[#concurrent_flat_set] Class Template concurrent_flat_set :idprefix: concurrent_flat_set_ `boost::concurrent_flat_set` — A hash table that stores unique values and allows for concurrent element insertion, erasure, lookup and access without external synchronization mechanisms. Even though it acts as a container, `boost::concurrent_flat_set` does not model the standard C++ https://en.cppreference.com/w/cpp/named_req/Container[Container^] concept. In particular, iterators and associated operations (`begin`, `end`, etc.) are not provided. Element access is done through user-provided _visitation functions_ that are passed to `concurrent_flat_set` operations where they are executed internally in a controlled fashion. Such visitation-based API allows for low-contention concurrent usage scenarios. The internal data structure of `boost::concurrent_flat_set` is similar to that of `boost::unordered_flat_set`. As a result of its using open-addressing techniques, `value_type` must be move-constructible and pointer stability is not kept under rehashing. Synopsis --- Description *Template Parameters* _Key_ `Key` must be https://en.cppreference.com/w/cpp/named_req/MoveInsertable[MoveInsertable^] into the container and https://en.cppreference.com/w/cpp/named_req/Erasable[Erasable^] from the container. _Hash_ A unary function object type that acts a hash function for a `Key`. It takes a single argument of type `Key` and returns a value of type `std::size_t`. _Pred_ A binary function object that induces an equivalence relation on values of type `Key`. It takes two arguments of type `Key` and returns a value of type `bool`. _Allocator_ An allocator whose value type is the same as the table\'s value type. `std::allocator_traits<Allocator>::pointer` and `std::allocator_traits<Allocator>::const_pointer` must be convertible to/from `value_type*` and `const value_type*`, respectively. The elements of the table are held into an internal _bucket array_. An element is inserted into a bucket determined by its hash code, but if the bucket is already occupied (a _collision_), an available one in the vicinity of the original position is used. The size of the bucket array can be automatically increased by a call to `insert`/`emplace`, or as a result of calling `rehash`/`reserve`. The _load factor_ of the table (number of elements divided by number of buckets) is never greater than `max_load_factor()`, except possibly for small sizes where the implementation may decide to allow for higher loads. If `link:../../../../../container_hash/doc/html/hash.html#ref_hash_is_avalanchinghash[hash_is_avalanching]<Hash>::value` is `true`, the hash function is used as-is; otherwise, a bit-mixing post-processing stage is added to increase the quality of hashing at the expense of extra computational cost. --- Concurrency Requirements and Guarantees Concurrent invocations of `operator()` on the same const instance of `Hash` or `Pred` are required to not introduce data races. For `Alloc` being either `Allocator` or any allocator type rebound from `Allocator`, concurrent invocations of the following operations on the same instance `al` of `Alloc` are required to not introduce data races: Copy construction from `al` of an allocator rebound from `Alloc` `std::allocator_traits<Alloc>::allocate` `std::allocator_traits<Alloc>::deallocate` `std::allocator_traits<Alloc>::construct` `std::allocator_traits<Alloc>::destroy` In general, these requirements on `Hash`, `Pred` and `Allocator` are met if these types are not stateful or if the operations only involve constant access to internal data members. With the exception of destruction, concurrent invocations of any operation on the same instance of a `concurrent_flat_set` do not introduce data races — that is, they are thread-safe. If an operation *op* is explicitly designated as _blocking on_ `x`, where `x` is an instance of a `boost::concurrent_flat_set`, prior blocking operations on `x` synchronize with *op*. So, blocking operations on the same `concurrent_flat_set` execute sequentially in a multithreaded scenario. An operation is said to be _blocking on rehashing of_ ``__x__`` if it blocks on `x` only when an internal rehashing is issued. When executed internally by a `boost::concurrent_flat_set`, the following operations by a user-provided visitation function on the element passed do not introduce data races: Read access to the element. Non-mutable modification of the element. Mutable modification of the element: ** Within a container function accepting two visitation functions, always for the first function. ** Within a non-const container function whose name does not contain `cvisit`, for the last (or only) visitation function. Any `boost::concurrent_flat_set operation` that inserts or modifies an element `e` synchronizes with the internal invocation of a visitation function on `e`. Visitation functions executed by a `boost::concurrent_flat_set` `x` are not allowed to invoke any operation on `x`; invoking operations on a different `boost::concurrent_flat_set` instance `y` is allowed only if concurrent outstanding operations on `y` do not access `x` directly or indirectly. --- Configuration Macros `BOOST_UNORDERED_DISABLE_REENTRANCY_CHECK` In debug builds (more precisely, when link:../../../../../assert/doc/html/assert.html#boost_assert_is_void[`BOOST_ASSERT_IS_VOID`^] is not defined), __container reentrancies__ (illegaly invoking an operation on `m` from within a function visiting elements of `m`) are detected and signalled through `BOOST_ASSERT_MSG`. When run-time speed is a concern, the feature can be disabled by globally defining this macro. --- `BOOST_UNORDERED_ENABLE_STATS` Globally define this macro to enable xref:reference/stats.adoc#stats[statistics calculation] for the table. Note that this option decreases the overall performance of many operations. --- Constants ```cpp static constexpr size_type bulk_visit_size; ``` Chunk size internally used in xref:concurrent_flat_set_bulk_visit[bulk visit] operations. Constructors Default Constructor ```c++ concurrent_flat_set(); ``` Constructs an empty table using `hasher()` as the hash function, `key_equal()` as the key equality predicate and `allocator_type()` as the allocator. Postconditions:;; `size() == 0` Requires:;; If the defaults are used, `hasher`, `key_equal` and `allocator_type` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^]. --- Bucket Count Constructor ```c++ explicit concurrent_flat_set(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); ``` Constructs an empty table with at least `n` buckets, using `hf` as the hash function, `eql` as the key equality predicate, and `a` as the allocator. Postconditions:;; `size() == 0` Requires:;; If the defaults are used, `hasher`, `key_equal` and `allocator_type` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^]. --- Iterator Range Constructor Constructs an empty table with at least `n` buckets, using `hf` as the hash function, `eql` as the key equality predicate and `a` as the allocator, and inserts the elements from `[f, l)` into it. Requires:;; If the defaults are used, `hasher`, `key_equal` and `allocator_type` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^]. --- Copy Constructor ```c++ concurrent_flat_set(concurrent_flat_set const& other); ``` The copy constructor. Copies the contained elements, hash function, predicate and allocator. If `Allocator::select_on_container_copy_construction` exists and has the right signature, the allocator will be constructed from its result. Requires:;; `value_type` is copy constructible Concurrency:;; Blocking on `other`. --- Move Constructor ```c++ concurrent_flat_set(concurrent_flat_set&& other); ``` The move constructor. The internal bucket array of `other` is transferred directly to the new table. The hash function, predicate and allocator are moved-constructed from `other`. If statistics are xref:concurrent_flat_set_boost_unordered_enable_stats[enabled], transfers the internal statistical information from `other` and calls `other.reset_stats()`. Concurrency:;; Blocking on `other`. --- Iterator Range Constructor with Allocator ```c++ template<class InputIterator> concurrent_flat_set(InputIterator f, InputIterator l, const allocator_type& a); ``` Constructs an empty table using `a` as the allocator, with the default hash function and key equality predicate and inserts the elements from `[f, l)` into it. Requires:;; `hasher`, `key_equal` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^]. --- Allocator Constructor ```c++ explicit concurrent_flat_set(Allocator const& a); ``` Constructs an empty table, using allocator `a`. --- Copy Constructor with Allocator ```c++ concurrent_flat_set(concurrent_flat_set const& other, Allocator const& a); ``` Constructs a table, copying ``other``\'s contained elements, hash function, and predicate, but using allocator `a`. Concurrency:;; Blocking on `other`. --- Move Constructor with Allocator ```c++ concurrent_flat_set(concurrent_flat_set&& other, Allocator const& a); ``` If `a == other.get_allocator()`, the elements of `other` are transferred directly to the new table; otherwise, elements are moved-constructed from those of `other`. The hash function and predicate are moved-constructed from `other`, and the allocator is copy-constructed from `a`. If statistics are xref:concurrent_flat_set_boost_unordered_enable_stats[enabled], transfers the internal statistical information from `other` iff `a == other.get_allocator()`, and always calls `other.reset_stats()`. Concurrency:;; Blocking on `other`. --- Move Constructor from unordered_flat_set ```c++ concurrent_flat_set(unordered_flat_set<Key, Hash, Pred, Allocator>&& other); ``` Move construction from a xref:#unordered_flat_set[`unordered_flat_set`]. The internal bucket array of `other` is transferred directly to the new container. The hash function, predicate and allocator are moved-constructed from `other`. If statistics are xref:concurrent_flat_set_boost_unordered_enable_stats[enabled], transfers the internal statistical information from `other` and calls `other.reset_stats()`. Complexity:;; O(`bucket_count()`) --- Initializer List Constructor Constructs an empty table with at least `n` buckets, using `hf` as the hash function, `eql` as the key equality predicate and `a`, and inserts the elements from `il` into it. Requires:;; If the defaults are used, `hasher`, `key_equal` and `allocator_type` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^]. --- Bucket Count Constructor with Allocator ```c++ concurrent_flat_set(size_type n, allocator_type const& a); ``` Constructs an empty table with at least `n` buckets, using `hf` as the hash function, the default hash function and key equality predicate and `a` as the allocator. Postconditions:;; `size() == 0` Requires:;; `hasher` and `key_equal` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^]. --- Bucket Count Constructor with Hasher and Allocator ```c++ concurrent_flat_set(size_type n, hasher const& hf, allocator_type const& a); ``` Constructs an empty table with at least `n` buckets, using `hf` as the hash function, the default key equality predicate and `a` as the allocator. Postconditions:;; `size() == 0` Requires:;; `key_equal` needs to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^]. --- Iterator Range Constructor with Bucket Count and Allocator Constructs an empty table with at least `n` buckets, using `a` as the allocator and default hash function and key equality predicate, and inserts the elements from `[f, l)` into it. Requires:;; `hasher`, `key_equal` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^]. --- Iterator Range Constructor with Bucket Count and Hasher Constructs an empty table with at least `n` buckets, using `hf` as the hash function, `a` as the allocator, with the default key equality predicate, and inserts the elements from `[f, l)` into it. Requires:;; `key_equal` needs to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^]. --- initializer_list Constructor with Allocator ```c++ concurrent_flat_set(std::initializer_list<value_type> il, const allocator_type& a); ``` Constructs an empty table using `a` and default hash function and key equality predicate, and inserts the elements from `il` into it. Requires:;; `hasher` and `key_equal` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^]. --- initializer_list Constructor with Bucket Count and Allocator ```c++ concurrent_flat_set(std::initializer_list<value_type> il, size_type n, const allocator_type& a); ``` Constructs an empty table with at least `n` buckets, using `a` and default hash function and key equality predicate, and inserts the elements from `il` into it. Requires:;; `hasher` and `key_equal` need to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^]. --- initializer_list Constructor with Bucket Count and Hasher and Allocator ```c++ concurrent_flat_set(std::initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a); ``` Constructs an empty table with at least `n` buckets, using `hf` as the hash function, `a` as the allocator and default key equality predicate,and inserts the elements from `il` into it. Requires:;; `key_equal` needs to be https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^]. --- Destructor ```c++ ~concurrent_flat_set(); ``` Note:;; The destructor is applied to every element, and all memory is deallocated --- Assignment Copy Assignment ```c++ concurrent_flat_set& operator=(concurrent_flat_set const& other); ``` The assignment operator. Destroys previously existing elements, copy-assigns the hash function and predicate from `other`, copy-assigns the allocator from `other` if `Alloc::propagate_on_container_copy_assignment` exists and `Alloc::propagate_on_container_copy_assignment::value` is `true`, and finally inserts copies of the elements of `other`. Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/CopyInsertable[CopyInsertable^] Concurrency:;; Blocking on `*this` and `other`. --- Move Assignment ```c++ concurrent_flat_set& operator=(concurrent_flat_set&& other) noexcept(boost::allocator_traits<Allocator>::is_always_equal::value || boost::allocator_traits<Allocator>::propagate_on_container_move_assignment::value); ``` The move assignment operator. Destroys previously existing elements, swaps the hash function and predicate from `other`, and move-assigns the allocator from `other` if `Alloc::propagate_on_container_move_assignment` exists and `Alloc::propagate_on_container_move_assignment::value` is `true`. If at this point the allocator is equal to `other.get_allocator()`, the internal bucket array of `other` is transferred directly to `*this`; otherwise, inserts move-constructed copies of the elements of `other`. If statistics are xref:concurrent_flat_set_boost_unordered_enable_stats[enabled], transfers the internal statistical information from `other` iff the final allocator is equal to `other.get_allocator()`, and always calls `other.reset_stats()`. Concurrency:;; Blocking on `*this` and `other`. --- Initializer List Assignment ```c++ concurrent_flat_set& operator=(std::initializer_list<value_type> il); ``` Assign from values in initializer list. All previously existing elements are destroyed. Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/CopyInsertable[CopyInsertable^] Concurrency:;; Blocking on `*this`. --- Visitation [c]visit ```c++ template<class F> size_t visit(const key_type& k, F f); template<class F> size_t visit(const key_type& k, F f) const; template<class F> size_t cvisit(const key_type& k, F f) const; template<class K, class F> size_t visit(const K& k, F f); template<class K, class F> size_t visit(const K& k, F f) const; template<class K, class F> size_t cvisit(const K& k, F f) const; ``` If an element `x` exists with key equivalent to `k`, invokes `f` with a const reference to `x`. Returns:;; The number of elements visited (0 or 1). Notes:;; The `template<class K, class F>` overloads only participate in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type. --- Bulk visit ```c++ template<class FwdIterator, class F> size_t visit(FwdIterator first, FwdIterator last, F f); template<class FwdIterator, class F> size_t visit(FwdIterator first, FwdIterator last, F f) const; template<class FwdIterator, class F> size_t cvisit(FwdIterator first, FwdIterator last, F f) const; ``` For each element `k` in the range [`first`, `last`), if there is an element `x` in the container with key equivalent to `k`, invokes `f` with a const reference to `x`. Although functionally equivalent to individually invoking xref:concurrent_flat_set_cvisit[`[c\\]visit`] for each key, bulk visitation performs generally faster due to internal streamlining optimizations. It is advisable that `std::distance(first,last)` be at least xref:#concurrent_flat_set_constants[`bulk_visit_size`] to enjoy a performance gain: beyond this size, performance is not expected to increase further. Requires:;; `FwdIterator` is a https://en.cppreference.com/w/cpp/named_req/ForwardIterator[LegacyForwardIterator^] ({cpp}11 to {cpp}17), or satisfies https://en.cppreference.com/w/cpp/iterator/forward_iterator[std::forward_iterator^] ({cpp}20 and later). For `K` = `std::iterator_traits<FwdIterator>::value_type`, either `K` is `key_type` or else `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs. In the latter case, the library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type. Returns:;; The number of elements visited. --- [c]visit_all ```c++ template<class F> size_t visit_all(F f); template<class F> size_t visit_all(F f) const; template<class F> size_t cvisit_all(F f) const; ``` Successively invokes `f` with const references to each of the elements in the table. Returns:;; The number of elements visited. --- Parallel [c]visit_all ```c++ template<class ExecutionPolicy, class F> void visit_all(ExecutionPolicy&& policy, F f); template<class ExecutionPolicy, class F> void visit_all(ExecutionPolicy&& policy, F f) const; template<class ExecutionPolicy, class F> void cvisit_all(ExecutionPolicy&& policy, F f) const; ``` Invokes `f` with const references to each of the elements in the table. Execution is parallelized according to the semantics of the execution policy specified. Throws:;; Depending on the exception handling mechanism of the execution policy used, may call `std::terminate` if an exception is thrown within `f`. Notes:;; Only available in compilers supporting C++17 parallel algorithms. + + These overloads only participate in overload resolution if `std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>` is `true`. + + Unsequenced execution policies are not allowed. --- [c]visit_while ```c++ template<class F> bool visit_while(F f); template<class F> bool visit_while(F f) const; template<class F> bool cvisit_while(F f) const; ``` Successively invokes `f` with const references to each of the elements in the table until `f` returns `false` or all the elements are visited. Returns:;; `false` iff `f` ever returns `false`. --- Parallel [c]visit_while ```c++ template<class ExecutionPolicy, class F> bool visit_while(ExecutionPolicy&& policy, F f); template<class ExecutionPolicy, class F> bool visit_while(ExecutionPolicy&& policy, F f) const; template<class ExecutionPolicy, class F> bool cvisit_while(ExecutionPolicy&& policy, F f) const; ``` Invokes `f` with const references to each of the elements in the table until `f` returns `false` or all the elements are visited. Execution is parallelized according to the semantics of the execution policy specified. Returns:;; `false` iff `f` ever returns `false`. Throws:;; Depending on the exception handling mechanism of the execution policy used, may call `std::terminate` if an exception is thrown within `f`. Notes:;; Only available in compilers supporting C++17 parallel algorithms. + + These overloads only participate in overload resolution if `std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>` is `true`. + + Unsequenced execution policies are not allowed. + + Parallelization implies that execution does not necessary finish as soon as `f` returns `false`, and as a result `f` may be invoked with further elements for which the return value is also `false`. --- Size and Capacity empty ```c++ [[nodiscard]] bool empty() const noexcept; ``` Returns:;; `size() == 0` --- size ```c++ size_type size() const noexcept; ``` Returns:;; The number of elements in the table. Notes:;; In the presence of concurrent insertion operations, the value returned may not accurately reflect the true size of the table right after execution. --- max_size ```c++ size_type max_size() const noexcept; ``` Returns:;; `size()` of the largest possible table. --- Modifiers emplace ```c++ template<class... Args> bool emplace(Args&&... args); ``` Inserts an object, constructed with the arguments `args`, in the table if and only if there is no element in the table with an equivalent key. Requires:;; `value_type` is constructible from `args`. Returns:;; `true` if an insert took place. Concurrency:;; Blocking on rehashing of `*this`. Notes:;; Invalidates pointers and references to elements if a rehashing is issued. --- Copy Insert ```c++ bool insert(const value_type& obj); ``` Inserts `obj` in the table if and only if there is no element in the table with an equivalent key. Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/CopyInsertable[CopyInsertable^]. Returns:;; `true` if an insert took place. + Concurrency:;; Blocking on rehashing of `*this`. Notes:;; Invalidates pointers and references to elements if a rehashing is issued. --- Move Insert ```c++ bool insert(value_type&& obj); ``` Inserts `obj` in the table if and only if there is no element in the table with an equivalent key. Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/MoveInsertable[MoveInsertable^]. Returns:;; `true` if an insert took place. Concurrency:;; Blocking on rehashing of `*this`. Notes:;; Invalidates pointers and references to elements if a rehashing is issued. --- Transparent Insert ```c++ template<class K> bool insert(K&& k); ``` Inserts an element constructed from `std::forward<K>(k)` in the container if and only if there is no element in the container with an equivalent key. Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/EmplaceConstructible[EmplaceConstructible^] from `k`. Returns:;; `true` if an insert took place. Concurrency:;; Blocking on rehashing of `*this`. Notes:;; Invalidates pointers and references to elements if a rehashing is issued. + + This overload only participates in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type. --- Insert Iterator Range ```c++ template<class InputIterator> size_type insert(InputIterator first, InputIterator last); ``` Equivalent to [listing,subs=\"+macros,+quotes\"] Returns:;; The number of elements inserted. --- Insert Initializer List ```c++ size_type insert(std::initializer_list<value_type> il); ``` Equivalent to [listing,subs=\"+macros,+quotes\"] Returns:;; The number of elements inserted. --- emplace_or_[c]visit ```c++ template<class... Args, class F> bool emplace_or_visit(Args&&... args, F&& f); template<class... Args, class F> bool emplace_or_cvisit(Args&&... args, F&& f); ``` Inserts an object, constructed with the arguments `args`, in the table if there is no element in the table with an equivalent key. Otherwise, invokes `f` with a const reference to the equivalent element. Requires:;; `value_type` is constructible from `args`. Returns:;; `true` if an insert took place. Concurrency:;; Blocking on rehashing of `*this`. Notes:;; Invalidates pointers and references to elements if a rehashing is issued. + + The interface is exposition only, as C++ does not allow to declare a parameter `f` after a variadic parameter pack. --- Copy insert_or_[c]visit ```c++ template<class F> bool insert_or_visit(const value_type& obj, F f); template<class F> bool insert_or_cvisit(const value_type& obj, F f); ``` Inserts `obj` in the table if and only if there is no element in the table with an equivalent key. Otherwise, invokes `f` with a const reference to the equivalent element. Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/CopyInsertable[CopyInsertable^]. Returns:;; `true` if an insert took place. + Concurrency:;; Blocking on rehashing of `*this`. Notes:;; Invalidates pointers and references to elements if a rehashing is issued. --- Move insert_or_[c]visit ```c++ template<class F> bool insert_or_visit(value_type&& obj, F f); template<class F> bool insert_or_cvisit(value_type&& obj, F f); ``` Inserts `obj` in the table if and only if there is no element in the table with an equivalent key. Otherwise, invokes `f` with a const reference to the equivalent element. Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/MoveInsertable[MoveInsertable^]. Returns:;; `true` if an insert took place. + Concurrency:;; Blocking on rehashing of `*this`. Notes:;; Invalidates pointers and references to elements if a rehashing is issued. --- Transparent insert_or_[c]visit ```c++ template<class K, class F> bool insert_or_visit(K&& k, F f); template<class K, class F> bool insert_or_cvisit(K&& k, F f); ``` Inserts an element constructed from `std::forward<K>(k)` in the container if and only if there is no element in the container with an equivalent key. Otherwise, invokes `f` with a const reference to the equivalent element. Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/EmplaceConstructible[EmplaceConstructible^] from `k`. Returns:;; `true` if an insert took place. Concurrency:;; Blocking on rehashing of `*this`. Notes:;; Invalidates pointers and references to elements if a rehashing is issued. + + These overloads only participate in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type. --- Insert Iterator Range or Visit ```c++ template<class InputIterator,class F> size_type insert_or_visit(InputIterator first, InputIterator last, F f); template<class InputIterator,class F> size_type insert_or_cvisit(InputIterator first, InputIterator last, F f); ``` Equivalent to [listing,subs=\"+macros,+quotes\"] Returns:;; The number of elements inserted. --- Insert Initializer List or Visit ```c++ template<class F> size_type insert_or_visit(std::initializer_list<value_type> il, F f); template<class F> size_type insert_or_cvisit(std::initializer_list<value_type> il, F f); ``` Equivalent to [listing,subs=\"+macros,+quotes\"] Returns:;; The number of elements inserted. --- emplace_and_[c]visit ```c++ template<class... Args, class F1, class F2> bool emplace_and_visit(Args&&... args, F1&& f1, F2&& f2); template<class... Args, class F1, class F2> bool emplace_and_cvisit(Args&&... args, F1&& f1, F2&& f2); ``` Inserts an object, constructed with the arguments `args`, in the table if there is no element in the table with an equivalent key, and then invokes `f1` with a const reference to the newly created element. Otherwise, invokes `f2` with a const reference to the equivalent element. Requires:;; `value_type` is constructible from `args`. Returns:;; `true` if an insert took place. Concurrency:;; Blocking on rehashing of `*this`. Notes:;; Invalidates pointers and references to elements if a rehashing is issued. + + The interface is exposition only, as C++ does not allow to declare parameters `f1` and `f2` after a variadic parameter pack. --- Copy insert_and_[c]visit ```c++ template<class F1, class F2> bool insert_and_visit(const value_type& obj, F1 f1, F2 f2); template<class F1, class F2> bool insert_and_cvisit(const value_type& obj, F1 f1, F2 f2); ``` Inserts `obj` in the table if and only if there is no element in the table with an equivalent key, and then invokes `f1` with a const reference to the newly created element. Otherwise, invokes `f2` with a const reference to the equivalent element. Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/CopyInsertable[CopyInsertable^]. Returns:;; `true` if an insert took place. + Concurrency:;; Blocking on rehashing of `*this`. Notes:;; Invalidates pointers and references to elements if a rehashing is issued. --- Move insert_and_[c]visit ```c++ template<class F1, class F2> bool insert_and_visit(value_type&& obj, F1 f1, F2 f2); template<class F1, class F2> bool insert_and_cvisit(value_type&& obj, F1 f1, F2 f2); ``` Inserts `obj` in the table if and only if there is no element in the table with an equivalent key, and then invokes `f1` with a const reference to the newly created element. Otherwise, invokes `f2` with a const reference to the equivalent element. Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/MoveInsertable[MoveInsertable^]. Returns:;; `true` if an insert took place. + Concurrency:;; Blocking on rehashing of `*this`. Notes:;; Invalidates pointers and references to elements if a rehashing is issued. --- Transparent insert_and_[c]visit ```c++ template<class K, class F1, class F2> bool insert_and_visit(K&& k, F1 f1, F2 f2); template<class K, class F1, class F2> bool insert_and_cvisit(K&& k, F1 f1, F2 f2); ``` Inserts an element constructed from `std::forward<K>(k)` in the container if and only if there is no element in the container with an equivalent key, and then invokes `f1` with a const reference to the newly created element. Otherwise, invokes `f2` with a const reference to the equivalent element. Requires:;; `value_type` is https://en.cppreference.com/w/cpp/named_req/EmplaceConstructible[EmplaceConstructible^] from `k`. Returns:;; `true` if an insert took place. Concurrency:;; Blocking on rehashing of `*this`. Notes:;; Invalidates pointers and references to elements if a rehashing is issued. + + These overloads only participate in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type. --- Insert Iterator Range and Visit ```c++ template<class InputIterator,class F1, class F2> size_type insert_and_visit(InputIterator first, InputIterator last, F1 f1, F2 f2); template<class InputIterator,class F1, class F2> size_type insert_and_cvisit(InputIterator first, InputIterator last, F1 f1, F2 f2); ``` Equivalent to [listing,subs=\"+macros,+quotes\"] Returns:;; The number of elements inserted. --- Insert Initializer List and Visit ```c++ template<class F1, class F2> size_type insert_and_visit(std::initializer_list<value_type> il, F1 f1, F2 f2); template<class F1, class F2> size_type insert_and_cvisit(std::initializer_list<value_type> il, F1 f1, F2 f2); ``` Equivalent to [listing,subs=\"+macros,+quotes\"] Returns:;; The number of elements inserted. --- erase ```c++ size_type erase(const key_type& k); template<class K> size_type erase(const K& k); ``` Erases the element with key equivalent to `k` if it exists. Returns:;; The number of elements erased (0 or 1). Throws:;; Only throws an exception if it is thrown by `hasher` or `key_equal`. Notes:;; The `template<class K>` overload only participates in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type. --- erase_if by Key ```c++ template<class F> size_type erase_if(const key_type& k, F f); template<class K, class F> size_type erase_if(const K& k, F f); ``` Erases the element `x` with key equivalent to `k` if it exists and `f(x)` is `true`. Returns:;; The number of elements erased (0 or 1). Throws:;; Only throws an exception if it is thrown by `hasher`, `key_equal` or `f`. Notes:;; The `template<class K, class F>` overload only participates in overload resolution if `std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>` is `false`. + + The `template<class K, class F>` overload only participates in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type. --- erase_if ```c++ template<class F> size_type erase_if(F f); ``` Successively invokes `f` with references to each of the elements in the table, and erases those for which `f` returns `true`. Returns:;; The number of elements erased. Throws:;; Only throws an exception if it is thrown by `f`. --- Parallel erase_if "```c++ template<class ExecutionPolicy, class F> void erase_if(ExecutionPolicy&& policy, F f); ```" Invokes `f` with references to each of the elements in the table, and erases those for which `f` returns `true`. Execution is parallelized according to the semantics of the execution policy specified. Throws:;; Depending on the exception handling mechanism of the execution policy used, may call `std::terminate` if an exception is thrown within `f`. Notes:;; Only available in compilers supporting C++17 parallel algorithms. + + This overload only participates in overload resolution if `std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>` is `true`. + + Unsequenced execution policies are not allowed. --- swap ```c++ void swap(concurrent_flat_set& other) noexcept(boost::allocator_traits<Allocator>::is_always_equal::value || boost::allocator_traits<Allocator>::propagate_on_container_swap::value); ``` Swaps the contents of the table with the parameter. If `Allocator::propagate_on_container_swap` is declared and `Allocator::propagate_on_container_swap::value` is `true` then the tables\' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior. Throws:;; Nothing unless `key_equal` or `hasher` throw on swapping. Concurrency:;; Blocking on `*this` and `other`. --- clear ```c++ void clear() noexcept; ``` Erases all elements in the table. Postconditions:;; `size() == 0`, `max_load() >= max_load_factor() * bucket_count()` Concurrency:;; Blocking on `*this`. --- merge ```c++ template<class H2, class P2> size_type merge(concurrent_flat_set<Key, H2, P2, Allocator>& source); template<class H2, class P2> size_type merge(concurrent_flat_set<Key, H2, P2, Allocator>&& source); ``` Move-inserts all the elements from `source` whose key is not already present in `*this`, and erases them from `source`. Returns:;; The number of elements inserted. Concurrency:;; Blocking on `*this` and `source`. --- Observers get_allocator ``` allocator_type get_allocator() const noexcept; ``` Returns:;; The table\'s allocator. --- hash_function ``` hasher hash_function() const; ``` Returns:;; The table\'s hash function. --- key_eq ``` key_equal key_eq() const; ``` Returns:;; The table\'s key equality predicate. --- Set Operations count "```c++ size_type count(const key_type& k) const; template<class K> size_type count(const K& k) const; ```" Returns:;; The number of elements with key equivalent to `k` (0 or 1). Notes:;; The `template<class K>` overload only participates in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type. + + In the presence of concurrent insertion operations, the value returned may not accurately reflect the true state of the table right after execution. --- contains "```c++ bool contains(const key_type& k) const; template<class K> bool contains(const K& k) const; ```" "Returns:;; A boolean indicating whether or not there is an element with key equal to `k` in the table. Notes:;; The `template<class K>` overload only participates in overload resolution if `Hash::is_transparent` and `Pred::is_transparent` are valid member typedefs. The library assumes that `Hash` is callable with both `K` and `Key` and that `Pred` is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the `Key` type. + + In the presence of concurrent insertion operations, the value returned may not accurately reflect the true state of the table right after execution." --- Bucket Interface bucket_count ```c++ size_type bucket_count() const noexcept; ``` Returns:;; The size of the bucket array. --- Hash Policy load_factor ```c++ float load_factor() const noexcept; ``` Returns:;; `static_cast<float>(size())/static_cast<float>(bucket_count())`, or `0` if `bucket_count() == 0`. --- max_load_factor ```c++ float max_load_factor() const noexcept; ``` Returns:;; Returns the table\'s maximum load factor. --- Set max_load_factor ```c++ void max_load_factor(float z); ``` Effects:;; Does nothing, as the user is not allowed to change this parameter. Kept for compatibility with `boost::unordered_set`. --- max_load ```c++ size_type max_load() const noexcept; ``` Returns:;; The maximum number of elements the table can hold without rehashing, assuming that no further elements will be erased. Note:;; After construction, rehash or clearance, the table\'s maximum load is at least `max_load_factor() * bucket_count()`. This number may decrease on erasure under high-load conditions. + + In the presence of concurrent insertion operations, the value returned may not accurately reflect the true state of the table right after execution. --- rehash ```c++ void rehash(size_type n); ``` Changes if necessary the size of the bucket array so that there are at least `n` buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the `bucket_count()` associated with the table. When `size() == 0`, `rehash(0)` will deallocate the underlying buckets array. Invalidates pointers and references to elements, and changes the order of elements. Throws:;; The function has no effect if an exception is thrown, unless it is thrown by the table\'s hash function or comparison function. Concurrency:;; Blocking on `*this`. --- reserve ```c++ void reserve(size_type n); ``` Equivalent to `a.rehash(ceil(n / a.max_load_factor()))`. Similar to `rehash`, this function can be used to grow or shrink the number of buckets in the table. Invalidates pointers and references to elements, and changes the order of elements. Throws:;; The function has no effect if an exception is thrown, unless it is thrown by the table\'s hash function or comparison function. Concurrency:;; Blocking on `*this`. --- Statistics get_stats ```c++ stats get_stats() const; ``` Returns:;; A statistical description of the insertion and lookup operations performed by the table so far. Notes:;; Only available if xref:reference/stats.adoc#stats[statistics calculation] is xref:concurrent_flat_set_boost_unordered_enable_stats[enabled]. --- reset_stats ```c++ void reset_stats() noexcept; ``` Effects:;; Sets to zero the internal statistics kept by the table. Notes:;; Only available if xref:reference/stats.adoc#stats[statistics calculation] is xref:concurrent_flat_set_boost_unordered_enable_stats[enabled]. --- Deduction Guides A deduction guide will not participate in overload resolution if any of the following are true: - It has an `InputIterator` template parameter and a type that does not qualify as an input iterator is deduced for that parameter. - It has an `Allocator` template parameter and a type that does not qualify as an allocator is deduced for that parameter. - It has a `Hash` template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter. - It has a `Pred` template parameter and a type that qualifies as an allocator is deduced for that parameter. A `size_­type` parameter type in a deduction guide refers to the `size_­type` member type of the container type deduced by the deduction guide. Its default value coincides with the default value of the constructor selected. __iter-value-type__ Equality Comparisons operator ```c++ template<class Key, class Hash, class Pred, class Alloc> bool operator==(const concurrent_flat_set<Key, Hash, Pred, Alloc>& x, const concurrent_flat_set<Key, Hash, Pred, Alloc>& y); ``` Returns `true` if `x.size() == y.size()` and for every element in `x`, there is an element in `y` with the same key, with an equal value (using `operator==` to compare the value types). Concurrency:;; Blocking on `x` and `y`. Notes:;; Behavior is undefined if the two tables don\'t have equivalent equality predicates. --- operator! ```c++ template<class Key, class Hash, class Pred, class Alloc> bool operator!=(const concurrent_flat_set<Key, Hash, Pred, Alloc>& x, const concurrent_flat_set<Key, Hash, Pred, Alloc>& y); ``` Returns `false` if `x.size() == y.size()` and for every element in `x`, there is an element in `y` with the same key, with an equal value (using `operator==` to compare the value types). Concurrency:;; Blocking on `x` and `y`. Notes:;; Behavior is undefined if the two tables don\'t have equivalent equality predicates. --- Swap ```c++ template<class Key, class Hash, class Pred, class Alloc> void swap(concurrent_flat_set<Key, Hash, Pred, Alloc>& x, concurrent_flat_set<Key, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y))); ``` Equivalent to [listing,subs=\"+macros,+quotes\"] --- erase_if ```c++ template<class K, class H, class P, class A, class Predicate> typename concurrent_flat_set<K, H, P, A>::size_type erase_if(concurrent_flat_set<K, H, P, A>& c, Predicate pred); ``` Equivalent to [listing,subs=\"+macros,+quotes\"] Serialization ``concurrent_flat_set``s can be archived/retrieved by means of link:../../../../../serialization/index.html[Boost.Serialization^] using the API provided by this library. Both regular and XML archives are supported. Saving an concurrent_flat_set to an archive Saves all the elements of a `concurrent_flat_set` `x` to an archive (XML archive) `ar`. Requires:;; `value_type` is serializable (XML serializable), and it supports Boost.Serialization `save_construct_data`/`load_construct_data` protocol (automatically suported by https://en.cppreference.com/w/cpp/named_req/DefaultConstructible[DefaultConstructible^] types). Concurrency:;; Blocking on `x`. --- Loading an concurrent_flat_set from an archive Deletes all preexisting elements of a `concurrent_flat_set` `x` and inserts from an archive (XML archive) `ar` restored copies of the elements of the original `concurrent_flat_set` `other` saved to the storage read by `ar`. Requires:;; `x.key_equal()` is functionally equivalent to `other.key_equal()`. Concurrency:;; Blocking on `x`.