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: unordered_set, unordered_map, unordered_multiset, unordered_multimap. 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.

std::unordered_map and std::unordered_set are essentially (respectively) equivalent to java.util.HashMap and java.util.HashSet from Java, or std::collections::HashMap and std::collections::HashSet from Rust.

History

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).^[1] 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++^[2] 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_*.^[3] Later, they were incorporated into the C++11 revision of the C++ standard.^[4] An implementation is also available in the Boost C++ Libraries as <boost/unordered_map.hpp>.^[5]

Overview of functions

The containers are defined in headers named after the names of the containers, e.g., unordered_set is defined in header <unordered_set>. All containers satisfy the requirements of the Container concept, which means they have begin(), end(), size(), max_size(), empty(), and swap() methods.

	`unordered_set` (C++11)	`unordered_map` (C++11)	`unordered_multiset` (C++11)	`unordered_multimap` (C++11)	Description
	(constructor)	(constructor)	(constructor)	(constructor)	Constructs the container from variety of sources
	(destructor)	(destructor)	(destructor)	(destructor)	Destructs the set and the contained elements
	`operator=`	`operator=`	`operator=`	`operator=`	Assigns values to the container
	`get_allocator`	`get_allocator`	`get_allocator`	`get_allocator`	Returns the allocator used to allocate memory for the elements
Element access	—	`at`	—	—	Accesses specified element with bounds checking.
Element access	—	`operator[]`	—	—	Accesses specified element without bounds checking.
Iterators	`begin`	`begin`	`begin`	`begin`	Returns an iterator to the beginning of the container
Iterators	`end`	`end`	`end`	`end`	Returns an iterator to the end of the container
Capacity	`empty`	`empty`	`empty`	`empty`	Checks whether the container is empty
	`size`	`size`	`size`	`size`	Returns number of elements in the container.
	`max_size`	`max_size`	`max_size`	`max_size`	Returns the maximum possible number of elements in the container
Modifiers	`clear`	`clear`	`clear`	`clear`	Clears the contents.
	`insert`	`insert`	`insert`	`insert`	Inserts elements.
	`emplace`	`emplace`	`emplace`	`emplace`	Constructs elements in-place (C++11)
	`emplace_hint`	`emplace_hint`	`emplace_hint`	`emplace_hint`	Constructs elements in-place using a hint (C++11)
	`erase`	`erase`	`erase`	`erase`	Erases elements.
	`swap`	`swap`	`swap`	`swap`	Swaps the contents with another container.
Lookup	`count`	`count`	`count`	`count`	Returns the number of elements matching specific key.
	`find`	`find`	`find`	`find`	Finds an element with specific key.
	`equal_range`	`equal_range`	`equal_range`	`equal_range`	Returns a range of elements matching specific key.
Bucket interface	...
Hash policy	...
Observers	`hash_function`	`hash_function`	`hash_function`	`hash_function`	Returns the function used to create hash of a key
Observers	`key_eq`	`key_eq`	`key_eq`	`key_eq`	Returns key comparison function.

Usage example

import std;

const std::unordered_map<std::string, int> 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;
 
struct Vector3 {
    int i;
    int j;
    int k;
};

struct HashVector3 {
    size_t operator()(const Vector3& x) const {
        return std::hash<int>()(x.i) ^ std::hash<int>()(x.j) ^ std::hash<int>()(x.k);
    }
};

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

std::unordered_map<Vector3, int, HashVector3> myPointToIntMap;

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

namespace std {
    template <>
    class hash<Vector3> {
    public:
        size_t operator()(const Vector3& x) const {
            return hash<int>()(x.i) ^ hash<int>()(x.j) ^ hash<int>()(x.k);
        }
    };
}

//...
std::unordered_map<Vector3, int> myPointToIntMap;