std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::at

 

C++

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)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (C++11)

vector

vector<bool>

inplace_vector (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

 

std::flat_map

Member types

Member functions

flat_map::flat_map flat_map::operator= Element access flat_map::at flat_map::operator[] Iterators flat_map::beginflat_map::cbegin flat_map::endflat_map::cend flat_map::rbeginflat_map::crbegin flat_map::rendflat_map::crend Lookup flat_map::count flat_map::find flat_map::contains flat_map::equal_range    flat_map::lower_bound flat_map::upper_bound Capacity flat_map::size flat_map::max_size flat_map::empty Modifiers flat_map::clear flat_map::erase flat_map::swap flat_map::emplace flat_map::extract flat_map::replace flat_map::insert flat_map::insert_range flat_map::insert_or_assign flat_map::emplace_hint flat_map::try_emplace Observers flat_map::key_comp flat_map::keys flat_map::value_comp flat_map::values

Non-member functions

operator==operator<=>         swap(std::flat_map) erase_if(std::flat_map)

Helper classes

uses_allocator<std::flat_map>

Tags

sorted_unique       sorted_unique_t

Deduction guides

 

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

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

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

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