can pimpl idiom's impl class has destructor？

Question

I learn pimpl in effective modern cpp,after do some search,no one has talk about the pimpl idiom's impl class's destructor implement, is it unnecessary?

//in widget.h
#include <memory>
class Widget{
public:
    Widget();
private:
    struct Impl;
    std::unique_ptr<Impl> pImpl;
};
//in widget.cpp
........
struct Widget::Impl{
  std::string name;                // Widget::Impl
  std::vector<double> data;
};


struct Widget::~Impl()   //error!!!,how can we implement it
{
};

Answer 1

The PIMPL idiom's implementation interface structure cannot be declared as a private field of a class. Also, the declaration of a unique_ptr to an incomplete datatype is not allowed in C++ so we have to opt for plain old pointers to declare the PIMPL and manually new and delete it appropriately.

The destructor of the struct Impl can be defined like this in widget.cpp :

Widget::Impl::~Impl()
{

};

The final code may look something like this:

widget.h

class Widget
{
public:
    Widget();
    ~Widget();
    struct Impl;
private:
    Impl* pImpl;
};

widget.cpp

struct Widget::Impl
{
  Impl();
  std::string name;                // Widget::Impl
  std::vector<double> data;

  ~Impl();
};

//Widget Impl Constructor  
Widget::Impl::Impl()
{

}

//Widget Impl Destructor
Widget::Impl::~Impl()
{

};

//Widget Constructor
Widget::Widget() : pImpl(nullptr)
{
    pImpl = new Impl();
}

//Widget Destructor
Widget::~Widget()
{
    delete pImpl;
    pImpl = nullptr;
}

Answer 2

thanks for demo code post by @Retired Ninja,it helps a lot,the point is,we need a declaration of ~Impl in the cpp file:

Widget::Widget():pImpl(std::make_unique<Impl>())
{
}

 struct Widget::Impl {
     std::string name;
     std::vector<double> data;
     ~Impl();// need this !!
 };

 Widget::Impl::~Impl(){
 };

 Widget::~Widget() {
 }