*Impl stuff

[cx500tc]

Poco is a great C++ learning too for me.

That being said I have been trying to understand the rational behind the usage of the *Impl in many Poco object:

For example Pipe has a pointer to PipeImpl, same goes for Socket which has a socket to SocketImpl and so forth. Why this design choice versus inheritance.

At first I though this was a way to speed up compilation by using a forward declaration of the Implement object but each .h file includes it's Impl.h counterpart so that's not it.

- Mario

[guenter]

The reason why the pImpl idiom is used for sockets and pipes is that Impl objects are reference counted. This allows us to use a Socket or Pipe objects as value objects, supporting copy constructor and assignment.

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StreamSocket s1("localhost", 1234);
StreamSocket s2 = s1;
StreamSocket s3;
...
s3 = s1;


Basically, Socket, StreamSocket, Pipe, etc. are nothing more than fancy reference counting smart pointers for the respective *Impl objects. The main advantage is cleaner code, greater ease of use and less proneness to certain programming errors.

[guenter]

The #include "*Impl.h" is mostly necessary because we are using inline forwarding member functions like

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inline int Pipe: :writeBytes(const void* buffer, int length)
{
   return _pImpl->writeBytes(buffer, length);
}