what's the difference about p.a and p->a where p is pointer?

Is there 开发者_运维知识库any difference about p.a and p->a where p is pointer? or they are just same thing.

The . operator is actually the operator for structure member access.

struct Foo
{
    int bar;
    int baz;
} aFoo;

aFoo.bar = 3;

If you have a pointer to a struct, (very common) you can access its members using pointer dereferencing and the . operator.

struct Foo *p;
p = &aFoo;

(*p).baz = 4;

The parentheses are needed because . has higher precendence than *. The above dereferencing a member of a structure pointed to by something is extremely common, so -> was introduced as a shorthand.

p->baz = 4; // identical to (*p).baz = 4

If p is a pointer, you never see p.anything in plain C, at least not in anything that compiles. However, it is used to denote property access in Objective-C which was a mistake IMO.

Yes, you can't do p.a on a pointer, the dot operator requires an actual instance, i.e. a value of the proper struct type.

Note that "under the hood", p->a is equivalent to (*p).a, but easier to type and read. Nobody ever uses the latter form, but it's sometimes handy as a way of understanding what the arrow operator does.

try (*p).a or (&p)->a where p is a pointer and an instance respectively ;-)

If p is a pointer then p.a isn't valid syntax.

It's compile time error.. p->a is valid only.