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[#concurrent_node_set]

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Class Template concurrent_node_set

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:idprefix: concurrent_node_set_

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`boost::concurrent_node_set` — A node-based hash table that stores unique values and allows for concurrent element insertion, erasure, lookup and access without external synchronization mechanisms.

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Even though it acts as a container, `boost::concurrent_node_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_node_set` operations where they are executed internally in a controlled fashion. Such visitation-based API allows for low-contention concurrent usage scenarios.

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The internal data structure of `boost::concurrent_node_set` is similar to that of `boost::unordered_node_set`. Unlike `boost::concurrent_flat_set`, pointer stability and node handling functionalities are provided, at the expense of potentially lower performance.

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Synopsis

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Description

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*Template Parameters*

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_Key_

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`Key` must be https://en.cppreference.com/w/cpp/named_req/MoveInsertable[MoveInsertable^] into the container

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and https://en.cppreference.com/w/cpp/named_req/Erasable[Erasable^] from the container.

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_Hash_

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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`.

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_Pred_

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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`.

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_Allocator_

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An allocator whose value type is the same as the table's value type.

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`std::allocator_traits<Allocator>::pointer` and `std::allocator_traits<Allocator>::const_pointer` must be convertible to/from `value_type*` and `const value_type*`, respectively.