> Hi Vitaliy,
> I apologise for not posting anything yet about this subject, but I am
> trying to process all the information about it.
> The problem I have at the moment is that I just discovered that the CSS
> C compiler does not deal very well with function pointers especially in
> structs, hence is delaying my research into the subject.
> The first issue I had to overcome was the fact that I am not only
> writing a program using OOP I am also writing the infrastructure to
> generate the objects and actually that is the most important part. I
> will also start by not having inheritance but I can see how that could
> be done. The other issue I had is the "VTable" I could not understand
> its function.

A "VTable" is a tool used to support virtual functions. Rather than put all 
the function pointers directly into your object struct, you put them into a 
separate struct and point each object of the class at the separate, VTable, 
struct. For example:

// Forward declare the actual class object

typedef struct MyClass;

// Now declare the structure containing pointers to the virtual methods
typedef struct MyClass_VTable
{
    void (*Initialize) (struct MyClass *this, int a, int b);
    int (*DoSomething)(struct MyClass *this, int a);
} MyClass_VTable;

// Now declare the actual object
typedef struct MyClass
{
    MyClass_VTable *pVtable;
    int        a_field;
    int        another_field;
} MyClass;

// Now declare the methods for the class
void MyClass_Initialize(struct MyClass *this, int a, int b);
int MyClass_DoSomething(struct MyClass *int a);

// Create an instance of the VTable struct and initialize
// it to point to the functions for the class
MyClass_VTable MyClass_vtable = {
    MyClass_Initialize,
    MyClass_DoSomething
};

// Now to create an instance of the MyClass object

MyClass    myObject = {
    &MyClass_vtable,
    0,
    0
};

// To call a virtual function

void someFunction(void)
{
    ...
    myObject.pVtable->Initialize(&myObject, 3, 5);
}

> So this is how I am doing it:
>
> I have a function called checksumCalculator() which takes two parameter,
> an int bufferSize and an array of chars buffer which is the packet. It
> returns a char checksumCal which represents the checksum.
>
> To transform it into a class I created three, setSize, setBuffer,
> calculatechecksum. I also have three fields, bufferSize, buffer and
> checksumCal.
>
> So far all seems ok, but then what I envisage is a struct with the three
> fields and three function pointers one for each of the methods. When an
> instance of the class is created the three fields are initialised and
> the function address passed to the pointers. Is this what you do?
> I found that when I try this I can not manipulate the fields of the
> struct with the class methods, how did you get around that?
>
> I am probably making a mess out of this, so apologies in advance.
>
> Best Regards
> Luis

You don't need a VTABLE or function pointers in a struct if you do not use 
"virtual" functions. In this case your object is just the struct with three 
fields in it.

-- Bob Ammerman
RAm Systems

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