(C++17) | ||||
| Sequence | ||||
(C++11) | ||||
(C++26) | ||||
(C++26) | ||||
(C++11) | ||||
| Associative | ||||
| Unordered associative | ||||
(C++11) | ||||
(C++11) | ||||
(C++11) | ||||
(C++11) | ||||
| Adaptors | ||||
(C++23) | ||||
(C++23) | ||||
(C++23) | ||||
(C++23) | ||||
| Views | ||||
(C++20) | ||||
(C++23) | ||||
| Tables | ||||
| Iterator invalidation | ||||
| Member function table | ||||
| Non-member function table |
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 typestd::out_of_range is thrown.
Compare istransparent. It allows calling this function without constructing an instance ofKey.Contents |
| key | - | the key of the element to find |
| x | - | a value of any type that can be transparently compared with a key |
A reference to the mapped value of the requested element.
Logarithmic in the size of the container.
#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(conststd::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(conststd::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 found2) out_of_range::what(): map::at: key not found
| access or insert specified element (public member function)[edit] | |
| finds element with specific key (public member function)[edit] |
