Convert a member function pointer to standard C function with multiple arguments [duplicate]

Question

I am trying to convert a member function pointer to standard C function pointer without success.

I tried different methods but I miss something.

My problem is that I need to call a library's function that takes as argument a functor:

void setFunction(void(*cbfun)(float*,int,int,int,int)){ ... }

inside a class in this way:

class base_t {

  public:

    void setCallback(){

      setFunction(&_callback);

    }

  private:

    void _callback(float * a, int b, int c, int d, int e) { ... }

};

Unfortunately, the _callback() function cannot be static.

I'm also tried to use std::bind but without fortune.

Is there any way I can pass the member to the function?

@BoPersson I think this is a good duplicate as well.

Arnav Borborah
– Arnav Borborah

2018-02-07 23:12:50 +00:00
Commented Feb 7, 2018 at 23:12 — Arnav Borborah
– Arnav Borborah, Commented Feb 7, 2018 at 23:12

R Sahu · Accepted Answer · 2018-02-07 23:36:39Z

2

I am trying to convert a member function pointer to standard C function pointer without success.

Short answer: You cannot.

Longer answer: Create a wrapper function to be used as a C function pointer and call the member function from it. Remember that you will need to have an object to be able make that member function call.

Here's an example:

void setFunction(void(*cbfun)(float*,int,int,int,int)){ ... }

class base_t;
base_t* current_base_t = nullptr;
extern "C" void callback_wrapper(float * a, int b, int c, int d, int e);

class base_t {

  public:

    void setCallback(){   
       current_base_t = this;
       setFunction(&callback_wrapper);   
    }

  private:

    void _callback(float * a, int b, int c, int d, int e) { ... }

};    

void callback_wrapper(float * a, int b, int c, int d, int e)
{
   if ( current_base_t != nullptr )
   {
      current_base_t->_callback(a, b, c, d, e);
   }
}

edited Feb 7, 2018 at 23:36

answered Feb 7, 2018 at 23:10

R Sahu

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3 Comments

Ben Voigt Over a year ago

Per the question, you will also need "extern "C". As-is, it is not a "standard C function" and its address cannot be passed to a C library.

R Sahu Over a year ago

@BenVoigt, good call. Thanks for the suggestion.

thewoz Over a year ago

Thanks for the answer. This solution is correct also in case of a multi threads application?

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