How to wrap a C++ library - oop

Question

I'm developing an application which is using a library and I would like to wrap this library so that it does not goes to deep into my application code. Thanks to that I could change the library I'm using just by re-implementing my wrapper classes.
Suppose that I have a library LibA. It gives me 2 objects to work with, LibAObj1 and LibAObj2. LibAObj2 has a method using LibAObj1.
Here can be a simple definition of their declaration
class LibAObj1 {};
class LibAObj2
{
void action(LibAObj1 &obj);
};
Now I would like to define an interface that my application can use to wrap those objects in my application code
For instance:
class ItfLibAObj1 {};
class ItfLibAObj2
{
public:
void action(ItfLibAObj1 &obj) = 0;
};
The problem comes whenever I want to implement my interface ItfLibAObj2.
class ImplLibAObj2 : public ItfLibAObj2
{
public:
void action(ItfLibAObj1 &itfObj)
{
<how to get my LibAObj1 from itfObj>?
obj.action(LibAObj1);
}
private:
LibObj2 obj;
}
The question is actually in the pseudo code. How to get my LibAObj1 contained in my ItfLibAObj1 reference? I could add a getter function in LibAObj1 interface to return a void pointer that I would cast but I don't find that elegant in C++.
Is there any kind of design pattern I could use to solve my problem? Or do I just have a design issue?
Note that I'm not wishing to select which library to use at run time.
Thanks a lot for your help.
Kind regards

Answer 1

You problem perfectly explains why in Proxy Pattern, both the Real and Proxy must implement the same interface:
And your code should look like this:
// interfaces
class ItfLibAObj1 {};
class ItfLibAObj2
{
public:
void action(ItfLibAObj1 &obj) = 0;
};
// real
class RealLibAObj1 : public ItfLibAObj1 {};
class RealLibAObj2 : public ItfLibAObj2
{
void action(ItfLibAObj1 &obj)
{
...
}
};
// proxy
class ProxyLibAObj1 : public ItfLibAObj1
{
private:
RealLibAObj1 real;
};
class ProxyLibAObj2 : public ItfLibAObj2
{
private:
RealLibAObj2 real;
void action(ItfLibAObj1 &obj)
{
// do something
real.action(obj); // delegate to the real
// do something
}
};
However, if the whole purpose of your "wrapping" is adding a new layer between your core/real and the outside (client), please consider the Facade Pattern which provides a simpler interface to the client, instead of merely mimic the classes/methods of the core.