如何选择带有类型参数的成员变量? [英] How do I select a member variable with a type parameter?

问题描述

我有一个缓存对象，可以缓存许多不同类型的对象，如下所示:

I have a cache object that caches a number of different types of objects, as illustrated below:

class Cache
{
public:
    ObjectTable<ObjTypeA> m_objACache;
    ObjectTable<ObjTypeB> m_objBCache;
    ObjectTable<ObjTypeC> m_objCCache;
};

我目前正在使用缓存的(糟糕)方式是直接访问缓存类属性"m_objACache"和"m_objBCache"，如下所示:

The (horrible) way I'm currently using the cache at the moment is directly accessing the cache class properties "m_objACache" and "m_objBCache" like this:

Cache c;
c.m_objACache.getObjectWithid(objectBuffer, 1);
c.m_objACache.getObjectWithid(objectBuffer, 2);
c.m_objBCache.getObjectWithid(objectBuffer, 3);

等.

我想做的是这样的:-

class Cache
{
public:
    template <typename T>
    void getObjectWithId(T &objectBuffer, int id)
    {
        ObjectTable<T>.getObjectWithId(objectBuffer, id);
    }
};

但是显然这行不通，因为我有"ObjectTable<T>"的地方，我需要一个变量名，但是我不能模板类变量-那么有什么办法可以做到这一点?还是声明所有变量并按如下方式访问它:

But obviously that does not work because where I have "ObjectTable<T>" I need a variable name, but I cannot template class variables - so is there a way I can do this? Or is it going to be a case if declaring all of the variables and accessing it like this:

class Cache
{
public:
    void getObjectWithId(ObjTypeA &objectBuffer, int id)
    {
        m_objACache.getObjectWithId(objectBuffer, id);
    }

    void getObjectWithId(ObjTypeB &objectBuffer, int id)
    {
        m_objBCache.getObjectWithId(objectBuffer, id);
    }

    void getObjectWithId(ObjTypeC &objectBuffer, int id)
    {
        m_objCCache.getObjectWithId(objectBuffer, id);
    }

protected:
    ObjectTable<ObjTypeA> m_objACache;
    ObjectTable<ObjTypeB> m_objBCache;
    ObjectTable<ObjTypeC> m_objCCache;
};

似乎很冗长..

ObjectTable可以使用的每个对象类型都有一个通用的基类，因此可能有其他方法可能不可避免地涉及向下转换，但我希望可以找到更好的方法.

Each object type that an ObjectTable can be used for has a common base class, so there could be some other way of doing this that may inevitably involve downcasting, but I'm hoping I can find a better way.

谢谢！

推荐答案

也许像这样吗?

class Cache
{
 // An "envelope" type which up-casts to the right ObjectTable<T> 
 // if we have a type parameter T. 
 struct ObjectTables : ObjectTable<ObjTypeA>,  
                       ObjectTable<ObjTypeB>, 
                       ObjectTable<ObjTypeC> {};

 ObjectTables tables; 
public:

    template <typename T>
    void getObjectWithId(T &objectBuffer, int id)
    { 
        // C++ does the work here
        ObjectTable<T> &o=tables;
        t.getObjectWithId(objectBuffer, id);
    }
};

此外，它很容易扩展.如果需要支持更多类型，只需投入更多的ObjectTables<>.

Also, it's easy to extend. Just throw in more ObjectTables<> if you need to support more types.

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