Using arrow -> and dot . operators together in C

Question

I was under the impression that it was possible to access data from a sub-node of a linked list or similar structure by using the arrow and dot operators together like so:

typedef struct a{
int num;
struct a *left;
struct a *right;
}tree;

tree *sample;
...
if(sample->left.num > sample->right.num)
    //do something

but when I try to implement this, using -> and . to access data from a sub node I get the error "request for member num in something not a structure or union".

Answer 1

Use -> for pointers; use . for objects.

In your specific case you want

if (sample->left->num > sample->right->num)

because all of sample, sample->left, and sample->right are pointers.

If you convert any of those pointers in the pointed to object; use . instead

struct a copyright;
copyright = *(sample->right);
// if (sample->left->num > copyright.num)
if (*(sample->left).num > copyright.num)

Answer 2

Since I don't see it mentioned explicitly:

• Use -> to dereference the pointer on its left hand side and access the member on its right hand side.
• Use . to access the member on its right hand side of the variable on its left hand side.

Answer 3

sample->left gives a struct a*, not a struct a, so we're dealing with pointers. So you still have to use ->.

You can, however, use sample->left->num.

Answer 4

. is for accessing the members of a struct (or union) e.g.

struct S {
int x;
}

S test;
test.x;

-> is a shorter way to write (*pointer_to_struct).struct_member

Answer 5

sample->left and sample->right are also pointers, so you want:

if (sample->left->num > sample->right->num) {
    // do something
}