Thread: Best place to forward declare PIMPL class?

Originally Posted by MutantJohn

Are there other practical reasons that separate this from just declaring in a header file and then implementing in a source file?

Some practical reasons are:
- Compile time (changing private members no longer causes a recompile in all dependent source files)
- Hiding private members from public declarations (less "noise" in the public class declaration)
- Binary compatibility (even if you add variables to the impl object, the size of the pointer won't change, so you won't break binary compatibility)

Was not expecting all these replies. I will add that I am learning about the PIMPL (compiler firewall) idiom to improve my knowledge of common practices. I've noticed a lot of Qt code that uses this idiom. Even if you end up working in where it's not necessary/desirable, it's good to know. You're improving your ability to parse other programmers' code mentally at the very least.

I also should add that I was able to answer one of my own questions. If the class used for the implementation object is declared locally (that is within the visible class), then it is considered a local class and has some restrictions outlined in this link. c++ - Why aren't static data members allowed in local classes? - Stack Overflow

Originally Posted by jiggunjer

Sorry just checking: so the client can't declare the PIMPL class in their code if they want to use it anyway? Not saying they should, just wondering if it is possible.

If you define/forward-declare it as a private member of the visible class, it should not be accessible through other parts of the hypothetical API / product that you are building. You might provide documentation for your own or other developers' use, but going with the general spirit of "hiding the implementation details" you would only provide information/docs about the interface (accessor functions, etc) to the common user of the product / API / whatever.

EDIT: Also my original phrasing in the title may (?) be an abuse of terms. As far as I understand it, the term 'pimpl' comes from 'pointer to implementation', and it is a common variable name given to the pointer that points to the internal implementation class.