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 struct {
    double x;
    double y;
    double z;   
} Vector3d;

Now I want to assign a CMAcceleration variable to a Vector3d variable (copying the whole struct). How can I do this?

I tried the following but get these compiler errors:

vector = acceleration;           // "incompatible type"
vector = (Vector3d)acceleration; // "conversion to non-scalar type requested"

Of course I can resort to set all members individually:

vector.x = acceleration.x;
vector.y = acceleration.y;
vector.z = acceleration.z;

but that seems rather inconvenient.

What's the best solution?

Answer 1

You could use a pointer to do the typecast;

vector = *((Vector3d *) &acceleration);

Answer 2

You use an utility function for that:

void AccelerationToVector( struct CMAcceleration* from, struct Vector3d* to )
{
     to->x = from->x;
     to->y = from ->y;
     to->z = from->z;
}

Answer 3

That's your only solution (apart from wrapping it into a function):

vector.x = acceleration.x;
vector.y = acceleration.y;
vector.z = acceleration.z;

You could actually cast it, like this (using pointers)

Vector3d *vector = (Vector3d*) &acceleration;

but this is not in the specs and therefore the behaviour depends on the compiler, runtime and the big green space monster.

Answer 4

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!

Answer 5

Why dont you use.

typedef CMAcceleration Vector3d;

(instead of creating a whole new structure)

in that case vector = acceleration; compiles just fine.