NAME¶

std::ranges::take_view::begin - std::ranges::take_view::begin

Synopsis¶

constexpr auto begin() requires (!__SimpleView<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 overload (1)) or const V (for overload (2)).

Let base_ be the underlying view, count_ be the number passed to the constructor (0
if default initialized).

The underlying random_access_range
view satisfies
... yes no
std::counted_iterator(ranges::begin(base_),
yes ranges::begin(base_)
sized_range
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 made explicit
integer-class types might fail

See also¶

end returns an iterator or a sentinel to the end
(C++20) (public member function)
counted_iterator iterator adaptor that tracks the distance to the end of the range
(C++20) (class template)
operator== compares a sentinel with an iterator returned from take_view::begin
(C++20) (function)