Pimpl and smart pointers

Sometimes I realise that I don’t know obvious things. My discovery of today is about Pimpl and smart pointers. For some reason I thought that the forward class declaration is only possible if the class is referenced by a pointer or a reference (#1), that is used in a form of T* or T& (#2). Therefore, when I wanted Pimpl, I didn’t use smart pointers because it seemed that it needed the full class definition.


#include <memory>
class A_pimpl;
class A {
  std::unique_ptr<A_pimpl> p;

I believed to some reason that it will not work because the class A_pimpl is partially defined. I was surprised as a child when tried and realised that it works! The fact #1 is not equivalent to the fact #2.

Now in A.cpp I can write:

#include "module.h"
#include "pimpl.h"

A::A() : p(new A_pimpl()) {}
A::~A() {}

Everything above also works for std::shared_ptr (C++ 2011), boost::scoped_ptr, and boost::shared_ptr.


It is important that class A must an explicitly provided destructor. Moreover, its body must in A.cpp, not in the header. Otherwise, it will be a compilation error, for example, “error C2338: can’t delete an incomplete type”.