Namespaces
Variants
Views
Actions

std::ranges::take_view<V>::begin

From cppreference.com
< cpp‎ | ranges‎ | take view
 
 
Ranges library
Range access
Range conversions
(C++23)

Range primitives



Dangling iterator handling
Range concepts
Views

Range factories
Range adaptors
Range generators
Range adaptor objects
Range adaptor closure objects
Helper items
(until C++23)(C++23)


 
 
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.

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).

Contents

[edit] Parameters

(none)

[edit] 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 number passed to the constructor (0 if default initialized).

The underlying view 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_)

[edit] Example

#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

[edit] 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

[edit] See also

returns an iterator or a sentinel to the end
(public member function) [edit]
iterator adaptor that tracks the distance to the end of the range
(class template) [edit]
compares a sentinel with an iterator returned from take_view::begin
(function) [edit]