Pointers problem..

Question

What is the difference b/w

struct {
    float *p; 
}*ptr=s;

*ptr->p++

and

(*ptr->p)++;

I understand that the former points to the next address while the latter increments the value by 1 but I cannot get how it is happening.....

Answer 1

It's all about precedence.

In the first example you are incrementing the location that *p points to.

In the second example you are dereferencing *p and incrementing the value.

Answer 2

Due to C operator precedence,

*ptr->p++;

is equivalent to

*(ptr->p++);

so it actually increments the pointer, but dereferences the original address, due to the way postfix ++ works.

However, since nothing is done to the dereferenced address, the statement is equivalent to

ptr->p++;

Answer 3

It might be better to write it as follows:

struct S
{
    float p;
};

S* ptr;

This way you have a named struct containing a float. It is named S. You then declare an S pointer called ptr.

 1) ptr++;
 2) ptr->p++;
 3) (ptr->p)++;
 4) (ptr++)->p++;

in 1) you increment the pointer by sizeof( S ).
in 2) you increment the float in the struct.
in 3) you increment the float in the struct.
in 4) you increment the pointer by sizeof( S ) and then increment the float in the struct.