std::unordered_map - cppreference.com (original) (raw)

std::unordered_map is an associative container that contains key-value pairs with unique keys. Search, insertion, and removal of elements have average constant-time complexity.

Internally, the elements are not sorted in any particular order, but organized into buckets. Which bucket an element is placed into depends entirely on the hash of its key. Keys with the same hash code appear in the same bucket. This allows fast access to individual elements, since once the hash is computed, it refers to the bucket containing the element.

Two keys are considered equivalent if the map's key equality predicate returns true when passed those keys. If two keys are equivalent, the hash function must return the same value for both keys.

std::unordered_map meets the requirements of Container, AllocatorAwareContainer, UnorderedAssociativeContainer.

Contents

• 1 Iterator invalidation
  • 1.1 Notes
• 2 Template parameters
• 3 Member types
• 4 Member functions
  • 4.1 Iterators
  • 4.2 Capacity
  • 4.3 Modifiers
  • 4.4 Lookup
  • 4.5 Bucket interface
  • 4.6 Hash policy
  • 4.7 Observers
• 5 Non-member functions
• 6 Deduction guides
• 7 Notes
• 8 Example
• 9 Defect reports
• 10 See also

[edit] Iterator invalidation

[edit] Notes

• The swap functions do not invalidate any of the iterators inside the container, but they do invalidate the iterator marking the end of the swap region.
• References and pointers to either key or data stored in the container are only invalidated by erasing that element, even when the corresponding iterator is invalidated.

[edit] Template parameters

[edit] Member types

[edit] Member functions

[edit] Non-member functions

[edit] Notes

[edit] Example

#include #include #include   int main() { // Create an unordered_map of three strings (that map to strings) std::unordered_map<std::string, std::string> u = { {"RED", "#FF0000"}, {"GREEN", "#00FF00"}, {"BLUE", "#0000FF"} };   // Helper lambda function to print key-value pairs auto print_key_value = [](const auto& key, const auto& value) { std::cout << "Key:[" << key << "] Value:[" << value << "]\n"; };   std::cout << "Iterate and print key-value pairs of unordered_map, being\n" "explicit with their types:\n"; for (const std::pair<const std::string, std::string>& n : u) print_key_value(n.first, n.second);   std::cout << "\nIterate and print key-value pairs using C++17 structured binding:\n"; for (const auto& [key, value] : u) print_key_value(key, value);   // Add two new entries to the unordered_map u["BLACK"] = "#000000"; u["WHITE"] = "#FFFFFF";   std::cout << "\nOutput values by key:\n" "The HEX of color RED is:[" << u["RED"] << "]\n" "The HEX of color BLACK is:[" << u["BLACK"] << "]\n\n";   std::cout << "Use operator[] with non-existent key to insert a new key-value pair:\n"; print_key_value("new_key", u["new_key"]);   std::cout << "\nIterate and print key-value pairs, using auto;\n" "new_key is now one of the keys in the map:\n"; for (const auto& n : u) print_key_value(n.first, n.second); }

Possible output:

Iterate and print key-value pairs of unordered_map, being explicit with their types: Key:[BLUE] Value:[#0000FF] Key:[GREEN] Value:[#00FF00] Key:[RED] Value:[#FF0000]   Iterate and print key-value pairs using C++17 structured binding: Key:[BLUE] Value:[#0000FF] Key:[GREEN] Value:[#00FF00] Key:[RED] Value:[#FF0000]   Output values by key: The HEX of color RED is:[#FF0000] The HEX of color BLACK is:[#000000]   Use operator[] with non-existent key to insert a new key-value pair: Key:[new_key] Value:[]   Iterate and print key-value pairs, using auto; new_key is now one of the keys in the map: Key:[new_key] Value:[] Key:[WHITE] Value:[#FFFFFF] Key:[BLACK] Value:[#000000] Key:[BLUE] Value:[#0000FF] Key:[GREEN] Value:[#00FF00] Key:[RED] Value:[#FF0000]

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also

| | collection of key-value pairs, hashed by keys (class template) [edit] | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | collection of key-value pairs, sorted by keys, keys are unique (class template) [edit] | | | adapts two containers to provide a collection of key-value pairs, sorted by unique keys (class template) [edit] |