How to increment a pointer address and pointer's value?

c pointers
Dinesh picture Dinesh · Nov 21, 2011 · Viewed 253.2k times · Source

Let us assume,

int *p;
int a = 100;
p = &a;

What will the following code will do actually and how? 

p++;
++p;
++*p;
++(*p);
++*(p);
*p++;
(*p)++;
*(p)++;
*++p;
*(++p);

I know, this is kind of messy in terms of coding, but I want to know what will actually happen when we code like this.

Note : Lets assume that the address of a=5120300, it is stored in pointer p whose address is 3560200. Now, what will be the value of p & a after the execution of each statement?

Answer

felipemaia picture felipemaia · Nov 21, 2011

First, the ++ operator takes precedence over the * operator, and the () operators take precedence over everything else.

Second, the ++number operator is the same as the number++ operator if you're not assigning them to anything. The difference is number++ returns number and then increments number, and ++number increments first and then returns it.

Third, by increasing the value of a pointer, you're incrementing it by the sizeof its contents, that is you're incrementing it as if you were iterating in an array.

So, to sum it all up:

ptr++;    // Pointer moves to the next int position (as if it was an array)
++ptr;    // Pointer moves to the next int position (as if it was an array)
++*ptr;   // The value of ptr is incremented
++(*ptr); // The value of ptr is incremented
++*(ptr); // The value of ptr is incremented
*ptr++;   // Pointer moves to the next int position (as if it was an array). But returns the old content
(*ptr)++; // The value of ptr is incremented
*(ptr)++; // Pointer moves to the next int position (as if it was an array). But returns the old content
*++ptr;   // Pointer moves to the next int position, and then get's accessed, with your code, segfault
*(++ptr); // Pointer moves to the next int position, and then get's accessed, with your code, segfault

As there are a lot of cases in here, I might have made some mistake, please correct me if I'm wrong.

EDIT:

So I was wrong, the precedence is a little more complicated than what I wrote, view it here: http://en.cppreference.com/w/cpp/language/operator_precedence

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