c++ - 有什么方法可以将元素从ordered_index和multi_index中移出？

从C ++ 17开始，std::set具有extract()成员函数，该成员函数支持将节点从容器中移出。这是示例代码：

#include <iostream>
#include <cassert>
#include <set>

struct trace {
    trace() {
        std::cout << this << ":" << " construct" << std::endl;
    }
    ~trace() {
        std::cout << this << ":" << " destruct" << std::endl;
    }
    trace(trace& other) {
        std::cout << this << ":" << " copy construct from " << &other << std::endl;
    }
    trace(trace&& other) {
        std::cout << this << ":" << " move construct from " << &other << std::endl;
    }
    trace& operator=(trace const& other) {
        std::cout << this << ":" << " copy assign from    " << &other << std::endl;
        return *this;
    }
    trace& operator=(trace&& other) {
        std::cout << this << ":" << " move assign from    " << &other << std::endl;
        return *this;
    }
};

inline bool operator<(trace const& lhs, trace const& rhs) {
    std::cout << &lhs << " < " << &rhs << std::endl;
    return &lhs < &rhs;
}

int main () {
    std::set<trace> s;
    s.insert(trace());
    s.insert(trace());
    s.insert(trace());
    auto it = s.begin();
    ++it;

    assert(s.size() == 3);
    std::cout << "[[extract]]" << std::endl;
    trace t = std::move(s.extract(it).value());
    assert(s.size() == 2);
}

正在运行的演示：https://wandbox.org/permlink/ZZHkZV1DUZpM3YrU

我得到以下结果：

0x7ffd30bbfbd0: construct
0x55edd361d2a0: move construct from 0x7ffd30bbfbd0
0x7ffd30bbfbd0: destruct
0x7ffd30bbfbc8: construct
0x7ffd30bbfbc8 < 0x55edd361d2a0
0x55edd361d2a0 < 0x7ffd30bbfbc8
0x7ffd30bbfbc8 < 0x55edd361d2a0
0x55edd361d2d0: move construct from 0x7ffd30bbfbc8
0x7ffd30bbfbc8: destruct
0x7ffd30bbfbc0: construct
0x7ffd30bbfbc0 < 0x55edd361d2a0
0x7ffd30bbfbc0 < 0x55edd361d2d0
0x55edd361d2d0 < 0x7ffd30bbfbc0
0x7ffd30bbfbc0 < 0x55edd361d2d0
0x55edd361d300: move construct from 0x7ffd30bbfbc0
0x7ffd30bbfbc0: destruct
[[extract]]
0x7ffd30bbfbb0: move construct from 0x55edd361d2d0
0x55edd361d2d0: destruct
0x7ffd30bbfbb0: destruct
0x55edd361d300: destruct
0x55edd361d2a0: destruct

在[[extract]]之后，容器将无法访问对象移动。没事。

我正在寻找一种在boost::multi_index上执行相同操作的方法。

我尝试了与以下问答相同的方法：
Move element from boost multi_index array

我尝试了相同的方法：

namespace mi = boost::multi_index;

using mi_trace = mi::multi_index_container<
    trace,
    mi::indexed_by<
        mi::ordered_unique<
            mi::identity<trace>
        >
    >
>;

int main () {
    mi_trace mi;

    mi.insert(trace());
    mi.insert(trace());
    mi.insert(trace());

    auto it = mi.begin();
    ++it;

    assert(mi.size() == 3);
    std::optional<trace> target;
    std::cout << "[[modify]]" << std::endl;
    if (mi.modify(
            it,
            [&](auto& e) {
                target.emplace(std::move(e));
            }
        )
    ) {
        std::cout << "[[erase]]" << std::endl;
        mi.erase(it);
        assert(mi.size() == 2);
    }
}

正在运行的演示：https://wandbox.org/permlink/eKpGDpMBbx5aRz9O

并得到以下输出：

[[modify]]
0x7fffe1f77c66: move construct from 0x55fdda3272e0
0x55fdda3272b0 < 0x55fdda3272e0
0x55fdda3272e0 < 0x55fdda327310
[[erase]]
0x55fdda3272e0: destruct
0x7fffe1f77c66: destruct
0x55fdda3272b0: destruct
0x55fdda327310: destruct

返回修改的lambda后，从对象（multi_index）移动的0x55fdda3272e0容器访问。我认为这是为了重新排序。 modify()不知道我将modify()与erase()一起使用。对于random_access_index，它可以很好地工作，因为容器不需要重新排序，但对于ordered_index不起作用。

有什么办法可以从ordered_index ed multi_index中移动元素吗？

最佳答案

您可以使用以下内容从multi_index_container中提取值，同时确保在提取值后立即擦除该元素：

struct extract_value_exception{};

template<typename MultiIndexContainerIndex>
auto extract_value(
    MultiIndexContainerIndex& i,
    typename MultiIndexContainerIndex::iterator it)
{
    using value_type = typename MultiIndexContainerIndex::value_type;

    std::optional<value_type> o;
    try{
        i.modify(it, [&](value_type& x){
            o.emplace(std::move(x));
            throw extract_value_exception{};
        });
    }
    catch(const extract_value_exception&){}
    return std::move(*o);
}

完整的示例如下。

Live On Wandbox

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/identity.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <cassert>
#include <iostream>
#include <optional>

struct trace {
    trace() {
        std::cout << this << ":" << " construct" << std::endl;
    }
    ~trace() {
        std::cout << this << ":" << " destruct" << std::endl;
    }
    trace(trace& other) {
        std::cout << this << ":" << " copy construct from " << &other << std::endl;
    }
    trace(trace&& other) {
        std::cout << this << ":" << " move construct from " << &other << std::endl;
    }
    trace& operator=(trace const& other) {
        std::cout << this << ":" << " copy assign from    " << &other << std::endl;
        return *this;
    }
    trace& operator=(trace&& other) {
        std::cout << this << ":" << " move assign from    " << &other << std::endl;
        return *this;
    }
};

inline bool operator<(trace const& lhs, trace const& rhs) {
    std::cout << &lhs << " < " << &rhs << std::endl;
    return &lhs < &rhs;
}

struct extract_value_exception{};

template<typename MultiIndexContainerIndex>
auto extract_value(
    MultiIndexContainerIndex& i,
    typename MultiIndexContainerIndex::iterator it)
{
    using value_type = typename MultiIndexContainerIndex::value_type;

    std::optional<value_type> o;
    try{
        i.modify(it, [&](value_type& x){
            o.emplace(std::move(x));
            throw extract_value_exception{};
        });
    }
    catch(const extract_value_exception&){}
    return std::move(*o);
}

int main () {
    boost::multi_index_container<trace> s;
    s.insert(trace());
    s.insert(trace());
    s.insert(trace());
    auto it = s.begin();
    ++it;

    assert(s.size() == 3);
    std::cout << "[[extract]]" << std::endl;
    trace t = extract_value(s, it);
    assert(s.size() == 2);
}

输出量

0x7ffd5feed89d: construct
0x21f7190: move construct from 0x7ffd5feed89d
0x7ffd5feed89d: destruct
0x7ffd5feed89e: construct
0x7ffd5feed89e < 0x21f7190
0x21f7190 < 0x7ffd5feed89e
0x21f71c0: move construct from 0x7ffd5feed89e
0x7ffd5feed89e: destruct
0x7ffd5feed89f: construct
0x7ffd5feed89f < 0x21f7190
0x7ffd5feed89f < 0x21f71c0
0x21f71c0 < 0x7ffd5feed89f
0x21f71f0: move construct from 0x7ffd5feed89f
0x7ffd5feed89f: destruct
[[extract]]
0x7ffd5feed836: move construct from 0x21f71c0
0x21f71c0: destruct
0x7ffd5feed867: move construct from 0x7ffd5feed836
0x7ffd5feed836: destruct
0x7ffd5feed867: destruct
0x21f7190: destruct
0x21f71f0: destruct

关于c++ - 有什么方法可以将元素从ordered_index和multi_index中移出？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/57335172/