std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::at
From cppreference.com
< cpp | container | unordered map
T& at( const Key& key ); |
(1) | (since C++11) |
|
const T& at(
const Key& key )
const
;
|
(2) | (since C++11) |
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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
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)
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
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) |
Example
| This section is incomplete Reason: no example |
See also
| access or insert specified element (public member function) |
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| finds element with specific key (public member function) |