[C] Multiple Indirection

[MrRaza]

Can anyone expand this subject to me clearly, it's a little confusing. I think for the most part that it is basically where you have a pointer point to another pointer that points to the target value? am i right? Should I show a code snipplet so you understand what I'm saying?

[iago]

You're exactly right, it's just a pointer to a pointer.  

Code Select Expand

int a = 6;
int *b = a;
int **c = b;

printf("%d%d%d\n", a, *b, **c);
**c++;
*b *= 4;
printf("%d%d%d\n", a, *b, **c);


If I'm right, that should output
666
282828

(thought I probably made some horrible mistake)

[Skywing]

You're exactly right, it's just a pointer to a pointer.  

Code Select Expand

int a = 6;
int *b = a;
int **c = b;

printf("%d%d%d\n", a, *b, **c);
**c++;
*b *= 4;
printf("%d%d%d\n", a, *b, **c);


If I'm right, that should output
666
282828

(thought I probably made some horrible mistake)

I think you'll need to use the address-of operator to initialize b and c like that.

[iago]

Yup, skywing's right, it should be this:

Code Select Expand

int a = 6;
int *b = &a;
int **c = &b;

printf("%d%d%d\n", a, *b, **c);
**c++;
*b *= 4;
printf("%d%d%d\n", a, *b, **c);


[Yoni]

If you turn that **c++ into (**c)++, sure

[MrRaza]

I also need some help with Pointers-to-functions(Function pointers), I don't really know where to start. Could someone give em some example code and explain it? If not, dont flame/reply.

[Yoni]

The simplest pseudo-syntax (?) for a pointer to a function in C is:

Code Select Expand

return-type (call-convention * pointer-name)(parameter-list);
where call-convention is a keyword such as __cdecl, __stdcall, or __fastcall (in VC++).

Example:

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int add(int a, int b) { return a + b; }
int subtract(int a, int b) { return a - b; }

int math(int a, int b, char op)
{
 int (* func)(int a, int b) = NULL;
 /* This declares a variable called 'func', and initializes it to NULL
 ** It is a pointer to a function that has 2 int parameters,
 ** and returns int
 */
 
 switch(op) {
 case '+':
   func = &add; // The & is optional, but recommended...
   break;
 case '-':
   func = subtract; // So the compiler should accept this, but might raise a warning
   break;
 default:
   return 0; // or whatever
 }
 
 return func(a, b); // Call it like you call a function
 // return (*func)(a, b); // This style is also allowed
}

You can use a type-definition to simplify the above code:

Code Select Expand

typedef int (* MathBinaryOperation)(int a, int b);
/* The type 'MathBinaryOperation' is a pointer to a function
** that has 2 int parameters and returns int
*/
int add(int a, int b) { return a + b; }
int subtract(int a, int b) { return a - b; }

int math(int a, int b, char op)
{
 MathBinaryOperation func = NULL; // Same as before, but better looking syntax
 
 switch(op) {
 case '+':
   func = &add;
   break;
 case '-':
   func = subtract;
   break;
 default:
   return 0;
 }
 
 return func(a, b);
}