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std::ranges::views::drop_while, std::ranges::drop_while_view

Defined in header <ranges> 
template< ranges::view V, class Pred >
    requires ranges::input_range<V> &&
             std::is_object_v<Pred> &&
             std::indirect_unary_predicate<const Pred, ranges::iterator_t<V>>
class drop_while_view :
    public ranges::view_interface<drop_while_view<V, Pred>>
 (1) (since C++20) 
namespace views {
    inline constexpr /* unspecified */ drop_while = /* unspecified */;
}
 (2) (since C++20)
Call signature 
template< ranges::viewable_range R, class Pred >
    requires /* see below */
constexpr ranges::view auto drop_while( R&& r, Pred&& pred );
 (since C++20) 
template< class Pred >
constexpr /*range adaptor closure*/ drop_while( Pred&& pred );
 (since C++20)
1) A range adaptor that represents view of elements from an underlying sequence, beginning at the first element for which the predicate returns false.
2) RangeAdaptorObject. The expression views::drop_while(e, f) is expression-equivalent to drop_while_view(e, f) for any suitable subexpressions e and f.

drop_while_view models the concepts contiguous_range, random_access_range, bidirectional_range, forward_range, input_range, and common_range when the underlying view V models respective concepts. It also models sized_range if ranges::forward_range<V> and std::sized_sentinel_for<ranges::sentinel_t<V>, ranges::iterator_t<V>> are modeled.

Member functions

(C++20)
 constructs a drop_while_view
(public member function)
(C++20)
 returns a copy of the underlying (adapted) view
(public member function)
(C++20)
 returns a reference to the stored predicate
(public member function)
(C++20)
 returns an iterator to the beginning
(public member function)
(C++20)
 returns an iterator or a sentinel to the end
(public member function)
Inherited from std::ranges::view_interface
(C++20)
 returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range.
(public member function of std::ranges::view_interface<D>)
(C++23)
 returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D>)
(C++23)
 returns a sentinel for the constant iterator of the range.
(public member function of std::ranges::view_interface<D>)
(C++20)
 returns whether the derived view is not empty. Provided if ranges::empty is applicable to it.
(public member function of std::ranges::view_interface<D>)
(C++20)
 gets the address of derived view's data. Provided if its iterator type satisfies contiguous_iterator.
(public member function of std::ranges::view_interface<D>)
(C++20)
 returns the number of elements in the derived view. Provided if it satisfies forward_range and its sentinel and iterator type satisfy sized_sentinel_for.
(public member function of std::ranges::view_interface<D>)
(C++20)
 returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
(C++20)
 returns the last element in the derived view. Provided if it satisfies bidirectional_range and common_range.
(public member function of std::ranges::view_interface<D>)
(C++20)
 returns the nth element in the derived view. Provided if it satisfies random_access_range.
(public member function of std::ranges::view_interface<D>)

Deduction guides

Helper templates

template< class T, class Pred >
inline constexpr bool enable_borrowed_range<std::ranges::drop_while_view<T, Pred>> =
    std::ranges::enable_borrowed_range<T>;
 (since C++20)

This specialization of std::ranges::enable_borrowed_range makes drop_while_view satisfy borrowed_range when the underlying view satisfies it.

Notes

In order to provide the amortized constant time complexity required by the range concept, the result of begin is cached within the drop_while_view object. If the underlying range is modified after the first call to begin(), subsequent uses of the drop_while_view object might have unintuitive behavior.

Example

#include <iomanip>
#include <iostream>
#include <ranges>
#include <string>
#include <string_view>
 
using std::operator""sv;
 
[[nodiscard]]
constexpr bool is_space(char ch) noexcept
{
    switch (ch)
    {
        case ' ':
        case '\t':
        case '\n':
        case '\v':
        case '\r':
        case '\f':
        return true;
    }
    return false;
};
 
[[nodiscard]]
constexpr std::string_view trim_left(std::string_view const in) noexcept
{
    return in | std::views::drop_while(is_space);
}
 
[[nodiscard]]
std::string trim(std::string_view const in)
{
    auto view
        = in
        | std::views::drop_while(is_space)
        | std::views::reverse
        | std::views::drop_while(is_space)
        | std::views::reverse
        ;
    return {view.begin(), view.end()};
}
 
int main()
{
    static_assert(trim_left(" \n C++23") == "C++23"sv);
 
    const auto s = trim(" \f\n\t\r\vHello, C++20!\f\n\t\r\v ");
    std::cout << "s = " << std::quoted(s) << '\n';
 
    static constexpr auto v = {0, 1, 2, 3, 4, 5};
    for (int n : v | std::views::drop_while([](int i) { return i < 3; }))
        std::cout << n << ' ';
}

Output:

s = "Hello, C++20!"
3 4 5

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3494 C++20 drop_while_view was never a borrowed_range it is a borrowed_range if its underlying view is

See also

(C++20)
 a view consisting of elements of another view, skipping the first N elements
(class template) (range adaptor object)

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