Question

Windows SDK contains a set of typedefs:

typedef long LONG;

typedef struct tagPOINT
{
    LONG  x;
    LONG  y;
} POINT;

typedef struct tagRECT
{
    LONG    left;
    LONG    top;
    LONG    right;
    LONG    bottom;
} RECT;

then, there’s a WinAPI function that expects a pointer to an array of POINT structs and the length of that array:

void ThatFunction( POINT* points, int numberOfElements );

and we have the following code:

RECT rect = ...//obtained from somewhere
ThatFunction( reinterpret_cast<POINT*>( &rect ), 2 );

so that RECT is being treated as an array of two POINT structures.

Is such cast safe?

Answer 1

For this particular Win32 structure, yes. You should of course make this assumption explicit:

static_assert(sizeof(struct tagRECT) == 2 * sizeof(POINT));
static_assert(offsetof(struct tagRECT, right) == sizeof(POINT));