C++11 - Possible dilemma with pimpl-idiom and unique_ptr?

Question

As I was trying to take advantage of pimpl-idiom and smart pointers to implement my own wrapper around platform-specific GUI components, I encountered a problem I'm unable to solve. The problem is probably with pimpl-idiom and *unique_ptr*, since I don't know what else could be wrong. All errors seem to be linking errors. I get various undefined reference errors while using the code (link below): http://www.cplusplus.com/forum/general/96020/

Here is the list of errors:

> obj\debug_windows\helixirrwidgets\src\helixirrwidgets\Window-windows.o||In
> function `Window':|
> E:\new2\projects\programming\test2\test2\..\..\helixirrwidgets\src\helixirrwidgets\Window.hpp|15|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper>
> >::PointerSmartSafe(HelixirrWidgets::Window::Helper*)'| E:\new2\projects\programming\test2\test2\..\..\helixirrwidgets\src\helixirrwidgets\Window.hpp|15|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper>
> >::PointerSmartSafe(HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper> > const&)'|
> obj\debug_windows\helixirrwidgets\src\helixirrwidgets\Window-windows.o||In
> function `Window':|
> E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|88|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper>
> >::PointerSmartSafe(HelixirrWidgets::Window::Helper*)'| E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|91|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper>
> >::PointerSmartSafe(HelixirrWidgets::Window::Helper*)'| obj\debug_windows\helixirrwidgets\src\helixirrwidgets\Window-windows.o||In
> function `ZN15HelixirrWidgets6Window12save_changesEv':|
> E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|98|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper> >::operator->()
> const'|
> E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|100|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper> >::operator->()
> const'|
> E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|101|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper> >::operator->()
> const'|
> E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|106|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper> >::operator->()
> const'|
> E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|107|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper> >::operator->()
> const'|
> E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|109|undefined
> reference to `GetStockObject@4'|
> E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|111|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper> >::operator->()
> const'|
> E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|112|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper> >::operator->()
> const'|
> E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|114|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper> >::operator->()
> const'|
> E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|118|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper> >::operator->()
> const'|
> E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|119|undefined
> reference to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper> >::operator->()
> const'|
> obj\debug_windows\helixirrwidgets\src\helixirrwidgets\Window-windows.o:E:\new2\projects\programming\helixirrwidgets\src\helixirrwidgets\Window-windows.cpp|120|more
> undefined references to
> `HelixirrWidgets::PointerSmartSafe<HelixirrWidgets::Window::Helper,
> std::default_delete<HelixirrWidgets::Window::Helper> >::operator->()
> const' follow| ||=== Build finished: 16 errors, 0 warnings ===|

How can I overcome this problem?

Answer 1

The problem does not have anything to do with the PIMPL idiom, nor with the fact that you are using std::unique_ptr<>.

Rather, it seems your code does not #include the PointerSmartSafe.inl file, which contains the definitions for all the member functions of the PointerSmartSafe<> class template.

Therefore, the compiler won't be able to implicitly instantiate those functions when calls to them are encountered, and those functions are not instantiated in PointerSmartSafe.inl either (only defined).

Therefore, the linker will complain that those symbols are not found. If you need more details, you may find this Q&A on StackOverflow quite informative.

---

To fix the problem, add an:

#include "PointerSmartSafe.hpp"

directive at the beginning of PointerSmartSafe.inl, and let all translation units which currently #include the PointerSmartSafe.hpp header #include the PointerSmartSafe.inl file instead.

Or, if you want to make this simpler, just move the content of PointerSmartSafe.inl at the end of PointerSmartSafe.hpp, and at that point you can even delete the PointerSmartSafe.inl file.