function_pointer_1

[hooshdar3]

Hello.
Assuming the following definitions:

int add(int a, int b)
{return a + b;}
int (*op)(int a, int b)=  add;

Why is the call

*op = add + 1;

wrong after the line

int (*op)(int a, int b)= add;

?

Thanks in advance!

[ne555]

*op = add + 1; ¿what do you think you are doing there?

[hooshdar3]

I am after the reason for the syntax error.All of them.

[BlackSheep]

You dereference a function pointer, and then try to assign it to a function (ie. the address of that function) plus one? This makes no sense. What are you trying to accomplish?

[hooshdar3]

I am not.the exam question had four options.On of which was this.
I now look for the reqasons for the error.

[BlackSheep]

You can't do pointer arithmetic with a function's address.
Also, I believe dereferencing a function pointer will just give you the address right back.

*p;
**p;
***p;
****p;
//They all do the exact same thing.
//You can do this too:
int a = (******p)(1,1);

[JLBorges]

> I now look for the reqasons for the error.

The binary + operator is available only for pointers that point to a complete object.
In particular. T* pointer ; pointer + 1 ; requires that T is a complete type - sizeof(T) is known at compile-time.

A function is not an object.

struct A ;

int main()
{
    typedef int function_type( int, int ) ;
    function_type* p1 /* = ... */ ;
    void* p2 /* = ... */
    A* p3 /* = ... */

    sizeof( function_type ) ; // *** error - not an object 
    sizeof( void ) ; // *** error - void is deemed to be an incomplete type
    sizeof( A ) ; // *** error - A is an incomplete type

    function_type a1[1] ; // *** error
    void a2[1] ; // *** error 
    A a3[1] ; // *** error - A is an incomplete type

    p1 + 1 ; // *** error - p1 does not point to an object
    p2 + 1 ; // *** error - p2 does not point to a complete object
    p3 + 1 ; // *** error - p3 does not point to a complete object
}