All Unordered-Container Facilities

All Unordered-Container Facilities

A compact scan page for hash-based containers, hashing support, bucket interfaces, and practical lookup rules.

How to use this reference page

Use reference pages to confirm names, categories, nearby facilities, and the constraints that matter before writing or reviewing code.

Scan the top of the page first to identify the primary types, functions, or algorithm families involved.
Use the nearby-page links when your question is really about a companion header, related algorithm family, or broader subsystem.
Validate tricky behavior with a small compileable example before relying on memory for details like invalidation, ordering, allocation, or lifetime rules.

All Unordered-Container Facilities

Standard unordered containers

std::unordered_map
std::unordered_multimap
std::unordered_set
std::unordered_multiset

Hashing and equality support

std::hash
key equality predicates
transparent hashing/equality where supported by the chosen functors

Lookup and modification operations

find
contains
count
equal_range
insert, emplace, try_emplace, insert_or_assign
erase, extract, merge

Bucket and load-factor controls

bucket_count
bucket
load_factor
max_load_factor
rehash
reserve

Practical rules

Reserve space when approximate element count is known.
Use unordered containers only when ordering is not part of the API contract.
Bad hashes can dominate performance; treat custom hashing as part of container design.

Example in practice

#include <map>
#include <string>

int main() {
    std::map<std::string, int> counts{std::pair{"Ada", 1}, std::pair{"Bjarne", 2}};
    auto [it, inserted] = counts.insert({"Grace", 3});
    return inserted ? it->second : 1;
}