Re: [boost] pimpl library proposal

Subject: Re: [boost] pimpl library proposal
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2010-12-03 13:36:08

Michael Bailey wrote:
> my_object.hpp
>
> class my_object
> : public boost::pimpl<class my_object_impl>
> {
> public:
>   my_object() {}
>   void do_something();
>   int do_other_thing( double ) const;
>   void lots_of_args( int, int, int, int );
> };
>
>
> my_object.cpp
>
> class my_object_impl
> {
> public:
>   void do_something() {...}
>   int do_other_thing( double a1 ) const {...}
>   void lots_of_args( int, int, int, int ) {...}
> private:
>   ...
> };
>
> BOOST_PIMPL_CTOR( my_object );
> BOOST_PIMPL_METHOD( 0, my_object, do_something );
> BOOST_PIMPL_METHOD( 1, my_object, do_other_thing, const );
> BOOST_PIMPL_METHOD( 4, my_object, lots_of_args );

I would write:

my_object.hpp

class my_object_impl;

class my_object
{
   my_object_impl* pimpl;
public:
   my_object() {}
   ~my_object();
   void do_something();
   int do_other_thing( double ) const;
   void lots_of_args( int, int, int, int );
};

my_object.cpp

class my_object_impl
{
public:
   void do_something() {...}
   int do_other_thing( double a1 ) const {...}
   void lots_of_args( int, int, int, int ) {...}
private:
   ...
};

my_object::my_object(): pimpl(new my_object_impl) {}
my_object::~my_object() { delete pimpl; }
void my_object::do_something() { pimpl->do_something(); }
int my_object::do_other_thing(double d) const { return 
pimpl->do_other_thing(d); }
void my_object::lots_of_args(int a, int b, int c, int d) { 
pimpl->lots_of_args(a,b,c,d); }

which really isn't much different in terms of the amount of typing, 
except for the forwarding.  Can you show what your BOOST_PIMPL_METHOD 
expands to?  I can't immediately guess how you manage to not tell it 
all the argument and return types.

Re using the shared_ptr, this mainly gains the safety of otherwise 
forgetting to implement the dtor, and has speed and space 
disadvantages.  I'm not convinced that is really useful, and so I would 
decouple it from the concise forwarding feature.

Generally, I think that implementing pimpls falls more into the 
category of stuff that's best done with some sort of tool, e.g. a 
"pimpl wizzard" in your IDE, rather than a library.

Regards,  Phil.