Warning: the code here only showcases the storage aspect, it is a skeleton, no dynamic aspect (construction, copy, move, destruction) has been taken into account. I would suggest an approach using the C++0x new class aligned_storage, which is precisely meant for having raw storage. // header class Foo { public: private: struct Impl; Impl& impl() … Read more
If we go to the cppreference document for std::unique_ptr: std::unique_ptr may be constructed for an incomplete type T, such as to facilitate the use as a handle in the Pimpl idiom. If the default deleter is used, T must be complete at the point in code where the deleter is invoked, which happens in the … Read more
I’d say that whether you do it per-class or on an all-or-nothing basis depends on why you go for the pimpl idiom in the first place. My reasons, when building a library, have been one of the following: Wanted to hide implementation in order to avoid disclosing information (yes, it was not a FOSS project … Read more
I’ve been making some objects using the pimpl idiom, but I’m not sure whether to used shared_ptr or unique_ptr. Definitely unique_ptr or scoped_ptr. Pimpl is not a pattern, but an idiom, which deals with compile-time dependency and binary compatibility. It should not affect the semantics of the objects, especially with regard to its copying behavior. … 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
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
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
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