I believe that your test_help.cpp actually sees the ~Help() destructor that you declared default. In that destructor, the compiler tries to generate the unique_ptr destructor, too, but it needs the Impl declaration for that. So if you move the destructor definition to the Help.cpp, this problem should be gone. — EDIT — You can define … Read more
When writing a C++ class, it’s appropriate to think about whether it’s going to be A Value Type Copy by value, identity is never important. It’s appropriate for it to be a key in a std::map. Example, a “string” class, or a “date” class, or a “complex number” class. To “copy” instances of such a … Read more
You are correct; the example seems to be missing an explicit template instantiation. When I try to run the example with a constructor and destructor for widget::impl on MSVC 2010 SP1, I get a linker error for pimpl<widget::impl>::pimpl() and pimpl<widget::impl>::~pimpl(). When I add template class pimpl<widget::impl>;, it links fine. In other words, GotW #101 eliminates … Read more
You define and document for your types what a ‘valid’ state is and what operation can be performed on moved-from objects of your types. Moving an object of a standard library type puts the object into an unspecified state, which can be queried as normal to determine valid operations. 17.6.5.15 Moved-from state of library types … Read more
Introduction The PIMPL is a private class that contains all of the implementation-specific data of the parent class. Qt provides a PIMPL framework and a set of conventions that need to be followed when using that framework. Qt’s PIMPLs can be used in all classes, even those not derived from QObject. The PIMPL needs to … Read more
So, I am wondering it this technique is really used in practice? Should I use it everywhere, or with caution? Of course it is used. I use it in my project, in almost every class. Reasons for using the PIMPL idiom: Binary compatibility When you’re developing a library, you can add/modify fields to XImpl without … Read more
I think most people refer to this as the Handle Body idiom. See James Coplien’s book Advanced C++ Programming Styles and Idioms. It’s also known as the Cheshire Cat because of Lewis Caroll’s character that fades away until only the grin remains. The example code should be distributed across two sets of source files. Then … Read more