Re: [boost] New New Callbacks Uploaded

From: Kevlin Henney (kevlin_at_[hidden])
Date: 2000-11-23 02:12:16

In message <v03110703b6421d10642f@[153.32.222.4]>, Jesse Jones
<jejones_at_[hidden]> writes
>>This means that we have three choices:
>>
>>(1) Impose this restriction.
>
>In other words the responsibility for making the functor call thread safe
>relies on the functor itself. The functor can do this by not having any
>mutable state or by using thread synchronization objects.

Exactly. It would be part and parcel of the function object
requirements.

>>(2) Separate out the mutability aspect into a separate class, ie
>>function_ptr and const_function_ptr. The former is not reference counted
>>and the latter requires deep, physical const-ness but can be reference
>>counted.
>
>Or perhaps the body copy semantics can be special cased more intelligently.
>Maybe using some sort of traits mechanism. For example, it's pretty silly
>to clone a functor wrapping a method call.

I presume that the default would have to err on the side of safety.
Although is no need to copy member or non-member function wrappers,
there is an issue over the amount of specialisation involved to meet the
expectations, eg pretty much all function object types in the standard
and I guess many parts of the Lambda library. How much coupling do we
want? I think that the amount of configuration involved would put me off
this solution.

>>(3) Not implement reference in the function_ptr at all.
>>
>>IMHO, (3) is now the most reasonable option. If we wish to have
>>reference counting of an object, then we can provide it using a smart
>>pointer. The only problem with this is that most smart pointers,
>>including shared_ptr, are restricted to the narrow view of supporting
>>only operator* and operator-> wrt pointer-ness.
>
>Interesting idea, although it seems like the utility of the non-copyable
>callback is pretty limited.

There's no restriction of the copyability. If you want reference
counting, and you want to manage the risks of inter-thread sharing, you
would write something like

        shared_ptr< function_ptr<void> > s(new_function_ptr(f));
        ...
        s();

Kevlin
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