Unordered associative containers (C++)

In the programming language C++, unordered associative containers are a group of class templates in the C++ Standard Library that implement hash table variants. Being templates, they can be used to store arbitrary elements, such as integers or custom classes. The following containers are defined in the current revision of the C++ standard:,  ,  ,. Each of these containers differ only on constraints placed on their elements.

The unordered associative containers are similar to the associative containers in the C++ Standard Library but have different constraints. As their name implies, the elements in the unordered associative containers are not ordered. This is due to the use of hashing to store objects. The containers can still be iterated through like a regular associative container.

History
The first widely used implementation of hash tables in the C++ language was,  ,  ,   class templates of the Silicon Graphics (SGI) Standard Template Library (STL). Due to their usefulness, they were later included in several other implementations of the C++ Standard Library (e.g., the GNU Compiler Collection's (GCC) libstdc++ and the Visual C++ (MSVC) standard library).

The  class templates were proposed into C++ Technical Report 1 (C++ TR1) and were accepted under names. Later, they were incorporated into the C++11 revision of the C++ standard. An implementation is also available in the Boost C++ Libraries as.

Overview of functions
The containers are defined in headers named after the names of the containers, e.g.,  is defined in header. All containers satisfy the requirements of the Container concept, which means they have,  ,  ,  ,  , and   methods.

Custom hash functions
To use custom objects in std::unordered_map, a custom hash function must be defined. This function takes a const reference to the custom type and returns a size_t

The user defined function can be used as is in std::unordered_map, by passing it as a template parameter

Or can be set as the default hash function by specializing the std::hash function