how to use ScreenToClient in win32, not MFC? - winapi

Question

As MSDN said
BOOL ScreenToClient(
_In_ HWND hWnd,
LPPOINT lpPoint
);
the ScreenToClient's second para is a pointer to POINT,
and PINT said by MSDN is
typedef struct tagPOINT {
LONG x;
LONG y;
} POINT, *PPOINT;
it has only x and y. It's NOT like MFC ScreenToClient function, the para is a rect, and rect has width and height.
I am confused how to use win32 ScreenToClient function.

Answer 1

You can use MapWindowPoints() to convert a RECT in a single operation:
RECT r = ...;
MapWindowPoints(NULL, hWnd, (LPPOINT)&r, 2);

Answer 2

MFC actually has two methods, they're overloaded. One accepts a POINT structure, just like the Win32 function, the other accepts a RECT structure, both work the same way: it maps each point from screen-to-client.
If you have a RECT that you want to get client coordinates of without using MFC then just do it manually, like so:
RECT rect = GetMyRect();
POINT rectTL;
rectTL.x = rect.left;
rectTL.y = rect.top;
ScreenToClient( hWnd, &rectTL );
POINT rectBR;
rectBR.x = rect.right
rectBR.y = rect.bottom;
ScreenToClient( hWnd, &rectBR );
rect.left = rectTL.x;
rect.top = rectTL.y;
rect.right = rectBR.x;
rect.bottom = rectBR.y;

Answer 3

Note that RECT is
typedef struct _RECT {
LONG left;
LONG top;
LONG right;
LONG bottom;
} RECT, *PRECT;
and looks like two consecutive POINTs in memory. Therefore you can do what the MFC source code does, which is approx. the following (don't have the MFC source in front of me right now):
::ScreenToClient(hWnd, (POINT*)&rect->left);
::ScreenToClient(hWnd, (POINT*)&rect->right);
which is not the cleanest thing from a C point of view, but those structures are bound to remain binary compatible.