std::views::filter metodu

Giriş

Açıklaması şöyle

In order to provide the amortized constant time complexity required by range, filter_view::begin caches the result in *this. This modifies the internal state of *this and thus cannot be done in a const member function.

Örnek

Elimizde şöyle bir kod olsun. const v olan kod derlenmez.

auto fn_is_l = [](auto const c) { return c == 'l'; };

{
  auto v = "hello"sv | std::views::filter(fn_is_l);
  std::cout << *v.begin() << std::endl; // ok
}

{
  auto const v = "hello"sv | std::views::filter(fn_is_l);
  std::cout << *v.begin() << std::endl; // error
}

Örnek

Şöyle yaparız.

#include <vector>
#include <ranges>
#include <iostream>

int main()
{
  std::vector<int> ints{0,1,2,3,4,5};
  auto even = [](int i){ return 0 == i % 2; };
  auto square = [](int i) { return i * i; };

  for (int i : ints | std::views::filter(even) | std::views::transform(square)) {
    std::cout << i << ' ';
  }
}

$ g++-10 -Wall -Wextra -std=c++20 ranges.cpp 
$ ./a.out 
0 4 16