Why was MoveTo replaced with MoveToEx?

Raymond Chen

Raymond

Commenter Ulric asks,
Where did MoveTo(HDC, int, int) go?

Back in the 16-bit days, the function to move the current point
was called MoveTo,
and its return value was a DWORD which encoded the
previous position,
packing two 16-bit coordinates into a single 32-bit value.
As part of the transition to 32-bit Windows,
GDI switched to using 32-bit coordinates instead of the
wimpy 16-bit coordinates of old.
As a result, it was no longer possible to encode the original
position in a single DWORD.
Something new had to be developed.

That new thing was the MoveToEx function.
Instead of returning a single DWORD,
it accepted a final parameter which
received the previous coordinates.
If you didn’t care about the previous coordinates,
you could just pass NULL.
All of the GDI functions which used to pack two 16-bit coordinates
into a single DWORD
got Ex-ified in this way so they could accommodate
the new 32-bit coordinate system.

But why did the old MoveTo function go away?
Why not keep it around for source code compatibility?

I find this an interesting question,
since most people seem to think that maintaining source code
compability between the 32-bit and 64-bit versions of Windows
was an idea whose

stupidity

rivals
prosecuting a land war in Asia.
(If we had followed this advice,
people would just be asking,

why did you replace WinExec with the much harder-to-use
CreateProcess
?)
By the same logic, source code compatibility between 16-bit and 32-bit
Windows is equally absurd.
According to these people,
porting 16-bit code to
to 32-bit Windows is the best time to introduce
these sorts of incompatibilities,
in order to force people to rewrite their programs.

Anyway, the reason we lost MoveTo was that there
was no way to return 64 bits of information in a 32-bit integer.
Now it's true that in many cases, the caller doesn't actually care
about the previous position,
but of course the MoveTo function doesn't know that.
It returns a value; it doesn't know whether the caller is going to
use that return value or not.

I guess one way out would have been to change the return value of
MoveTo to void.
That way, people who didn't care about the return value would still
compile, while people who did try to use the return value
would get a compile error and have to switch to MoveToEx.

Yeah, I guess that could've been done,
but you could also have done that yourself:

#define MoveTo(hdc, x, y) ((void)MoveToEx(hdc, x, y, NULL))

I find it interesting that most people who write their own
MoveTo macro don't use the (void) cast.
In most cases, this is a mistake in porting from 16-bit Windows.
(I can tell because the macro is mixed in with a bunch of other
porting macros.)
However, in other cases, it could be intentional.
The authors of the macro may simply not have known about the old
16-bit days and simply expected their macro to be used as if it
were prototyped as
BOOL MoveTo(HDC, int, int).

These people will probably be baffled if they run across any
actual 16-bit Windows code that tried to extract the high word
from the return value of MoveTo.
"Why are you extracting the high word from a BOOL?"

Historical exercise:
Instead of adding a new parameter, why not just make
MoveToEx return an __int64?

Raymond Chen
Raymond Chen

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