Re: [boost] boost::function

From: Douglas Gregor (gregod_at_[hidden])
Date: 2001-07-09 20:31:08

On Monday 09 July 2001 06:58, you wrote:
> I'm playing with the boost:: function library for the first time, probably
> a naive question but, could someone explain to me why the mixins and
> policies classes are expected to be stateless i.e. always default
> constructed?
>
> Thanks,
>
> Richard
>
> COINCIDENCE: You weren't paying attention to the other half of what was
> going on. - The Hipcrime Vocab by Chad C. Mulligan.

Always default constructed and stateless are two different things.

Mixins add user-defined state. They are default-constructed because this 
appears to be common practice for mixins; however, it would be a small matter 
to include the ability for the user to specify an initial value. 

Policy classes aren't entirely stateless. They retain state over one 
invocation of a Boost.Function object (i.e., the same policy object will have 
its precall and postcall methods invoked for a single invocation of the 
Boost.Function object), but they do not retain state between invocations. 
Policies are default-constructed because there doesn't appear to be any 
useful information to pass to the policy constructor. 

An alternative to the policy precall/postcall methods was to construct the 
policy given a pointer to the Boost.Function object (that would serve as the 
"precall") and the destruction of the policy object would be equivalent to 
"postcall." This approach is reasonable, although there may be problems with 
exception safety. While the precall/postcall member functions can safely 
throw, this isn't necessarily true of the constructor/destructor policy type. 
It's safer not to take that risk...

Policies don't keep state between invocations because that would cause in 
increase in the size of Boost.Function objects, even for empty policies.

Mixins won't increase the size of Boost.Function objects because of the empty 
base class optimization. 

        Doug