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

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.

Data members

Member functions

Deduction guides

Helper templates

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 <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';
}

3 4 5

Defect reports

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

See also