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

On 11/29/13, 1:37 PM, Lars Viklund wrote:

On Fri, Nov 29, 2013 at 11:23:35AM +0100, Matthijs Möhlmann wrote:

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Hello all, I'm not sure if this is possible at all, but let me explain: It's not possible.

Ok, thank you.

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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 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