std::ranges:: set_difference, std::ranges:: set_difference_result

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定义于头文件 `<algorithm>`
调用签名
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 = { } ) ;	(1)	(C++20 起)
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 = { } ) ;	(2)	(C++20 起)
辅助类型
template < class I, class O > using set_difference_result = ranges:: in_out_result < I, O > ;	(3)	(C++20 起)

将来自已排序输入范围 [ first1 , last1 ) 中未在已排序输入范围 [ first2 , last2 ) 中找到的元素复制到起始于 result 的输出范围。

如果出现以下情况，则行为未定义：

输入范围未分别相对于 comp 和 proj1 或 proj2 进行排序，或
结果范围与任一输入范围存在重叠。

1) 使用给定的二元比较函数 comp 对元素进行比较。

2) 与 (1) 相同，但使用 r1 作为第一范围， r2 作为第二范围，如同使用 ranges:: begin ( r1 ) 作为 first1 ， ranges:: end ( r1 ) 作为 last1 ， ranges:: begin ( r2 ) 作为 first2 ，以及 ranges:: end ( r2 ) 作为 last2 。

本页面描述的函数式实体是 算法函数对象 （非正式称为 niebloids ），即：

在调用其中任何一个时，都不能指定显式模板实参列表。
它们对实参依赖查找均不可见。
当通过常规非限定查找将其中任何一个作为函数调用运算符左侧的名称找到时，会抑制实参依赖查找。

first1, last1	-	定义第一个已排序输入范围元素的迭代器-哨位对
first2, last2	-	定义第二个已排序输入范围元素的迭代器-哨位对
r1	-	第一个已排序输入范围
r2	-	第二个已排序输入范围
result	-	输出范围的起始位置
comp	-	应用于投影元素的比较器
proj1	-	应用于第一个范围元素的投影
proj2	-	应用于第二个范围元素的投影

返回值

{ last1, result_last } ，其中 result_last 表示已构造范围的末端。

复杂度

最多进行 $2\cdot(N 1 +N 2)-1$ 次比较和每次投影的应用，其中 $\scriptsize N_1$ $N 1$ 和 $\scriptsize N_2$ $N 2$ 分别为 ranges:: distance ( first1, last1 ) 和 ranges:: distance ( first2, last2 ) 。

可能的实现

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

运行此代码

#include <algorithm>
#include <cassert>
#include <iostream>
#include <iterator>
#include <string_view>
#include <vector>
auto print = [](const auto& v, std::string_view end = "")
{
    std::cout << "{ ";
    for (auto n{v.size()}; auto i : v)
        std::cout << i << (--n ? ", " : " ");
    std::cout << "} " << end;
};
struct Order // 包含一些非常有趣数据的结构体
{
    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\n");
    // 我们想知道哪些订单在旧状态和新状态之间被"削减"：
    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} }

参见

ranges::set_union (C++20)	计算两个集合的并集 (算法函数对象)
ranges::set_intersection (C++20)	计算两个集合的交集 (算法函数对象)
ranges::set_symmetric_difference (C++20)	计算两个集合的对称差 (算法函数对象)
ranges::includes (C++20)	若一个序列是另一个序列的子序列则返回 true (算法函数对象)
set_difference	计算两个集合的差集 (函数模板)

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External libraries

cppreference.net

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Variants

std::ranges:: set_difference, std::ranges:: set_difference_result

目录

参数

返回值

复杂度

可能的实现

示例

参见