std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::at

From cppreference.com

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros(C++20)
Language support library
Concepts library(C++20)
Diagnostics library
Memory management library
Metaprogramming library(C++11)
General utilities library
Containers library
Iterators library
Ranges library(C++20)
Algorithms library
Strings library
Text processing library
Numerics library
Date and time library
Input/output library
Filesystem library(C++17)
Concurrency support library(C++11)
Execution control library(C++26)
Technical specifications
Symbols index
External libraries

[edit]

Containers library

node-handle (C++17)
Sequence
array (C++11)
vector
vector<bool>
inplace_vector (C++26)
hive (C++26)
deque
forward_list (C++11)
list
Associative
set
multiset
map
multimap
Unordered associative
unordered_set (C++11)
unordered_multiset (C++11)
unordered_map (C++11)
unordered_multimap (C++11)
Adaptors
stack
queue
priority_queue
flat_set (C++23)
flat_multiset (C++23)
flat_map (C++23)
flat_multimap (C++23)
Views
span (C++20)
mdspan (C++23)
Tables
Iterator invalidation
Member function table
Non-member function table

unordered_map::unordered_map
unordered_map::~unordered_map
unordered_map::operator=
unordered_map::get_allocator
Iterators
unordered_map::beginunordered_map::cbegin
unordered_map::endunordered_map::cend
Capacity
unordered_map::size
unordered_map::max_size
unordered_map::empty
Modifiers
unordered_map::clear
unordered_map::erase
unordered_map::swap
unordered_map::extract (C++17)
unordered_map::merge (C++17)
unordered_map::insert
unordered_map::insert_range (C++23)
unordered_map::insert_or_assign (C++17)
unordered_map::emplace
unordered_map::emplace_hint
unordered_map::try_emplace (C++17)

Lookup
unordered_map::at
unordered_map::operator[]
unordered_map::count
unordered_map::find
unordered_map::contains (C++20)
unordered_map::equal_range
Bucket interface
unordered_map::begin(size_type)unordered_map::cbegin(size_type)
unordered_map::end(size_type)unordered_map::cend(size_type)
unordered_map::bucket_count
unordered_map::max_bucket_count
unordered_map::bucket_size
unordered_map::bucket
Hash policy
unordered_map::load_factor
unordered_map::max_load_factor
unordered_map::rehash
unordered_map::reserve
Observers
unordered_map::hash_function
unordered_map::key_eq

Non-member functions

operator==operator!=

(until C++20)

std::swap(std::unordered_map)
erase_if(std::unordered_map) (C++20)

Deduction guides(C++17)

[edit]

T& at(const Key& key);	(1)	(since C++11)
const T& at(const Key& key)const;	(2)	(since C++11)
template<class K> T& at(const K& x);	(3)	(since C++26)
template<class K> const T& at(const K& x)const;	(4)	(since C++26)

Returns a reference to the mapped value of the element with specified key. If no such element exists, an exception of typestd::out_of_range is thrown.

1,2) The key is equivalent tokey.

3,4) The key comparesequivalent to the valuex. The reference to the mapped value is obtained as if by expressionthis->find(x)->second.

The expressionthis->find(x) must be well-formed and have well-defined behavior, otherwise the behavior is undefined.

These overloads participate in overload resolution only ifHash andKeyEqual are bothtransparent. This assumes that suchHash is callable with bothK andKey type, and that theKeyEqual is transparent, which, together, allows calling this function without constructing an instance ofKey.

[edit]Parameters

key	-	the key of the element to find
x	-	a value of any type that can be transparently compared with a key

[edit]Return value

A reference to the mapped value of the requested element.

[edit]Exceptions

1,2)std::out_of_range if the container does not have an element with the specifiedkey.

3,4)std::out_of_range if the container does not have the specified element, that is, iffind(x)== end() istrue.

[edit]Complexity

Average case: constant, worst case: linear in size.

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_associative_heterogeneous_insertion`	`202311L`	(C++26)	Heterogeneous overloads for the remaining member functions inordered andunordered associativecontainers.(3,4)