Casting one C structure into another

Question

I have two identical (but differently named) C structures:

typedef struct {
      double x;
      double y;
      double z;
} CMAcceleration;


typedef struc         


        

Answer 1

A safe (albeit somewhat convoluted) way to do it would be to use a union:

union { CMAcceleration a, Vector3d v } tmp = { .a = acceleration };
vector = tmp.v;

Values are reinterpreted (since C99) when the accessed member is not the last set one. In this case, we set the acceleration and then we access the vector, so the acceleration is reinterpreted.

This is the way the NSRectToCGRect function is implemented, for example.

Answer 2

memcpy(&vector, &acceleration, sizeof(Vector3d));

Please note that this works only, if the physical layout of the structs in memory are identical. However, as @Oli pointed out, the compiler is not obliged to ensure this!