Generating a derived class instance from a string of its name. (i.e, reflection)

Question

If I have a base class:

class Base 
{
public:
virtual void Test()=0;
};

and, in a dynamically loaded module (.so/.dll), I implemented a class derived from this:

class SomethingFromBase : Base 
{
...
};

and, the user, once this library is loaded, asks to create an instance of SomethingFromBase (let's say we get the name from cin.), and, we have no knowledge of SomethingFromBase (i.e, no way to just do if(inputstr == "SomethingFrombase") { ... } is there any way to create an instance of SomethingFromBase?

I'm fairly certain this is impossible in (standard) C++, but, I'm always hoping SO will surprise me!

If this is possible with the addition of some library, I'd still like to know about it. Thanks!

Edit: See cdhowie's answer. Guides to implementing this technique: http://www.linuxjournal.com/article/3687?page=0,1 http://www.abstraction.net/ViewArticle.aspx?articleID=67

cdhowie · Accepted Answer · 2010-11-20 04:31:10Z

5

You typically achieve this by required that plugin libraries define some global variable of a struct type that contains pointers to various functions you can call. (Like setup, teardown, etc.) During the setup function, they would call back into your application to some "register" function where they could pass in a string representing the name of the class, and a factory function pointer that will create an instance when executed.

You stash this away in some map, and when the user enters a string, you look at the map to see if there is a factory function registered. If so, just call it.

So this is not "true" reflection, but you can manually hack it to some degree. See, for example, Pidgin, which allows the specification of protocol plugins that may supply many entries in the protocol list.

EDIT: Here is a nifty guide to implementing something similar. I'm more of a C person, so I can't vouch for it being truly awesome or anything, but at first glance it looks good. I've done similar stuff in C on Linux, and the basic approach works pretty well.

edited Nov 20, 2010 at 4:31

answered Nov 20, 2010 at 4:25

cdhowie

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3 Comments

asdfjklqwer Over a year ago

Yes, this is the only possible way I can see to achieve this. You need to maintain some sort of registry which stores each new type and associates it with an arbitrary string.

please delete me Over a year ago

Ah, nice and sensible, and practically no overhead, since its not "real" reflection. Cool stuff.

cdhowie Over a year ago

Yup, only the overhead of function pointers. Also, check out the guide I just added to my answer.

riderchap · Accepted Answer · 2010-11-20 05:23:16Z

Have a map store the objects of the class, keyed by the class name. All the classes that need to be created this way, need to be derived from some base class named something like "Creatable" The code to add the object of the class need to be given with the implementation of class.

//Creatable.h
#define IMPLEMENT_CREATABLE( ClassName ) \
  ObjectMap::Instance().Add( string(ClassName), new ClassName );

//ObjectMap.h. This is a singleton
class ObjectMap
{
  ...........
  map<string, Creatable *> myMap;
  ...........
public:
  void Add( const string &className, Creatable * );
  Creatable * Get( const string &className );
};

//My Class.h
class MyClass : public Creatable
{
  ................
};

//My Class.cpp
IMPLEMENT_CREATABLE(MyClass);


//Client.cpp
string className;
cin>>className;
Creatable *anObject = ObjectMap::Instance().Get( className );

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