Can I treat a struct like an array?

Question

I have a struct for holding a 4D vector

struct {
    float x;
    float y;
    float z;
    float w;
} vector4f

And I'm using a library that has some functions that operate on vectors but take float pointers as their arguments.

Is it legal to call something like doSomethingWithVectors( (float *) &myVector)?

Answer 1

You can write code that would make an attempt to treat it as an array, but the language makes no guarantees about the functionality of that code. The behavior is undefined.

In C language taking a storage region occupied by a value of one type and reinterpreting it as another type is almost always illegal. There are a few exceptions from that rule (which is why I said "almost"), like you can reinterpret any object as a char array, but in general it is explicitly illegal.

Moreover, the possible dangers are not purely theoretical, and it is not just about the possible alignment differences between arrays and structs. Modern compilers might (and do) rely on the aforementioned language rule in order to perform aliasing optimizations (read about strict aliasing semantics in GCC, for one example). In short, the compler is allowed to translate code under the assumption that memory occupied by a struct can never overlap memory occupied by an array of float. This often leads to unexpected results when people start using tricks like in your post.

Answer 2

It might work but it is not portable, the compiler is free to align things so that one float does not neccessarily immediately follow the other.