Re: Maintaining const in Impl classes

From:

Seungbeom Kim <musiphil@bawi.org>

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 7 Aug 2007 18:14:10 CST

Message-ID:

<f9a69q$mu0$1@news.Stanford.EDU>

Stefan Naewe wrote:

news.nospam.cechner@gmail.com schrieb:

Is there a well known mechanism for re-enabling the const part of the
Impl interface? I cant find any gotw addressing this issue...

Try this:

#include <iostream>

struct MyThing
{
  struct Impl {
    void A() const { std::cout << "Impl::A() const\n"; }
    void A() { std::cout << "Impl::A()\n"; };
  };

  MyThing() : pImpl_( new Impl() ) { }

  void A() const { std::cout << "MyThing::A() const\n"; get()->A(); }
  void A() { std::cout << "MyThing::A()\n"; get()->A(); }

private:

  const Impl* get() const { return pImpl_; }
  Impl* get() { return pImpl_; }

  Impl* pImpl_;
};

A nice first approach, but a problem is that you could easily forget to
use get() instead of pImpl_. A better idea would be to use:

template<typename T>
class pImplPtr
{
    T* ptr;

public:
    pImplPtr(T* p) : ptr(p) { }
    ~pImplPtr() { delete ptr; }

    operator T*() { return ptr; }
    operator const T*() const { return ptr; }
    T* operator->() { return ptr; }
    const T* operator->() const { return ptr; }

private:
    pImplPtr(const pImplPtr&); // prevent copying
    pImplPtr& operator=(const pImplPtr&); // prevent assignment
};

Then just change the declaration of pImpl_ from

    Impl* pImpl_;

to

    pImplPtr<Impl> pImpl_;

and every piece of code that uses pImpl_ is automatically affected.

--
Seungbeom Kim

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