C++ :: Implicit Conversions To / From Library Type Only In Implementation

1. I designed two classes: Processor and Data.

2. Data represents some data object the user can pass to Processor's public methods.

3. Internally, Processor needs to use InternalData type which is based on the content of Data (I can use Data's public interface to get the required information from it, or construct a Data object using its public constructor when needed, and that's how I have done it so far).

4. To avoid repeating code and localize changes required when Data's interface would change someday, I made conversion functions from Data to InternalData and back inside Processor, as private methods.

Now here comes the kicker:

5. But I'd like these conversions to be implicit inside Processor's methods instead of explicit. And only there.

6. These conversion functions are only for Processor implementation's use. They shouldn't be visible nor accessible from the outside world.

Where the problem lays:

7. InternalData is a library type. I don't have control over it and I cannot modify its interface.

That is, I cannot just add converting constructors or conversion operator member functions to them.You can consider it to be built-in type if you wish.

8. I don't want to put those converters inside Data class either, since it's not its business and it shouldn't know that Processor converts it to something else internally.

Long story short, I'd like to teach the Processor's implementation how to make type conversions between Data and InternalData implicitly, but no one else except Processor should be affected by it. Outside world shouldn't be able to do these conversions or even know about them being done inside Processor's implementation.Is there any way to do it in C++?

The core of the problem seems to be the fact that in C++ defining implicit conversions is possible only from/to a user-defined type when defining this type. I don't know of any way to define such conversions for some other type's internal use only. (Especially when I don't have control about one of these converted types.)