John Matthews wrote:
> --- In [email protected], "Bill Cunningham" <bill...@...> wrote:
>> I would like to start writing code and calling functions in C
>> code with a pointer to a function. This so I can learn
>> functions pointers.
>
> Simple example:
>
> #include <stdio.h>
>
> static void inc(int *p) /* increment */
> {
> (*p)++;
> }
>
> static void dec(int *p) /* decrement */
> {
> (*p)--;
> }
>
> int main(void)
> {
> void (*fnPtr)(int *p); /* function pointer */
> int i = 0;
>
> printf("i=%d\n", i);
> fnPtr = inc;
> fnPtr(&i);
> printf("i=%d\n", i);
> fnPtr = dec;
> fnPtr(&i);
> printf("i=%d\n", i);
>
> return 0;
> }
Now for a similar simple example in C++:
#include <stdio.h>
class Base
{
public:
Base() {}
virtual ~Base() {}
virtual void Alter(int &) const {}
};
class Inc : public Base
{
public:
Inc() {}
virtual ~Inc() {}
virtual void Alter(int &Num) const { Num++; }
};
class Dec : public Base
{
public:
Dec() {}
virtual ~Dec() {}
virtual void Alter(int &Num) const { Num--; }
};
void DoActionAndPrint(const Base &Action, int &Num)
{
Action.Alter(Num);
printf("Num = %d\n", Num);
}
int main(void)
{
Base A;
Inc B;
Dec C;
int Num = 0;
DoActionAndPrint(A, Num);
DoActionAndPrint(B, Num);
DoActionAndPrint(C, Num);
}
Notice what is passed to DoActionAndPrint() is a const reference to the
instance of the base class 'Base'. When Alter() is called, it calls the
most derived member function. Behind the scenes there is a set of
pointers to functions in what is called a 'vtable' so that calling the
function in the base class actually ends up calling the derived class
function (assuming correct use of the 'virtual' keyword across the chain
built by the base-derived relationship).
http://en.wikipedia.org/wiki/Virtual_method_table
--
Thomas Hruska
CubicleSoft President
Ph: 517-803-4197
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