Section hub

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

Since C++20

A range adaptor that represents view of elements from an underlying sequence, beginning at the first element for which the predicate returns false.

# Declarations

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>>

(since C++20)

namespace views {
inline constexpr /* unspecified */ drop_while = /* unspecified */;
}

(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)

# 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 <iostream>
#include <ranges>
#include <string>
#include <string_view>
 
using std::operator""sv;
 
[[nodiscard]]
constexpr bool is_space(char p) noexcept
{
    auto ne = [p](auto q) { return p != q; };
    return !!(" \t\n\v\r\f" | std::views::drop_while(ne));
};
 
[[nodiscard("trims the output")]]
constexpr std::string_view trim_left(std::string_view const in) noexcept
{
    auto view = in | std::views::drop_while(is_space);
    return {view.begin(), view.end()};
}
 
[[nodiscard("trims the output")]]
constexpr 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);
 
    constexpr auto src{" \f\n\t\r\vHello, C++20!\f\n\t\r\v "sv};
    static_assert(trim(src) == "Hello, C++20!");
 
    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 << ' ';
    std::cout << '\n';
}

# Defect reports

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

This hub groups the ranges library by user task rather than by raw reference tree shape. View types and adaptor objects are presented as the same conceptual item.