Question

this is my struct :

struct Node {
    struct Node* data;
    struct Node* links[4];
}

assuming there is no padding, does Node->links[-1] guaranteed to be pointing on Node::data ?

Answer 1

No guarantee; this is undefined behaviour:

• Compiler-dependent structure padding
• Standard only defines array indexing between 0 and length (inclusive)
• Possible strict-aliasing violation

In practice, it’s quite possible that you will end up pointing at data, but any attempts to access it will result in UB.