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Defined in header <ranges> | ||
template<class R> using keys_view=ranges::elements_view<R,0>; | (1) | (since C++20) |
namespace views{ inlineconstexprauto keys= ranges::elements<0>; | (2) | (since C++20) |
Takes aview oftuple-like values (e.g.std::tuple orstd::pair), and produces a view with avalue-type of thefirst element of the adapted view's value-type.
Contents |
keys_view can be useful for extractingkeys from associative containers, e.g.
std::map<std::string,int> map{{"one",1},{"two",2}}; for(autoconst& key: std::views::keys(map))std::cout<< key<<' ';// prints: one two
Displays values for each type ofquark in particle physics.
#include <iomanip>#include <iostream>#include <locale>#include <ranges>#include <string>#include <tuple>#include <vector> int main(){conststd::vector<std::tuple<std::string,double,bool>> quark_mass_charge{// name, MeV/c², has positive electric-charge:{"up",2.3,true},{"down",4.8,false},{"charm",1275,true},{"strange",95,false},{"top",173'210, true}, {"bottom", 4'180,false},}; std::cout.imbue(std::locale("en_US.utf8"));std::cout<<"Quark name: │ ";for(std::stringconst& name: std::views::keys(quark_mass_charge))std::cout<<std::setw(9)<< name<<" │ "; std::cout<<"\n""Mass MeV/c²: │ ";for(constdouble mass: std::views::values(quark_mass_charge))std::cout<<std::setw(9)<< mass<<" │ "; std::cout<<"\n""E-charge: │ ";for(constbool pos: std::views::elements<2>(quark_mass_charge))std::cout<<std::setw(9)<<(pos?"+2/3":"-1/3")<<" │ ";std::cout<<'\n';}
Output:
Quark name: │ up │ down │ charm │ strange │ top │ bottom │Mass MeV/c²: │ 2.3 │ 4.8 │ 1,275 │ 95 │ 173,210 │ 4,180 │E-charge: │ +2/3 │ -1/3 │ +2/3 │ -1/3 │ +2/3 │ -1/3 │
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3563 | C++20 | keys_view is unable to participate in CTAD due to its use ofviews::all_t | views::all_t removed |
takes aview consisting of pair-like values and produces aview of the second elements of each pair(class template)(range adaptor object)[edit] | |
takes aview consisting oftuple-like values and a number N and produces aview of Nth element of each tuple(class template)(range adaptor object)[edit] | |
| BLAS-like slice of a valarray: starting index, length, stride (class)[edit] |
