Ytukyg

I'm trying to create a class that inherits from a base class that uses the PIMPL idiom.

I have a base class base.h:

#include <memory>



class Base {

 public:

  class Impl;

  Base(std::unique_ptr<Base::Impl> impl);

  virtual ~Base();



 private:

  std::unique_ptr<Base::Impl> impl_;

};

which is implemented in base.cpp:

#include "base.h"



class Base::Impl {

 public:

  Impl() {}

  ~Impl() {}

};



Base::Base(std::unique_ptr<Base::Impl> impl) : impl_(std::move(impl)) {}

Base::~Base() {}

which is then derived in derived.cpp:

#include "base.h"



class Derived : public Base {

 public:

  Derived() : Base(nullptr) {}

  ~Derived() override {}

};

When I compile these three files, I get the following error:

In file included from /usr/include/c++/7/memory:80:0,

                 from base.h:1,

                 from derived.cpp:1:

/usr/include/c++/7/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = Base::Impl]’:

/usr/include/c++/7/bits/unique_ptr.h:268:17:   required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = Base::Impl; _Dp = std::default_delete<Base::Impl>]’

derived.cpp:5:27:   required from here

/usr/include/c++/7/bits/unique_ptr.h:76:22: error: invalid application of ‘sizeof’ to incomplete type ‘Base::Impl’

  static_assert(sizeof(_Tp)>0,

which kind of makes sense, since there is no ~Impl() for the derived class to use. After I add the following snippet to derived.cpp before the Derived class definition:

class Base::Impl {

 public:

  Impl() {}

  ~Impl() {}

};

it compiles fine. Is there any way for derived.cpp to use the Impl destructor from base.cpp without adding it to some other shared file that both base.cpp and derived.cpp use? I've also noticed that this issue only arises because of the constructor taking in a unique_ptr to Base::Impl. If I replace that constructor with an empty constructor that just assigns a new Impl to Base::impl_, I don't have to include the destructor like I had to above. What's different in that situation?