Re: [boost] Pimpl Again?

Subject: Re: [boost] Pimpl Again?
From: charleyb123 . (charleyb123_at_[hidden])
Date: 2016-05-31 21:30:57

On Tue, May 31, 2016 at 4:59 PM, Gavin Lambert <gavinl_at_[hidden]>
wrote:

> On 1/06/2016 09:12, charleyb123 . wrote:
>
>> This is why I'm such a fan of CRTP (curiously-recurring-template-pattern),
>> where it should work for this PIMPL design with no virtual and no
>> overhead.
>>
>> I've been quiet on this thread, but:
>>
>>    *- I like the PIMPL pattern very much
>>    *- I like your approach
>>    *- I want no overhead
>>    *- I vote CRTP
>>    *- CRTP also allows template-member-functions (requested by Chris, also
>> desired by me)
>>    *- I'm generally disappointed with namespaces
>>
>
> This already uses CRTP, eg:
>
> struct T : public pimpl<T>::shared {...};
>
> Is there some other application of this that you actually meant instead?
>

//"IMPL" interface consistent with "std_impl<>"
template<typename IMPL>
struct std_pimpl
{
  using P = typename IMPL::TYPE;
  IMPL* impl_;
  ...
  std_pimpl<IMPL>& clone(void) const {
     assert(impl_);
     return *new std_pimpl(IMPL::Clone(*impl_));
  }
  ...standard pimpl interface forwarding to IMPL::...
};

template<typename D, typename F>
struct std_impl
{
  using TYPE = F;
  F* impl_;

  const F& getImpl(void) const { assert(impl_); return *impl_; }

  D& clone(void) const {
    return D::Clone(static_cast<const D&>(*this));
  }

  static D& Clone(const D& clone_from) {
    //..default implementation
     return *new D(clone_from);
  }
  ...standard impl interface...
};

class Foo {};

struct FooImpl : public std_impl<FooImpl,Foo>
{
  using BASE = std_impl<FooImpl,Foo>;
  //...
  static F& Clone(const Foo& clone_from) {
     // Override BASE::Clone()
     return *new D(clone_from.getImpl().cloneNew());
  }
};

using FooPimp = std_pimpl<FooImpl>;

------------

In short, what I like:

*- some kind of "std_pimpl<>" provides a standardized interface, and
"plumbing"/implementation.
*- CRTP in the "std_impl<>" allows for a zero-cost compile-time
implementation-override.
*- If necessary, implementation can permit "std_pimpl<>" to be
parameterized with merely a "Foo" declaration.

--charley