在类中保存具有不同值类型的多个映射并支持基于类型的查找

时间:2015-11-09 12:55:39

标签: c++ tuples

我想使用具有不同值类型的多个映射来实现异构存储。我根据值类型索引地图。为此,我使用的是类型列表。这是一个简化的例子。

我的type-list课程如下所示。

namespace details
{
    struct null{};

    template<int N, typename E, typename T, typename ... Ts>
    struct index : index<N+1, E, Ts...> {};

    template<int N, typename E, typename ... Ts>
    struct index<N, E, E, Ts...>
    {
        constexpr static int value = N;
    };

    template<int N, typename E>
    struct index<N, E, null>
    {
        constexpr static int value = -1;
    };
}

template<typename ... Ts>
struct typelist
{
    using type = typelist<Ts...>;
    template<typename T>
    struct index : details::index<0, T, Ts..., details::null> {};
};

以下的存储类实现,一个模板类,其类型为T

#include <iostream>
#include <tuple>
#include <typeinfo>
#include <map>


template<typename T>
struct storage_impl
{
    //storage_impl(storage_impl const &) = delete;

    bool try_getting(int key, T &value)
    {
        auto search = _storage.find(key);
        if(search != _storage.end() )
        {
            value = search->second;
            return true;
        }
        return false;   
    }
    std::map<int,T> _storage;
};

最后,存储管理器看起来像是,它有几个storage_impl实例,我想根据类型对它们进行索引。

struct storage_manager{
    // tuple storing different storage_impls
    std::tuple< storage_impl<double> , storage_impl<std::string> > storages { storage_impl<double>{},storage_impl<std::string>{} };
    using item_types = typelist<double,std::string>;

    storage_manager(){}
    ~storage_manager(){}
    storage_manager(storage_manager const &) = delete;

    template<typename T>
    bool try_getting(int key, T &value)
    {
        return std::get<item_types::index<T>::value>(storages).try_getting(key,value);
    }
};

int main()
{ 
    storage_manager  mgr;
    double val1;
    std::cout<<mgr.try_getting(123,val1);
}

每件事都有效,直到storage_implcopyable。但我希望storage_implnon-copyable

如何与我的storage_impl non-copyable实现相同的目标。

P.S。 - 不想使用继承。

1 个答案:

答案 0 :(得分:0)

以下是我的工作代码,我已经将cache_impls声明为成员变量,并为每种情况声明了专用函数storage_ref。 我只实现了try_get,其他功能可以使用storage_ref实现。

#include <iostream>
#include <tuple>
#include <map>
#include <boost/noncopyable.hpp>

template<typename T>
struct storage_impl : public boost::noncopyable
{
    storage_impl(){}
    ~storage_impl(){}

    bool try_getting(int key, T &value)
    {
        auto search = _storage.find(key);
        if(search != _storage.end() )
        {
            value = search->second;
            return true;
        }
        return false;   
    }
    bool try_setting(int key, T const &value)
    {
        auto search = _storage.insert(std::make_pair(key,value));
        if(search.second == true )
        {
            return true;
        }
        return false;   
    }

    bool exists(int key)
    {
        auto search = _storage.find(key);
        if(search != _storage.end() )
        {
            return true;
        }
        return false; 
    }

    std::map<int,T> _storage;
};

struct storage_manager  : public boost::noncopyable
{
    storage_impl<double> double_store;
    storage_impl<std::string> string_store;

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

    template<typename T>
    typename item_return<T>::type storage_ref();

    template<typename T> 
    bool try_getting(int key, T &value)
    {
        return storage_ref<T>().try_getting(key,value);
    }
    template<typename T> 
    bool try_setting(int key, T const &value)
    {
        return storage_ref<T>().try_setting(key,value);
    }

    template<typename T> 
    bool exists(int key)
    {
        return storage_ref<T>().exists(key);
    }

    storage_manager(){}
    ~storage_manager(){}
};


//double specialization
template<>
struct storage_manager::item_return<double>{ using type = storage_impl<double>&; };

template<>
inline storage_impl<double>& storage_manager::storage_ref<double>()
{
    return double_store;
}

//std::string specialization
template<>
struct storage_manager::item_return<std::string>{ using type = storage_impl<std::string>&; };

template<>
inline storage_impl<std::string>& storage_manager::storage_ref<std::string>()
{
    return string_store;
}

int main()
{
    storage_manager mgr;
    double val1 = 90;
    std::cout<<mgr.try_getting(123,val1)<<'\n';

    std::cout<<mgr.try_setting(123,val1)<<'\n';

    std::cout<<mgr.exists<double>(123)<<'\n';

    std::string val2;
    std::cout<<mgr.try_getting(123,val2)<<'\n';  
}

期待替代品......