedef Hash hasher;
    typedef Equality key_equal;
    typedef Alloc allocator_type;
    typedef typename allocator_type::pointer pointer;
    typedef typename allocator_type::const_pointer const_pointer;
    typedef typename allocator_type::reference reference;
    typedef typename allocator_type::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;

    allocator_type get_allocator() const;

    // size and capacity
    bool empty() const;     // May take linear time!
    size_type size() const; // May take linear time!
    size_type max_size() const;

    // iterators
    iterator begin();
    const_iterator begin() const;
    iterator end();
    const_iterator end() const;
    const_iterator cbegin() const;
    const_iterator cend() const;
  
    // modifiers
    std::pair<iterator, bool> insert(const value_type& x);
    iterator insert(const_iterator hint, const value_type& x);
    template<class InputIterator>
    void insert(InputIterator first, InputIterator last);
    // C++11 specific
    std::pair<iterator, bool> insert(value_type&& x);
    iterator insert(const_iterator hint, value_type&& x);    
    void insert(std::initializer_list<value_type> il);
    // C++11 specific
    template<typename... Args>
    std::pair<iterator, bool> emplace(Args&&... args);
    template<typename... Args>
    iterator emplace_hint(const_iterator hint, Args&&... args);
      
    iterator unsafe_erase(const_iterator position);
    size_type unsafe_erase(const key_type& k);
    iterator unsafe_erase(const_iterator first, const_iterator last);
    void clear();

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // lookup
    iterator find(const key_type& k);
    const_iterator find(const key_type& k) const;
    size_type count(const key_type& k) const;
    std::pair<iterator, iterator> equal_range(const key_type& k);
    std::pair<const_iterator, const_iterator> equal_range(const key_type& k) const;

    // parallel iteration
    typedef implementation-defined range_type;
    typedef implementation-defined const_range_type;
    range_type range();
    const_range_type range() const;

    // bucket interface - for debugging
    size_type unsafe_bucket_count() const;
    size_type unsafe_max_bucket_count() const;
    size_type unsafe_bucket_size(size_type n);
    size_type unsafe_bucket(const key_type& k) const;
    local_iterator unsafe_begin(size_type n);
    const_local_iterator unsafe_begin(size_type n) const;
    local_iterator unsafe_end(size_type n);
    const_local_iterator unsafe_end(size_type n) const;
    const_local_iterator unsafe_cbegin(size_type n) const;
    const_local_iterator unsafe_cend(size_type n) const;

    // hash policy
    float load_factor() const;
    float max_load_factor() const;
    void max_load_factor(float z);
    void rehash(size_type n);

Members of concurrent_unordered_map

In the following synopsis, methods in bold may be concurrently invoked.

public:
    // construct/destroy/copy
    explicit concurrent_unordered_map(size_type n = implementation-defined,
    const Hasher& hf = hasher(),
    const key_equal& eql = key_equal(),
    const allocator_type& a = allocator_type());
    template <typename InputIterator>
    concurrent_unordered_map(InputIterator first, InputIterator last,
                              size_type n = implementation-defined,
                              const hasher& hf = hasher(),
                              const key_equal& eql = key_equal(),
                              const allocator_type& a = allocator_type());
    concurrent_unordered_map(const concurrent_unordered_map&);
    concurrent_unordered_map(const Alloc&);
    concurrent_unordered_map(const concurrent_unordered_map&, const Alloc&);
    //C++11 specific
    concurrent_unordered_map(concurrent_unordered_map&&);
    concurrent_unordered_map(concurrent_unordered_map&&, const Allocator&);
    concurrent_unordered_map(std::initializer_list<value_type> il,
                              size_type n = implementation-defined,
                              const Hasher& hf = hasher(),
                              const key_equal& eql = key_equal(),
                              const allocator_type& a = allocator_type());
    ~concurrent_unordered_map();

    concurrent_unordered_map& operator=(const concurrent_unordered_map&);
    //C++11 specific
    concurrent_unordered_map& operator=(concurrent_unordered_map&&);
    concurrent_unordered_map& operator=(std::initializer_list<value_type> il);

    void swap(concurrent_unordered_map&);

    mapped_type& operator[](const key_type& k);
    mapped_type& at( const key_type& k );
    const mapped_type& at(const key_type& k) const;

Members of concurrent_unordered_multimap

public:
    // construct/destroy/copy
    explicit concurrent_unordered_multimap(size_type n = implementation-defined,
                                           const Hasher& hf = hasher(),
                                           const key_equal& eql = key_equal(),
                                           const allocator_type& a = allocator_type());
    template <typename InputIterator>
    concurrent_unordered_multimap( InputIterator first, InputIterator last,
                                   size_type n = implementation-defined,
                                   const hasher& hf = hasher(),
                                   const key_equal& eql = key_equal(),
                                   const allocator_type& a = allocator_type());
    concurrent_unordered_multimap(const concurrent_unordered_multimap&);
    concurrent_unordered_multimap(const Alloc&);
    concurrent_unordered_multimap(const concurrent_unordered_multimap&, const Alloc&);
    //C++11 specific
    concurrent_unordered_multimap(concurrent_unordered_multimap&&);
    concurrent_unordered_multimap(concurrent_unordered_multimap&&, const Allocator&);
    concurrent_unordered_multimap(std::initializer_list<value_type> il,
                                  size_type n = implementation-defined,
                                  const Hasher& hf = hasher(),
                                  const key_equal& eql = key_equal(),
                                  const allocator_type& a = allocator_type());
    ~concurrent_unordered_multimap();

    concurrent_unordered_multimap& operator=(const concurrent_unordered_multimap&);
    //C++11 specific
    concurrent_unordered_multimap& operator=(concurrent_unordered_multimap&&);
    concurrent_unordered_multimap& operator=(std::initializer_list<value_type> il);

    void swap(concurrent_unordered_multimap&);
For more complete information about compiler optimizations, see our Optimization Notice.