std::ranges::take_view<V>::begin
From cppreference.com
constexpr auto begin() requires (!/*simple-view*/<V>);
|
(1) | (since C++20) |
constexpr auto begin() const requires ranges::range<const V>;
|
(2) | (since C++20) |
Returns an iterator to the first element of the take_view.
1) Returns a
std::counted_iterator or a ranges::iterator_t<V>.2) Returns a
std::counted_iterator or a ranges::iterator_t<const V>.Overload (1) does not participate in overload resolution if V is a simple view (that is, if V and const V are views with the same iterator and sentinel types).
Parameters
(none)
Return value
The result depends on the concepts satisfied by possibly const-qualified underlying view type Base, which is V for (1) or const V for (2).
Let base_ be the underlying view, count_ be the underlying counter (equals to 0 if take_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_)
| ||
Example
Run this code
#include <concepts>
#include <forward_list>
#include <iostream>
#include <ranges>
#include <string_view>
#include <type_traits>
using namespace std::literals;
int main()
{
{
static constexpr auto 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)> and
std::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)> and
std::is_same_v<decltype(iter),
std::counted_iterator<
std::forward_list<std::string_view>::iterator>>
);
}
}
Output:
∀x
Ax
Defect reports
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 |
See also
| returns an iterator or a sentinel to the end (public member function) | |
(C++20) |
iterator adaptor that tracks the distance to the end of the range (class template) |
(C++20) |
compares a sentinel with an iterator returned from take_view::begin (function) |
