std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>:: at
From cppreference.com
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T
&
at
(
const
Key
&
key
)
;
|
(1) | (since C++23) |
|
const
T
&
at
(
const
Key
&
key
)
const
;
|
(2) | (since C++23) |
|
template
<
class
K
>
T & at ( const K & x ) ; |
(3) | (since C++23) |
|
template
<
class
K
>
const T & at ( const K & x ) const ; |
(4) | (since C++23) |
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 the qualified-id
Compare
::
is_transparent
is valid and denotes a type. It 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
Logarithmic in the size of the container.
Example
Run this code
#include <cassert> #include <iostream> #include <flat_map> struct LightKey { int o; }; struct HeavyKey { int o[1000]; }; // The container must use std::less<> (or other transparent Comparator) to // access overloads (3,4). This includes standard overloads, such as // comparison between std::string and std::string_view. bool operator<(const HeavyKey& x, const LightKey& y) { return x.o[0] < y.o; } bool operator<(const LightKey& x, const HeavyKey& y) { return x.o < y.o[0]; } bool operator<(const HeavyKey& x, const HeavyKey& y) { return x.o[0] < y.o[0]; } int main() { std::flat_map<int, char> map{{1, 'a'}, {2, 'b'}}; assert(map.at(1) == 'a'); assert(map.at(2) == 'b'); try { map.at(13); } catch(const std::out_of_range& ex) { std::cout << "1) out_of_range::what(): " << ex.what() << '\n'; } #ifdef __cpp_lib_associative_heterogeneous_insertion // Transparent comparison demo. std::flat_map<HeavyKey, char, std::less<>> map2{{{1}, 'a'}, {{2}, 'b'}}; assert(map2.at(LightKey{1}) == 'a'); assert(map2.at(LightKey{2}) == 'b'); try { map2.at(LightKey{13}); } catch(const std::out_of_range& ex) { std::cout << "2) out_of_range::what(): " << ex.what() << '\n'; } #endif }
Possible output:
1) out_of_range::what(): map::at: key not found 2) out_of_range::what(): map::at: key not found
See also
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access or insert specified element
(public member function) |
|
|
finds element with specific key
(public member function) |