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

< cpp‎ | ranges
Ranges library
Range access
Range conversions
Range primitives

Dangling iterator handling
Range concepts


Range adaptor objects
Range adaptor closure objects
Helper items
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.


[edit] Expression-equivalent

Expression e is expression-equivalent to expression f, if

  • e and f have the same effects, and
  • either both are constant subexpressions or else neither is a constant subexpression, and
  • either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).

[edit] Member functions

constructs a drop_while_view
(public member function) [edit]
returns a copy of the underlying (adapted) view
(public member function) [edit]
returns a reference to the stored predicate
(public member function) [edit]
returns an iterator to the beginning
(public member function) [edit]
returns an iterator or a sentinel to the end
(public member function) [edit]
Inherited from std::ranges::view_interface
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>) [edit]
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>) [edit]
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>) [edit]
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>) [edit]
Returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>) [edit]
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>) [edit]
Returns the nth element in the derived view. Provided if it satisfies random_access_range.
(public member function of std::ranges::view_interface<D>) [edit]

[edit] Deduction guides

[edit] Helper templates

template<class T, class Pred>

inline constexpr bool enable_borrowed_range<std::ranges::drop_while_view<T, Pred>> =

(since C++20)

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

[edit] 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.

[edit] Example

#include <iomanip>
#include <iostream>
#include <ranges>
#include <string>
#include <string_view>
using std::operator""sv;
constexpr bool is_space(char ch) noexcept
    switch (ch)
        case ' ':
        case '\t':
        case '\n':
        case '\v':
        case '\r':
        case '\f':
        return true;
    return false;
constexpr std::string_view trim_left(std::string_view const in) noexcept
    return in | std::views::drop_while(is_space);
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 << ' ';


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

[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
LWG 3494 C++20 drop_while_view was never a borrowed_range it is a borrowed_range if its underlying view is

[edit] See also

a view consisting of elements of another view, skipping the first N elements
(class template) (range adaptor object) [edit]