C++：使用Boost MultiIndex判断指针类型的class

Question

这个问题的答案具有普遍的应用性，但我会用下面的例子来激发它：

我有以下模板class：

template <typename V>
class Collection
{
public:
    struct id{};
    struct name{};
    // A collection parameterized by type V that is indexed by a string 'name'
    // and a integer 'id'
    // The type V must implement 'get_id()' and a 'get_name()' functions
    typedef multi_index_container <
        V,
        indexed_by<
            ordered_unique<
                tag<id>, const_mem_fun<V, unsigned int, &V::get_id> >,
            ordered_unique<
                tag<name>, const_mem_fun<V, std::string, &V::get_name> >
        >
    > ni_collection;
>

我想修改此模板，以便我可以使用对象、它们的指针或它们的引用创建一个集合：Collection<Obj>、Collection<std::unique_ptr<Obj>> 或 Collection<Obj *>。

我将如何修改我的模板以实现此目的？

---更新--- 我在这里发布了一个相关问题：

综合两地优秀答案，终于达到了最初的目的。以下是我当前实施的详细信息：

template <typename V>
class Collection
{
private:
// A type-level function that returns the undecorated type of the object
// So unrwap_object_type<Widget *> = Widget
//    unwrap_object_type<std::unique_ptr<Widget>> = Widget
//    unwrap_object_type<Widget> = Widget
template<typename T, typename = void>
struct unwrap_object_type { typedef T type; };

template <typename T>
struct unwrap_object_type<T *, void> { typedef T type; };

template<typename T>
struct unwrap_object_type<T,
    typename std::conditional<false,
        typename T::element_type, void>::type>
{
  typedef typename T::element_type type;
};
////

// So that QHNETO_COLLECTION<Widget>, QHNETO_COLLECTION<Widet *>,
// and QHNETO_COLLECTION<std::unique_ptr<Widget>> are valid
typedef typename unwrap_object_type<V>::type W;

// Tags for the two indices (id and name) of the collection
struct id;
struct name;

// A collection parameterized by type V that is indexed by a string 'name'
// and a integer 'id'
// The type V must implement 'get_id()' and a 'get_name()' functions
typedef multi_index_container <
    V,
    indexed_by<
        ordered_unique<
            tag<id>,
            const_mem_fun<W, unsigned int, &W::get_id> >,
        ordered_unique<
            tag<name>,
            const_mem_fun<W, std::string, &W::get_name> >
    >
> ni_collection;

ni_collection m_collection;
};

Answer 1

您可以使用自定义 key extractor. E.g: Advanced features of Boost.MultiIndex key extractors

For reference elements, consider boost::reference_wrapper or std::reference_wrapper

Live On Coliru

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/mem_fun.hpp>
#include <boost/multi_index/ordered_index.hpp>

namespace bmi = boost::multi_index;

template <typename V> class Collection {
    struct id_extractor {
        typedef unsigned result_type;
        template <typename U> result_type operator()(U const&e) const { return e.get_id(); }
        template <typename U> result_type operator()(U*e)       const { return e->get_id(); }
    };
    struct name_extractor {
        typedef std::string result_type;
        template <typename U> result_type const& operator()(U const&e) const { return e.get_name(); }
        template <typename U> result_type const& operator()(U*e)       const { return e->get_name(); }
    };
  public:
    struct id;
    struct name;

    // A collection parameterized by type V that is indexed by a string 'name'
    // and a integer 'id'
    // The type V must implement 'get_id()' and a 'get_name()' functions
    typedef boost::multi_index_container<
        V, bmi::indexed_by<bmi::ordered_unique<bmi::tag<id>, name_extractor >,
                           bmi::ordered_unique<bmi::tag<name>, id_extractor > > >
        ni_collection;
};

struct Demo {
    unsigned _id;
    std::string _name;

    unsigned           get_id() const { return _id; }
    std::string const& get_name() const { return _name; }
};

int main() {
    Collection<Demo>::ni_collection works{ { 42, "LTUAE" }, { 4, "PI" } };
    Collection<Demo *>::ni_collection also_works{ new Demo{ 42, "LTUAE" }, new Demo{ 4, "PI" } };
}

Answer 2

详细说明@sehe 的回答：Boost.MultiIndex 预定义的键提取器自动处理解引用（例如，const_mem_fun<foo,bar,&foo::bar> 可以与 multi_index_container 的 foo* 一起使用).您可以利用此功能并编写以下内容（没有任何 user-provided 密钥提取器）：

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#include <boost/multi_index_container.hpp>
#include <boost/multi_index/mem_fun.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <memory>

namespace bmi = boost::multi_index;

template<typename T>
struct remove_pointer{using type=T;};

template<typename T>
struct remove_pointer<T*>{using type=T;};

template<typename T>
struct remove_pointer<std::shared_ptr<T>>{using type=T;};

template <typename V> class Collection {
  public:
    struct id;
    struct name;

    using W=typename remove_pointer<V>::type;

    typedef boost::multi_index_container<
        V,
        bmi::indexed_by<
            bmi::ordered_unique<
                bmi::tag<id>,
                bmi::const_mem_fun<W, unsigned int, &W::get_id>
            >,
            bmi::ordered_unique<
                bmi::tag<name>,
                bmi::const_mem_fun<W,const std::string&, &W::get_name> 
            > 
        >
    > ni_collection;
};

struct Demo {
    unsigned _id;
    std::string _name;

    Demo(unsigned _id,const std::string& _name):_id(_id),_name(_name){}

    unsigned           get_id() const { return _id; }
    std::string const& get_name() const { return _name; }
};

int main() {
    Collection<Demo>::ni_collection works{ { 42, "LTUAE" }, { 4, "PI" } };
    Collection<Demo *>::ni_collection also_works{ new Demo{ 42, "LTUAE" }, new Demo{ 4, "PI" } };
    Collection<std::shared_ptr<Demo>>::ni_collection this_too{ std::make_shared<Demo>( 42, "LTUAE" ), std::make_shared<Demo>( 4, "PI" ) };
}

唯一棘手的部分是 const_mem_fun 使用 W=std::remove_pointer<V>::type（即 V 如果 V 是普通类型或它指向的类型如果它是一个指针）。

已编辑： 而不是 std::remove_pointer<V>，更新后的代码使用手工制作的 remove_pointer 模板 class 部分专门用于理解 T* 和 std::shared_ptr<T>；您可以扩展它以涵盖例如 std::unique_ptr<T> 或您需要满足的任何其他智能指针 class。

C++：使用Boost MultiIndex判断指针类型的class

C++: Determine the class of a pointer type in the use of Boost MultiIndex

c++

boost

types

boost-multi-index