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cpointersincrementdereferenceoperator-precedence

Pointer Arithmetic: ++*ptr or *ptr++?

I am learning C language and quite confused the differences between ++*ptr and *ptr++.

For example:

int x = 19;
int *ptr = &x;

I know ++*ptr and *ptr++ produce different results but I am not sure why is that?

Solution

  • These statements produce different results because of the way in which the operators bind. In particular, the prefix ++ operator has the same precedence as *, and they associate right-to-left. Thus

    ++*ptr

    is parsed as

    ++(*ptr)

    meaning "increment the value pointed at by ptr,". On the other hand, the postfix ++ operator has higher precedence than the dereferrence operator *. Thefore

    *ptr++

    means

    *(ptr++)

    which means "increment ptr to go to the element after the one it points at, then dereference its old value" (since postfix ++ hands back the value the pointer used to have).

    In the context you described, you probably want to write ++*ptr, which would increment x indirectly through ptr. Writing *ptr++ would be dangerous because it would march ptr forward past x, and since x isn't part of an array the pointer would be dangling somewhere in memory (perhaps on top of itself!)

    Hope this helps!