C / C++ / MFC

1-Aug-02 1:50

Hi,
I have a C API that I want to wrapp in C++.

The idea I have is to have a C++ Base Class and have the C API call virtual functions of the base class, then the derived class can customise these functions.

The only way i can think of doing this is to have a single global pointer to the base class and have the C code call the member functions of that global object. So before I can call certain functions linked to the C API I have to make sure this global pointer points to the right instantiation of the derived class... Unsure | :~

Its not very elegant or clean, i was wondering if anybody had any better ideas.

Asim Hussain
e: asim@jawache.net
w: www.jawache.net

Funky Chicken1-Aug-02 2:21

Funky Chicken

1-Aug-02 2:21

Confused as to what you mean here:

1) You want a C++ class to wrap C functions. In this case, you dont need to do anything special
class MySleep
{
public:
void GoSleep (int x) { sleep (x);};
};

2) You want C functions to access C++ classes (callbacks, threads?)
If you can pass a parameter around then you can pass the 'this' pointer as a parameter to a static function which calls the appropriate virtual function of 'this'

class MyThread
{
public:
static void threadHelper (void *param);
virtual void onStart ();
};

void MyThread::threadHelper (void *param)
{
((MyThread *)param)->onStart ();
}

.
.
.
create_thread (MyThread::threadHelper, this);

There are undoubtedly many other ways of doing this as well

Jawache1-Aug-02 2:42

1-Aug-02 2:42

Thanks,

Yes it was section 2, I want the the C Functions to Call C++ Functions.

The problem is still that the C Functions need to know the 'this' parameter to pass to the static function, therefore you still have to specify a global variable for the C Functions to use.

I've now implemented a static member function that returns an instance of the derived class pointer in the base class. Its still not pretty since I still have to make sure the derived class sets that pointer to itself before the C Functions can call it, but its better than making sure a global pointer is set.

Cheers

Asim Hussain
e: asim@jawache.net
w: www.jawache.net

Daniel Lohmann1-Aug-02 4:03

Daniel Lohmann

1-Aug-02 4:03

It's still not completly clear what you try to achive.

You have C functions that need to call member functions of an object. Is their any special reason why this functions have to be C functions or you can not pass them the instance pointer of the object to call?

If the C function is a callback you want to redirect into a member function, you should take a look at this article[^].

--

Daniel Lohmann

http://www.losoft.de
(Hey, this page is worth looking! You can find some free and handy NT tools there Big Grin | :-D

)

Jawache1-Aug-02 5:18

1-Aug-02 5:18

I am extending and embedding the Python interpreter into my application,
Python is a scripting language, I have written extentions to it in C, I now want these C extention functions to call C++ functions of a base class.

class Base {
// .....
virtual void func (char * line) { };
//....

class Derived: public Base {
//....
virtual void func (char * line) { printf(line); }
//....

extern "C" static pyFunc (...) {
// Somehow call the func pointer of the base class
}

When the a certain function in the script is called it calls 'pyFunc' I want pyFunc to then call Base::func (and then the Dervied::func)

There will only ever be a single instance of the Derived class, so before I had instantiated it as a global object, then called the 'Derived.func' function from that global object in the C functions. I can't instatinate and object of the base class since only the base classes member functions will be called.

I want the C Functions to call functions of the base class but the functions deffered to those of the derived class.

It works but it is messy, i was just wondering if there was a cleaner way of doing it?

Asim Hussain
e: asim@jawache.net
w: www.jawache.net

Joaquín M López Muñoz1-Aug-02 7:15

Joaquín M López Muñoz

1-Aug-02 7:15

I'm not sure what your problem is. By the mechanism of virtual functions, if you call func on a Derived object, it is Derived::func that is executed. If you want the base implementation to be executed as well, imbed a call to it into the derived func:

class Derived: public Base {
//....
virtual void func (char * line) {
  Base::func(line);
  printf(line);
}

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

Daniel Lohmann1-Aug-02 8:13

Daniel Lohmann

1-Aug-02 8:13

Sounds to be a classical instantiation of the Singleton pattern. Therefore your approach with the static function to retrieve the one and only instance is quite fine. To make sure that there is never (even accidentally) another instance, you should declare the constructor as protected. The following shows the principle:

class Base
{
public:
    static Base& Instance()
    {
        ASSERT( Base::pInstance );
        return *Base::pInstance;
    }

    virtual void f(); 
    virtual void g();
    ...

protected:

    Base()
    {
        ...
    }
    
    // We don't need to implement this. Objects of class Base
    // are never copied. We just declare it to make sure, the
    // compiler does not produce one its own.
    Base( const Base& );

    static Base* pInstance;
};

class Derived : public Base
{
public:

     virtual void f();
     ...

    static void CreateTheInstance()
    {
        ASSERT( Base::pInstance == 0 );
        Base::pInstance = new Derived();
    }

protected:
    Derived()
    {
        ...
    }
};

During initialization or at some other appropriate point in time you have to call Derived::CreateTheInstance() to construct the one and only instance, that is afterwards accessable via Base::Instance().
Roll eyes | :rolleyes:

--

Daniel Lohmann

http://www.losoft.de
(Hey, this page is worth looking! You can find some free and handy NT tools there Big Grin | :-D

)

Jawache1-Aug-02 12:26

1-Aug-02 12:26

Excellent,

Thanks just what I wanted.

Asim Hussain
e: asim@jawache.net
w: www.jawache.net

Randomic functions ?

Cris1-Aug-02 1:47

Cris

1-Aug-02 1:47

I would like to use a function that returns a integer value between two values. I met the rand() function, but I need a value in a range.

[]'s
Cristiano.

Nish Nishant1-Aug-02 2:11

Nish Nishant

1-Aug-02 2:11

(rand()/RAND_MAX)*5 for [between 0-5]
(rand()/RAND_MAX)*n for [between 0-n]

Author of the romantic comedy

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Funky Chicken1-Aug-02 2:18

Funky Chicken

1-Aug-02 2:18

Or easier,

rand () % n

If you want a range between A and B then just shift everything along:

int value = a + (rand () % (b - a));

Cris1-Aug-02 2:46

Cris

1-Aug-02 2:46

This not work corretly. The returned value is zero.

[]'s

Chunky Chicken1-Aug-02 2:56

Chunky Chicken

1-Aug-02 2:56

The following will print 100 random values between 120 and 320

int getRange (int low, int high)
{
return low + (rand () % (high - low));
}

int main (int argc, char *argv[])
{
srand (time (NULL));

for (int x = 0; x < 100; x++)
printf ("Random Value %d = %d\n", x, getRange (120, 320));
return 0;
}

[CoY0te]1-Aug-02 3:25

[CoY0te]

1-Aug-02 3:25

Almost...
The problem is:
int rand(void);
so it should be:
rand()*n/RAND_MAX
or
((double)rand()/RAND_MAX)*n

Railgun is like a Gilette Mach 3 - it does the job with one, easy stroke.