Re: [Boost-users] [interceptors] How to intercept when a function is called

Subject: Re: [Boost-users] [interceptors] How to intercept when a function is called
From: Michael Powell (mwpowellhtx_at_[hidden])
Date: 2014-07-02 18:00:55

On Wed, Jul 2, 2014 at 2:41 PM, Jason Roehm <jasonr_at_[hidden]> wrote:
>
> On 07/02/2014 03:32 PM, Michael Powell wrote:
>>
>> Hello,
>>
>> I don't know if there is a first or second class library in Boost to
>> do with this already. Or at least a C++ mechanism I could leverage.
>>
>> Basically, I want to hook or intercept when a function call is made. I
>> do not want to change the underlying call, although I suppose that
>> hook could be done as well. For now, I just want a callback when that
>> function calls is made.
>>
>> For instance, a vector push_back listener. I want to listen when items
>> are push_back-ed onto the vector. The items should still land in the
>> vector. I just want the notification when that occurs.
>>
>> I've dug around for different "callback" flavors, but these just seem
>> to be wrappers to an underlying method pointer, bound function, along
>> these lines. I can do that easy enough but it doesn't buy me the
>> interception when the push back is actually called.
>>
>> The tricky part with push_back as well, is that it comes in two
>> flavors, so there needs to be a stand in lambda, something to declare
>> the actual kind of argument(s) I want. Then there's the matter of
>> Variadics. But for now I am not concerned with either of these.
>
>
> You might have a look at the execute-around pattern. There is an example
> here:
>
> https://gitorious.org/redistd/redistd/commit/8079aa6399d7555a21d1551830c2d229d6bcba4c
>
> Essentially, you wrap the object that you care about in another object that
> implicitly converts to a pointer to the underlying type, but the dereference
> operator is set up in such a way that you can invoke arbitrary callables
> before and after each member function invocation.

Poor choice of wording I think on my part. Not necessarily that an
instance is being called, although that's a compelling thing to think
about. I need to know, for instance, in this case, not only the moment
when vector is being push_back-ed, but also the item being inserted.

Which with a solution like the pointer or dereference overload, would
get a grand before-snapshot, but not just after.

> I don't see a way to easily make this function-specific, or for a way to
> determine inside the hook function which of the member functions was called.

It seems like I want to extend std::bind itself, if it were possibly.
I'll have a look at that. I can cook up a functor wrapper simple
enough. The trick is capturing just what I need and wiring up the
callback(s) around the actual bound function call.

Thank you...

> Jason
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