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

 

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std::unordered_map

Member types
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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)

 

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 type std::out_of_range is thrown.

1,2) The key is equivalent to key.

3,4) The key compares equivalent to the value x. The reference to the mapped value is obtained as if by expression this->find(x)->second.

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

These overloads participate in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type. This assumes that such Hash is callable with both K and Key type, and that the KeyEqual is transparent, which, together, allows calling this function without constructing an instance of Key.

Parameters

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

Return value

A reference to the mapped value of the requested element.

Exceptions

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

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

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 in ordered and unordered associative containers. (3,4)

See also

operator[]	access or insert specified element (public member function)
find	finds element with specific key (public member function)