RE: [boost] [smart_ptr] const-correctness as function argument

From: Yitzhak Sapir (yitzhaks_at_[hidden])
Date: 2002-10-02 09:04:57

1) Usually I declare my classes as
class some_class
{
  ...
private:
  class impl;
  boost::scoped_ptr<impl> m_impl;
  ...
};

It seems to me more natural that the implementation class of the class belongs to the class rather than sits outside of it.  Using this, it becomes harder to use a pimpl_base.  I also think having those two lines is a little clearer than inheriting from pimpl_base (but just a little).

2) I know I can write an adaptor for scoped_ptr to do this (calling it impl_ptr or whatever).  I suggested it before, and I'll probably do it soon for myself.  This post, I was thinking that since a scoped_ptr is the owner of the data, const functions on the pointer should give const data.  Maybe I'm just wrong or missing something :)  But since passing scoped_ptr's as an argument may be rare (passing p.get() may be a better option), is there really a need to have a operator->()const (and operator*() const) that returns a non-const T? (Even a rare use-case would suffice for me to convince me it's wrong to have them in scoped_ptr, I was just suggesting something).

> -----Original Message-----
> From: scleary_at_[hidden] [mailto:scleary_at_[hidden]]
> Sent: Wednesday, October 02, 2002 2:20 PM
> To: boost_at_[hidden]
> Subject: RE: [boost] [smart_ptr] const-correctness as 
> function argument
> 
> 
> OK, here's what I have never understood about this argument: 
> what's wrong
> with
>   template <typename T>
>   class pimpl_base
>   {
>     private:
>       scoped_ptr<T> pimpl_;
>     protected:
>       T * pimpl() { return pimpl_.get(); }
>       const T * pimpl() const { return pimpl_.get(); }
>       explicit pimpl_base(T * const npimpl):pimpl_(npimpl) { }
>   };
> 
>   class MyObjImpl;
>   class MyObj: pimpl_base<MyObjImpl>
>   {
>     ...
>   };
> 
> the pimpl_base supplies the const-correctness "necessary" for 
> its derived
> classes. Each derived class accesses the implementation via 
> "pimpl()->".
> 
> 	-Steve
> 
> > -----Original Message-----
> > From: Yitzhak Sapir [mailto:yitzhaks_at_[hidden]]
> > Sent: Wednesday, October 02, 2002 9:04 AM
> > To: Boost mailing list
> > Subject: RE: [boost] [smart_ptr] const-correctness as 
> > function argument
> > 
> > 
> > > -----Original Message-----
> > > From: David Abrahams [mailto:dave_at_[hidden]]
> >  
> > > Why is everyone barking up this tree instead of using 
> > > shared_ptr<T const>
> > > "as god intended"?
> > 
> > I don't have a problem with shared_ptr as it is, (and in my 
> > post, I covered reasons for leaving shared_ptr's as they are 
> > -- I don't support the viewpoint that shared_ptr should have 
> > two ->/* functions).  But I use scoped_ptr's to hold the 
> > pointers to a private implementation struct.  When I do this, 
> > I don't want to have non-const access to the struct in const 
> > functions.   Having a scoped_ptr<T const> won't help me in 
> > this regard because I do want non-const access in non-const 
> functions.
> > _______________________________________________
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