Re: Offsetof

mihirtr@gmail.com wrote:


The correct line should be:

my_ptr = &received_buff.mem;

If you only have the received_buff.mem pointer value, there is no way
to get the pointer to the Buffer structure. For an example, have a
look at the "container_of" macro in the Linux kernel.

Anyway, this is C code only and not how one would write C++. I'm not
even sure how portable this is across other C compilers (offsetof is
an gcc extension). In the Linux kernel (that's what I do for a living
and there is no C++, unfortunately), they use these kind of constructs
to emulate inheritance and virtual destructors:

/*
  * Parent class
  */

struct device {
         ...
};

/* constructor */
int device_register(struct device *dev)
{
         ...
}

/* virtual destructor */
void device_release(struct device *dev)
{
         /* call the virtual destructor */
         dev->release(dev);
}

/*
  * Child class
  */

struct amba_device {
         ...
         struct device dev; /* inheritance, kind of */
         ...
};

/* destructor prototype */
void amba_device_release(struct device *dev);

/* constructor */
int amba_device_register(struct amba_device *dev)
{
         /* register the virtual destructor */
         dev->dev.release = amba_device_release;

         /* call parent constructor */
         device_register(&dev->dev);
         ...
}

/* virtual destructor */
void amba_device_release(struct device *d);
{
         struct amba_device *dev = container_of(d, struct amba_device, dev);

         /* now do the clean-up on the amba_device */
         ...
}

So, why bother with the above when you get it in C++ directly using
inheritance? Of course, you would have to re-write parts of the code
but in the end it will save you a lot time by eliminating the pointer
arithmetics (and more bugs). In your case, maybe something like below:

struct RawBuffer
{
         Member mem;
};

struct Buffer : public RawBuffer
{
         SomeId id;
}

void processData(RawBuffer &buf)
{
         /* do something with buf.mem */
         ...
}

void someFunction()
{
         Buffer b;

         processData(b);
}

As you can see, you can call processData(Buffer) directly even if the
function only takes a RawBuffer argument since RawBuffer is a
superclass of Buffer.

--
Catalin

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