Re: [Boost-users] [Bind & Function] c function pointers

Subject: Re: [Boost-users] [Bind & Function] c function pointers
From: Matthijs Möhlmann (matthijs_at_[hidden])
Date: 2013-11-29 10:16:54

On 11/29/13, 1:37 PM, Lars Viklund wrote:
> On Fri, Nov 29, 2013 at 11:23:35AM +0100, Matthijs Möhlmann wrote:
>> Hello all,
>> I'm not sure if this is possible at all, but let me explain:
> It's not possible.
>
Ok, thank you.

>> I have the following function signature:
>> typedef int (*sasl_callback_ft)(void);
>>
>> That function in question gets called with unknown amount of parameters
>> (usually 3 or 4)
> That doesn't quite make sense. That signature is for a function that
> takes exactly zero arguments, both in C and C++.
That's strange, the SASL library uses that as Signature and I can assign a
function with 3/4 arguments to that.
>> boost::function<int(void *, int, const char **, unsigned *)> callback =
>>     boost::bind(&admin::callback_fun, this, _1, _2, _3, _4);
>>
>> Obviously the compiler doesn't allow to assign callback to a
>> sasl_callback_ft var
>> sasl_callback_ft fun = callback;
> Both boost::function and the return type of boost::bind are objects of a
> Callable type. They bear no relation to traditional function pointers
> except that you can call them with the same kind of syntax.
>
> A function pointer can be assigned a free function, a static member
> function or a stateless lambda.
>
> You may want to look into libraries that via code generation or other
> tricks produce a completely new function in memory, often called a
> "thunk" or a "trampoline", which forwards to the actual more complex
> callable.
>
> Another approach is to store state in some external/global/thread-local
> storage and assign a free function that forwards the call based on the
> stored state and any other state you can gleam from your library.
>
> Often, C-like APIs like the one you try to interact with tends to have a
> void* in the interface which you can use to tunnel the information
> needed to call the right thing. If this API doesn't have such a thing,
> you're in for a fun time.
>
Thank you for the suggestions, I'm going to look at that.

Regards, Matthijs