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: Edward Diener (eldiener_at_[hidden])
Date: 2014-07-02 18:15:06

On 7/2/2014 6:00 PM, Michael Powell wrote:
> 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.

The Boost signals2 library lets you setup a callback when an event 
happens. How you define that event and what is called when the event 
occurs is up to you and you set this up for yourself.