c++ - boost::multi_index 随机访问唯一索引

Question

我的应用程序中有一个 boost::multi_index 容器，它包含对象的共享指针 (std::shared_ptr)。正如我从文档中了解到的那样，它创建了具有对唯一索引的双向访问的树结构。是否可以告诉容器有一个数组结构，可以随机访问唯一索引(类似于 boost::flat_set)？

Answer 1

没有多索引

当然，您可以完全按照您的描述进行操作:

Live On Coliru

#include <boost/container/flat_set.hpp>
#include <memory>
#include <vector>
#include <iostream>

template <typename V, typename... Init>
auto make_shared_flat_set(Init&&... values) {
    return boost::container::flat_set<std::shared_ptr<V> > { 
        std::make_shared<V>(std::forward<Init>(values))...
    };
}

int main() {
    using sptr = std::shared_ptr<std::string>;
    auto const set = make_shared_flat_set<std::string>("one", "two", "three", "four", "Hungary");

    // the set-like interface: (ordered_unique)
    std::cout << "set: ";
    for (auto& e : set) std::cout << *e << " ";

    {
        // let's make a random access copy:
        std::vector<sptr> random_access(set.begin(), set.end());
        std::cout << "\ncopy: ";
        for (auto& e : random_access) std::cout << *e << " ";
    }

    {
        // let's make a random access view:
        using spref = std::reference_wrapper<sptr const>;
        std::vector<spref> random_access(set.begin(), set.end());
        std::cout << "\nview: ";
        for (sptr const& e : random_access) std::cout << *e << " ";
    }
}

打印

set: one two three four Hungary 
copy: one two three four Hungary 
view: one two three four Hungary 

使用多索引

Live On Coliru

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/identity.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/random_access_index.hpp>
#include <memory>
#include <iostream>

namespace bmi = boost::multi_index;

template <typename V> using Table = bmi::multi_index_container<std::shared_ptr<V>,
        bmi::indexed_by<
            bmi::random_access<bmi::tag<struct ra_idx> >,
            bmi::ordered_unique<bmi::tag<struct set_idx>, bmi::identity<V> >
        >
    >;

template <typename V, typename... Init>
auto make_shared_flat_set(Init&&... values) {
    return Table<std::string> { 
        std::make_shared<V>(std::forward<Init>(values))...
    };
}

int main() {
    auto const set = make_shared_flat_set<std::string>("one", "two", "three", "four", "Hungary");

    // the set-like interface: (ordered_unique)
    auto& as_set = set.get<set_idx>();

    std::cout << "set: ";
    for (auto& e : as_set) std::cout << *e << " ";

    // the random access interface:
    std::cout << "\nrandom access: ";
    auto& random_access = set.get<ra_idx>();
    for (auto& e : random_access) std::cout << *e << " ";
}

打印:

set: Hungary four one three two 
random access: one two three four Hungary 

奖励:

要将 random_access 索引放入与集合相同的顺序，请使用重新排列:

// put the random access in the same order as the set:
random_access.rearrange(make_ro_view(as_set).begin());

std::cout << "\nrandom access (rearranged): ";
for (auto& e : random_access) std::cout << *e << " ";

Live On Coliru

打印

set: Hungary four one three two 
random access: one two three four Hungary 
random access (rearranged): Hungary four one three two