VS2005 Pro SP1 and C4244

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

A Wieser - 28 Dec 2006 16:13 GMT

I've just upgraded to SP1, and am seeing C4244 warnings on code like the
following:

void fn()
{
short a = 1;
short b = 2;

a += b; // warning C4244: '+=' : conversion from 'int' to
'short', possible loss of data i:\2005\CPP\NDS\testmsl\Calsetng.cpp 168

// but
a = a + b; // doesn't give a warning.
}

I'm not sure if this was wrong before SP1 or not, but it's reported as fixed
here:
https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID
=99575

Does anyone else get this error with or without SP1?

Anthony Wieser
Wieser Software Ltd

Reply to this Message

David Lowndes - 28 Dec 2006 16:50 GMT

>I've just upgraded to SP1, and am seeing C4244 warnings on code like the
>following:
[quoted text clipped - 16 lines]
>
>Does anyone else get this error with or without SP1?

Anthony,

I'm surprised by the comment to that bug report. I believe this is one
of those C/C++ quirks that throws most of us at some time or another.

As far as I know it's expected behaviour not a bug.

Dave

Reply to this Message

Bruno van Dooren [MVP VC++] - 28 Dec 2006 21:05 GMT

>> a += b; // warning C4244: '+=' : conversion from 'int' to
>>'short', possible loss of data i:\2005\CPP\NDS\testmsl\Calsetng.cpp 168
[quoted text clipped - 16 lines]
>
> As far as I know it's expected behaviour not a bug.

This is documented in MSDN if you look for the += operator documentation
(you have to follow one or two links from there)
Basically, the C standard says that for those operators, the arguments are
automatically converted to int.
It's one of those things that must have seemed like a good idea at the
time...

Signature

Kind regards,
Bruno van Dooren
bruno_nos_pam_van_dooren@hotmail.com
Remove only "_nos_pam"

Reply to this Message

A Wieser - 28 Dec 2006 21:28 GMT

| >> a += b; // warning C4244: '+=' : conversion from 'int' to
| >>'short', possible loss of data i:\2005\CPP\NDS\testmsl\Calsetng.cpp 168
[quoted text clipped - 24 lines]
| It's one of those things that must have seemed like a good idea at the
| time...

The documentation seems a bit misleading:
it says here:
ms-help://MS.MSDNQTR.v80.en/MS.MSDN.v80/MS.VisualStudio.v80.en/dv_vclang/html/a6135e34-4fce-4c79-87b6-b6447db3507e.htm
"The assignment operators return the value of the object specified by the
left operand after the assignment. The resultant type is the type of the
left operand. "

Also, the documentation here:
ms-help://MS.MSDNQTR.v80.en/MS.MSDN.v80/MS.VisualStudio.v80.en/dv_vclang/html/b028cf35-2ff1-4f14-9027-fd53ebec8aa0.htm
says:
"+=
Add the value of the second operand to the value of the first operand;
store the result in the object specified by the first operand.

"

So the two lines below should be equivalent:

>> a += b; // warning C4244: '+=' : conversion from 'int' to

'short'.

| >> // but
| >> a = a + b; // doesn't give a warning.

Where's your reference that implied that it was ok to convert to int? I
can't find it anywhere (but maybe my google searches are wrong).

Anthony Wieser
Wieser Software Ltd

Reply to this Message

Bruno van Dooren [MVP VC++] - 29 Dec 2006 06:57 GMT

> The documentation seems a bit misleading:
> it says here:
[quoted text clipped - 9 lines]
> Add the value of the second operand to the value of the first operand;
> store the result in the object specified by the first operand.

Hi,

http://msdn2.microsoft.com/en-us/library/k630sk6z.aspx
bullet point 1 applies.

See here for the rules:
http://msdn2.microsoft.com/en-us/library/3t4w2bkb.aspx

The behavior of += is not explicitly specified, but the you can see that the
documentation allows for it. It is allowed by the C/C++ standards as well.

I still think it is idiotic that += does this, since it is supposed to be
equal to = +.

Signature

Kind regards,
Bruno van Dooren
bruno_nos_pam_van_dooren@hotmail.com
Remove only "_nos_pam"

Reply to this Message

A Wieser - 29 Dec 2006 07:26 GMT

|> The documentation seems a bit misleading:
| > it says here:

ms-help://MS.MSDNQTR.v80.en/MS.MSDN.v80/MS.VisualStudio.v80.en/dv_vclang/html/a6135e34-4fce-4c79-87b6-b6447db3507e.htm

ms-help://MS.MSDNQTR.v80.en/MS.MSDN.v80/MS.VisualStudio.v80.en/dv_vclang/html/b028cf35-2ff1-4f14-9027-fd53ebec8aa0.htm

| > says:
| > "+=
[quoted text clipped - 14 lines]
| I still think it is idiotic that += does this, since it is supposed to be
| equal to = +.

OK, I see here that it says:
If none of the above conditions are met, both operands are converted to type
int.

So that means the compiler really should be warning me in the second case
too, where I write

short a=1;
short b=2;

a = a + b;

That's even more depressing! So in the next version of the compiler, I'll
get even more warnings.

Maybe the next version will have C4416 defined.

// warning C4416: Ha Ha Ha! You're still using 16 bit values!

--

Anthony Wieser
Wieser Software Ltd

Reply to this Message

Rate this thread:

VS2005 Pro SP1 and C4244

Free Magazines