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constexprauto begin() requires(!/*simple-view*/<V>); | (1) | (since C++20) |
constexprauto begin()const requiresranges::range<const V>; | (2) | (since C++20) |
Returns an iterator to the first element of thetake_view.
Overload(1) does not participate in overload resolution ifV is asimple view (that is, ifV andconst V are views with the same iterator and sentinel types).
Contents |
(none)
The result depends on the concepts satisfied by possibly const-qualified underlying view typeBase, which isV for(1) orconst V for(2).
Letbase_ be the underlying view,count_ be the underlying counter (equals to0 iftake_view was default initialized).
| The underlying view type satisfies ... | random_access_range | ||
|---|---|---|---|
| yes | no | ||
sized_range | yes | ranges::begin(base_) | std::counted_iterator(ranges::begin(base_), ranges::range_difference_t<Base_>(this->size())) |
| no | std::counted_iterator(ranges::begin(base_), count_) | ||
#include <concepts>#include <forward_list>#include <iostream>#include <ranges>#include <string_view>#include <type_traits>usingnamespace std::literals; int main(){{staticconstexprauto v={"∀x"sv,"∃y"sv,"ε"sv,"δ"sv};auto view= std::ranges::take_view(v,8);auto iter= view.begin();std::cout<<*iter<<'\n'; static_assert( std::ranges::sized_range<decltype(v)> and std::ranges::random_access_range<decltype(v)> andstd::is_same_v<decltype(iter), decltype(std::ranges::begin(v))>);} {std::forward_list v={"Ax"sv,"Ey"sv,"p"sv,"q"sv};auto view= std::ranges::take_view(v,8);auto iter= view.begin();std::cout<<*iter<<'\n'; static_assert( not std::ranges::sized_range<decltype(v)> and not std::ranges::random_access_range<decltype(v)> andstd::is_same_v<decltype(iter),std::counted_iterator<std::forward_list<std::string_view>::iterator>>);}}
Output:
∀xAx
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| P2393R1 | C++20 | implicit conversions between signed and unsigned integer-class types might fail | made explicit |
| returns an iterator or a sentinel to the end (public member function)[edit] | |
(C++20) | iterator adaptor that tracks the distance to the end of the range (class template)[edit] |
(C++20) | compares a sentinel with an iterator returned fromtake_view::begin (function)[edit] |
