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) |
|
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
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) |
|
|
finds element with specific key
(public member function) |