How to cast C struct just another struct type if their memory size are equal?

Question

I have 2 matrix structs means equal data but have different form like these:

// Matrix type 1.
typedef float Scalar;
typedef struct { Scalar e[4]; } Vector;
         


        

Answer 1

Well, you could just declare CATransform3DMakeScale like this:

Matrix CATransform3DMakeScale (CGFloat sx, CGFloat sy, CGFloat sz);

C doesn't type-check to make sure your deceleration matches the original library. If the memory layout is the same, it will work. However, it's bad coding practice and you should include a lengthy comment justifying your actions. ;-)

Otherwise, there's no way around it: you have to use pointers or copy the data. This would work:

CATransform3D m3d = CATransform3DMakeScale ( 1, 2, 3 );
Matrix m;
memcpy(&m, &m3d, sizeof m);

As you've discovered, you can't cast the structure directly. You also can't do this:

Matrix m = *(Matrix*) &CATransform3DMakeScale ( 1, 2, 3 );

because C only allows you to use the & operator on an l-value.