[boost] Re: shared_ptr inheritance query

From: Dave Handley (dave_at_[hidden])
Date: 2005-01-31 15:25:19

Jonathan Wakely wrote:

>
> http://boost.org/libs/smart_ptr/enable_shared_from_this.html says:
>
>    Requires:
>        enable_shared_from_this<T> must be an accessible base class of T.

I've seen this - but even if I make the inheritance protected, VC++7.1 balks 
at it complaining that a conversion from the type T to 
enable_shared_from_this is available but inaccessible.

>
> Obviously using public inheritance works, but you might also be able to
> make it work by making shared_ptr<T> a friend of policy<T> so it can do
> the conversion. You'll also need to disable the BOOST_ASSERTs in
> enable_shared_from_this.hpp by defining BOOST_DISABLE_ASSERTS or by
> copying enable_shared_from_this.hpp and re-implementing the class
> without the assertions. I have no idea if this is supposed to work, nor
> if it will still work if the internals of shared_ptr or
> enable_shared_from_this change. Caveat emptor.
>

I've tried friendship as you recommend, between shared_ptr<T> and policy<T>, 
but again without success.

Interestingly, I actually get the error from a construction of a standard 
shared_ptr, without even using shared_from_this.  I've gone as far as 
commenting out every call to shared_from_this in my code.  If I inherit from 
enable_shared_from_this, the compiler complains on construction of a 
shared_ptr in the detail::sp_enable_shared_from_this line in the code below. 
If I also comment out enable_shared_from_this, everything compiles okay.

template<class Y>
explicit shared_ptr(Y * p): px(p), pn(p, checked_deleter<Y>()) // Y must be 
complete
{
    detail::sp_enable_shared_from_this(p, p, pn);
}

This is remarkably frustrating, since there are a large number of 
implementation details in my policy class that I am keen to hide from the 
API, and the API works fine for all my other policies, it is just the 
shared_ptr policy that is collapsing.

Dave