How do I use unique_ptr for pimpl?

Question

Here is a simplification of what I'm seeing when I try to use unique_ptr for pimpl. I chose unique_ptr because I really want the class to own the pointer - I want the lifetimes of the pimpl pointer and the class to be the same.

Anyway, here is the header:

#ifndef HELP #define HELP 1  #include <memory>  class Help {  public:    Help(int ii);   ~Help() = default;  private:    class Impl;   std::unique_ptr<Impl> _M_impl; };  #endif // HELP 

Here is the source:

#include "Help.h"  class Help::Impl { public:   Impl(int ii)   : _M_i{ii}   { }  private:    int _M_i; };  Help::Help(int ii) : _M_impl{new Help::Impl{ii}} { } 

I could compile these into a library just fine. But when I try to use it in a test program I get

ed@bad-horse:~/ext_distribution$ ../bin/bin/g++ -std=c++0x -o test_help test_help.cpp Help.cpp In file included from /home/ed/bin/lib/gcc/x86_64-unknown-linux-gnu/4.7.0/../../../../include/c++/4.7.0/memory:86:0,                  from Help.h:4,                  from test_help.cpp:3: /home/ed/bin/lib/gcc/x86_64-unknown-linux-gnu/4.7.0/../../../../include/c++/4.7.0/bits/unique_ptr.h: In instantiation of 'void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = Help::Impl]': /home/ed/bin/lib/gcc/x86_64-unknown-linux-gnu/4.7.0/../../../../include/c++/4.7.0/bits/unique_ptr.h:245:4:   required from 'void std::unique_ptr<_Tp, _Dp>::reset(std::unique_ptr<_Tp, _Dp>::pointer) [with _Tp = Help::Impl; _Dp = std::default_delete<Help::Impl>; std::unique_ptr<_Tp, _Dp>::pointer = Help::Impl*]' /home/ed/bin/lib/gcc/x86_64-unknown-linux-gnu/4.7.0/../../../../include/c++/4.7.0/bits/unique_ptr.h:169:32:   required from 'std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = Help::Impl; _Dp = std::default_delete<Help::Impl>]' Help.h:6:7:   required from here /home/ed/bin/lib/gcc/x86_64-unknown-linux-gnu/4.7.0/../../../../include/c++/4.7.0/bits/unique_ptr.h:63:14: error: invalid application of 'sizeof' to incomplete type 'Help::Impl' 

This is a well known safety feature. I've tried to follow.

My problem is that if I put Help::Impl declaration in a header it would seem to obviate any advantage of pimpl. The class layout is visible to users. The definition is hidden but I could have done that with the Help class and private members. Also, including the declaration of Impl brings in new headers that I would have liked to keep separate.

What am I missing? What do folks put in an Impl declaration and where? Am I doing the Help dtor wrong? Argh!

Answer 1

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 the destructor to be default in the cpp file, too:

Help::~Help() = default;