Casting double array to a struct of doubles

Question

Is it OK to cast a double array to a struct made of doubles?

struct A
{
   double x;
   double y;
   double z;
};

int main (int argc , char ** argv)
{
   do         


        

Answer 1

The standard gives little guarantees about memory layout of objects.

For classes/structs:

9.2./15: Nonstatic data members of a class with the same access control are allocated so that later members have higher addresses within a class object. The order of allocation of non-static data members with different access control is unspecified. Implementation alignment requirements might cause two adjacent members not to be allocated immediately after each other; so might requirements for space for managing virtual functions and virtual base classes.

For arrays, the elements are contiguous. Nothing is said about alignment, so it may or may not use same alignment rules than in struct :

8.3.4: An object of array type contains a contiguously allocated non-empty set of N subobjects of type T.

The only thing you can be sure of in your specific example is that a.x corresponds to arr[0], if using a reinterpret_cast:

9.2.21: A pointer to a standard-layout struct object, suitably converted using a reinterpret_cast, points to its initial member (...) and vice versa. [
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