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

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 2

Another version of the utility function making use of C99:

static inline struct Vector3d AccelerationToVector(struct CMAcceleration In)
{
    return (struct Vector3d){In.x, In.y, In.z};
}

With the compiler optimization turned up (e.g., -Os), this should turn into absolutely no object code when invoked.

Answer 3

You could use a pointer to do the typecast;

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

Answer 4

This is achieved easily through a union:

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

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

typedef union {
    CMAcceleration acceleration;
    Vector3d vector;
} view;

int main() {
    view v = (view) (Vector3d) {1.0, 2.0, 3.0};
    CMAcceleration accel = v.acceleration;
    printf("proof: %g %g %g\n", accel.x, accel.y, accel.z);
}

Answer 5

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 6

Why dont you use.

typedef CMAcceleration Vector3d;

(instead of creating a whole new structure)

in that case vector = acceleration; compiles just fine.