std::ranges::set_difference() 算法

自 C++20 起

简化

// (1)
constexpr set_difference_result<I1, O>
    set_difference( I1 first1, S1 last1, I2 first2, S2 last2, O result,
                    Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

// (2)
constexpr set_difference_result<ranges::borrowed_iterator_t<R1>, O>
    set_difference( R1&& r1, R2&& r2, O result, Comp comp = {},
                    Proj1 proj1 = {}, Proj2 proj2 = {} );

参数类型是泛型的，并具有以下约束

• I1, I2 - std::input_iterator
• S1, S2 - std::sentinel_for<I1>, std::sentinel_for<I2>
• R1, R2 - std::ranges::input_range
• O - std::weakly_incrementable
• Comp - （无）
• Proj1, Proj2 - (无)

所有重载的 Proj 和 Comp 模板参数具有以下默认类型：std::identity, ranges::less。

此外，每个重载都有以下约束

• (1) - mergeable<I1, I2, O, Comp, Proj1, Proj2>
• (2) - mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>, O, Comp, Proj1, Proj2>

(为便于阅读，此处省略了 std:: 命名空间)

详细

// (1)
template<
  std::input_iterator I1,
  std::sentinel_for<I1> S1,
  std::input_iterator I2,
  std::sentinel_for<I2> S2,
  std::weakly_incrementable O,
  class Comp = ranges::less,
  class Proj1 = std::identity,
  class Proj2 = std::identity
>
  requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
constexpr set_difference_result<I1, O>
    set_difference( I1 first1, S1 last1, I2 first2, S2 last2, O result,
                    Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );

// (2)
template<
  ranges::input_range R1,
  ranges::input_range R2,
  std::weakly_incrementable O,
  class Comp = ranges::less,
  class Proj1 = std::identity,
  class Proj2 = std::identity
>
  requires std::mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>,
                          O, Comp, Proj1, Proj2>
constexpr set_difference_result<ranges::borrowed_iterator_t<R1>, O>
    set_difference( R1&& r1, R2&& r2, O result, Comp comp = {},
                    Proj1 proj1 = {}, Proj2 proj2 = {} );

辅助类型定义如下：

template< class I, class O >
using set_difference_result = ranges::in_out_result<I, O>;

将已排序输入范围 [first1; last1) 中未在已排序输入范围 [first2; last2) 中找到的元素复制到始于 result 的输出范围。

• (1) 使用给定的二元比较函数 comp 比较元素。
• (2) 与 (1) 相同，但使用 r1 作为第一个范围，r2 作为第二个范围，如同使用 ranges::begin(r1) 作为 first1，ranges::end(r1) 作为 last1，ranges::begin(r2) 作为 first2，ranges::end(r2) 作为 last2。

未定义行为

行为未定义

如果

• 输入范围未按 comp 和 proj1 或 proj2 分别排序。
• 或结果范围与任何一个输入范围重叠。

本页描述的函数类实体是niebloids。

参数

first1 last1	第一个已排序的输入范围。
r r1	第一个已排序的输入范围。
first2 last2	第二个已排序的输入范围。
r2	第二个已排序的输入范围。
result	目标范围的开头。
comp	应用于投影元素的比较器。
proj1	应用于第一个范围中元素的投影。
proj2	应用于第二个范围中元素的投影。

返回值

一个类型为 ranges::set_difference_result 的值，按以下方式初始化

{
  last1,
  result_last
}

其中 result_last 是构造范围的末尾。

复杂度

给定 N1 为 ranges::distance(first1, last1)，N2 为 ranges::distance(first2, last12)

2 * (N1 + N2) − 1 次比较和每次投影的应用。

异常

(无)

可能的实现

set_difference(1) 和 set_difference(2)

struct set_difference_fn
{
    template<std::input_iterator I1, std::sentinel_for<I1> S1,
             std::input_iterator I2, std::sentinel_for<I2> S2,
             std::weakly_incrementable O, class Comp = ranges::less,
             class Proj1 = std::identity, class Proj2 = std::identity >
    requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
    constexpr ranges::set_difference_result<I1, O>
        operator()(I1 first1, S1 last1, I2 first2, S2 last2,
                   O result, Comp comp = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        while (!(first1 == last1 or first2 == last2))
        {
            if (std::invoke(comp, std::invoke(proj1, *first1), std::invoke(proj2, *first2)))
            {
                *result = *first1;
                ++first1;
                ++result;
            }
            else if (std::invoke(comp, std::invoke(proj2, *first2),
                                 std::invoke(proj1, *first1)))
            {
                ++first2;
            }
            else
            {
                ++first1;
                ++first2;
            }
        }
        return ranges::copy(std::move(first1), std::move(last1), std::move(result));
    }

    template<ranges::input_range R1, ranges::input_range R2,
             std::weakly_incrementable O, class Comp = ranges::less,
             class Proj1 = std::identity, class Proj2 = std::identity>
    requires std::mergeable<ranges::iterator_t<R1>, ranges::iterator_t<R2>,
                            O, Comp, Proj1, Proj2>
    constexpr ranges::set_difference_result<ranges::borrowed_iterator_t<R1>, O>
        operator()(R1&& r1, R2&& r2, O result, Comp comp = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r1), ranges::end(r1),
                       ranges::begin(r2), ranges::end(r2),
                       std::move(result), std::move(comp),
                       std::move(proj1), std::move(proj2));
    }
};

inline constexpr set_difference_fn set_difference {};

示例

Main.cpp

#include <algorithm>
#include <cassert>
#include <iostream>
#include <iterator>
#include <string_view>
#include <vector>

auto print = [](const auto& v, std::string_view end = "")
{
    for (std::cout << "{ "; auto i : v)
        std::cout << i << ' ';
    std::cout << "} " << end;
};

struct Order // a struct with some very interesting data
{
    int order_id {};

    friend std::ostream& operator<<(std::ostream& os, const Order& ord)
    {
        return os << "{" << ord.order_id << "},";
    }
};

int main()
{
    const auto v1 = {1, 2, 5, 5, 5, 9};
    const auto v2 = {2, 5, 7};
    std::vector<int> diff {};

    std::ranges::set_difference(v1, v2, std::back_inserter(diff));
    print(v1, "∖ ");
    print(v2, "= ");
    print(diff, "\n");

    // we want to know which orders "cut" between old and new states:
    const std::vector<Order> old_orders { {1}, {2}, {5}, {9}, };
    const std::vector<Order> new_orders { {2}, {5}, {7}, };
    std::vector<Order> cut_orders(old_orders.size() + new_orders.size());

    auto [old_orders_end, cut_orders_last] =
        std::ranges::set_difference(old_orders, new_orders,
                                    cut_orders.begin(), {},
                                    &Order::order_id, &Order::order_id);
    assert(old_orders_end == old_orders.end());

    std::cout << "old orders = "; print(old_orders, "\n");
    std::cout << "new orders = "; print(new_orders, "\n");
    std::cout << "cut orders = "; print(cut_orders, "\n");
    cut_orders.erase(cut_orders_last, end(cut_orders));
    std::cout << "cut orders = "; print(cut_orders, "\n");
}

输出

{ 1 2 5 5 5 9 } ∖ { 2 5 7 } = { 1 5 5 9 } 
old orders = { {1}, {2}, {5}, {9}, } 
new orders = { {2}, {5}, {7}, } 
cut orders = { {1}, {9}, {0}, {0}, {0}, {0}, {0}, } 
cut orders = { {1}, {9}, }