forward declaration возможен?

struct A;
A *pa;

#include <boost/property_tree/ptree_fwd.hpp>
#include <memory>

class CJSON
{
public:
    CJSON();
    ~CJSON();

private:
    std::unique_ptr<boost::property_tree::ptree> pt;
};

#include "cjson.hpp"
#include <boost/property_tree/ptree.hpp>

CJSON::CJSON() = default;
CJSON::~CJSON() = default;

В cjson.hpp:

#include <boost/property_tree/ptree_fwd.hpp>

hpp
class CJSON;


cpp
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/json_parser.hpp>

#include "DownloaderJSON.hpp"

class CJSON
{
private:
    boost::property_tree::ptree pt;

public:
    CJSON(const char * msg, const int lmsg)
    {
        std::stringstream ss;
        ss.write(msg, lmsg);

        boost::property_tree::read_json(ss, pt);
    }
};

In file included from /usr/include/c++/8/memory:80,
                 from /usr/include/c++/8/thread:39,
                 from Downloader.cpp:5:
/usr/include/c++/8/bits/unique_ptr.h: In instantiation of ‘typename std::_MakeUniq<_Tp>::__single_object std::make_unique(_Args&& ...) [with _Tp = CJSON; _Args = {char*&, const int&}; typename std::_MakeUniq<_Tp>::__single_object = std::unique_ptr<CJSON, std::default_delete<CJSON> >]’:
Downloader.cpp:149:49:   required from here
/usr/include/c++/8/bits/unique_ptr.h:831:30: error: invalid use of incomplete type ‘class CJSON’
     { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from Downloader.cpp:21:
DownloaderJSON.hpp:3:7: note: forward declaration of ‘class CJSON’
 class CJSON;
       ^~~~~
In file included from /usr/include/c++/8/memory:80,
                 from /usr/include/c++/8/thread:39,
                 from Downloader.cpp:5:
/usr/include/c++/8/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = CJSON]’:
/usr/include/c++/8/bits/unique_ptr.h:274:17:   required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = CJSON; _Dp = std::default_delete<CJSON>]’
Downloader.cpp:149:49:   required from here
/usr/include/c++/8/bits/unique_ptr.h:79:16: error: invalid application of ‘sizeof’ to incomplete type ‘CJSON’
  static_assert(sizeof(_Tp)>0,

Downloader.cpp:150:36: error: invalid use of incomplete type ‘class CJSON’
  auto pjson = new CJSON(msg, msgLen);
                                    ^
In file included from Downloader.cpp:21:
DownloaderJSON.hpp:3:7: note: forward declaration of ‘class CJSON’
 class CJSON;

In file included from /usr/include/c++/8/memory:80,
                 from /usr/include/c++/8/thread:39,
                 from Downloader.cpp:5:
/usr/include/c++/8/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = boost::property_tree::basic_ptree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >]’:
/usr/include/c++/8/bits/unique_ptr.h:274:17:   required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = boost::property_tree::basic_ptree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >; _Dp = std::default_delete<boost::property_tree::basic_ptree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> > >]’
DownloaderJSON.hpp:5:7:   required from ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = CJSON]’
/usr/include/c++/8/bits/unique_ptr.h:274:17:   required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = CJSON; _Dp = std::default_delete<CJSON>]’
Downloader.cpp:149:49:   required from here
/usr/include/c++/8/bits/unique_ptr.h:79:16: error: invalid application of ‘sizeof’ to incomplete type ‘boost::property_tree::basic_ptree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >’
  static_assert(sizeof(_Tp)>0,

// --------------------
// interface (widget.h)
struct widget
{
    // public members
private:
    struct impl; // forward declaration of the implementation class
    // One implementation example: see below for other design options and trade-offs
    std::experimental::propagate_const< // const-forwarding pointer wrapper
        std::unique_ptr<                // unique-ownership opaque pointer
            impl>> pImpl;               // to the forward-declared implementation class
};
 
// ---------------------------
// implementation (widget.cpp)
struct widget::impl
{
    // implementation details
};

/usr/include/c++/8/bits/unique_ptr.h:79:16: error: invalid application of ‘sizeof’ to incomplete type ‘boost::property_tree::basic_ptree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >’
  static_assert(sizeof(_Tp)>0,

hpp
class CJSON
{
private:
    struct impl;
    std::unique_ptr<impl> pimpl;

public:
    CJSON(const char *, const int);
    ~CJSON() {}
};

cpp
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/json_parser.hpp>

#include "DownloaderJSON.hpp"

struct CJSON::impl
{
    boost::property_tree::ptree pt;
};

CJSON::CJSON(const char * msg, const int lmsg)
    : pimpl(std::make_unique<impl>())
{
    std::stringstream ss;
    ss.write(msg, lmsg);
}
...

/usr/include/c++/8/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = CJSON::impl]’:
/usr/include/c++/8/bits/unique_ptr.h:274:17:   required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = CJSON::impl; _Dp = std::default_delete<CJSON::impl>]’
DownloaderJSON.hpp:15:14:   required from here
/usr/include/c++/8/bits/unique_ptr.h:79:16: error: invalid application of ‘sizeof’ to incomplete type ‘CJSON::impl’
  static_assert(sizeof(_Tp)>0,                ^~~~~~~~~~~

hpp
...
  ~CJSON() {}
...