Pimpl中的unique_ptr编译错误及解决方法

问题起源

实现Pimpl时，使用unique_ptr时，编译会报如下错误：

[build] /usr/include/c++/9/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = widget::impl]’:
[build] /usr/include/c++/9/bits/unique_ptr.h:292:17: required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = widget::impl; _Dp = std::default_deletewidget::impl]’
[build] /usr/include/c++/9/experimental/propagate_const:63:11: required from here
[build] /usr/include/c++/9/bits/unique_ptr.h:79:16: error: invalid application of ‘sizeof’ to incomplete type ‘widget::impl’
[build] 79 | static_assert(sizeof(_Tp)>0,
[build] | ^~~~~~~~~~~

样例代码：

#include 
#include 
#include 
 
// interface (widget.h)
class widget {
   
    class impl; 
    std::experimental::propagate_const<std::unique_ptr<impl>> pImpl;
 public:
    void draw();
    bool shown() const {
    return true; } // public API that implementation has to call
    widget(int);
    widget(widget&&); // defined in the implementation file
                      // Note: calling draw() on moved-from object is UB
    widget(const widget&) = delete;
    widget&