Unordered associative containers (C++)

The first widely used implementation of hash tables in the C++ language was hash_map, hash_set, hash_multimap, hash_multiset 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 hash_* class templates were proposed into C++ Technical Report 1 (C++ TR1) and were accepted under names unordered_*. 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., unordered_set is defined in header . All containers satisfy the requirements of the Container concept, which means they have begin(), end(), size(), max_size(), empty(), and swap() methods. ==Usage example==

import std; using std::string; using std::unordered_map; const unordered_map MONTHS { {"January", 31}, {"February", 28}, {"March", 31}, {"April", 30}, {"May", 31}, {"June", 30}, {"July", 31}, {"August", 31}, {"September", 30}, {"October", 31}, {"November", 30}, {"December", 31} }; int main(int argc, char* argv[]) { std::println("September -> {}", MONTHS["September"]); std::println("April -> {}", MONTHS["April"]); std::println("December -> {}", MONTHS["December"]); std::println("February -> {}", MONTHS["February"]); return 0; } ==Custom hash functions==

To use custom objects in std::unordered_map, a custom hasher must be defined. This function takes a const reference to the custom type and returns a size_t. import std; using std::hash; struct Vector3 { int i; int j; int k; }; struct HashVector3 { size_t operator()(const Vector3& x) const { return hash()(x.i) ^ hash()(x.j) ^ hash()(x.k); } }; The user defined function can be used as is in std::unordered_map, by passing it as a template parameter unordered_map myPointToIntMap; Or can be set as the default hash function by specializing std::hash: namespace std { template <> class hash { public: size_t operator()(const Vector3& x) const { return hash()(x.i) ^ hash()(x.j) ^ hash()(x.k); } }; } //... unordered_map myPointToIntMap; ==See also==