Some C++ Gotchas

2009 Q1 link clearance: Microsoft blogger edition

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From elsewhere in the collective.

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re: Some C++ Gotchas

Norman Diamond — Mon, 30 Jun 2008 03:41:23 GMT

Betas are betas. Now if betas were betas for a reason, so that bugs found in betas would not be present in releases, then betas would be really valuable. But betas would still be betas. You should know to download and install betas on machines that are dedicated to experimenting.

re: Some C++ Gotchas

Khris — Sun, 29 Jun 2008 12:49:58 GMT

I recommend that you do not download this beta. It totally damaged my SQL server and Framework up to the point that I cannot unistall and reinstall. It causes system problems. Its a real pain in my ass because I've tried everything to fix it and I've only attained partial progresss. I won't download a beta like this. I've heard rumors that Vista and new programs have problems but I didn't think I'd ever really have a problem because I know how to fix most problems but this is annoying. My system did not crash but everytime I start up it tells me that SQL is incomplete or corrupt. I like no problems but I guess I'm going to have to live with this one unless I reinstall all my programs which I don't want to do. frustration.

re: Some C++ Gotchas

Norman Diamond — Mon, 23 Jun 2008 09:50:06 GMT

"In C and C++, pointers are considered signed for the purposes of casting."

In the old days, before the "unsigned" keyword was invented, C hackers knew how to get one. On the PDP-11, "int" was signed 16 bits, and "char*" was unsigned 16 bits.

Now fast-forward to some popular implementations on modern hardware...

"(intptr_t) 0x80000000"

The result of the cast to intptr_t is signed because intptr_t is a signed type. If intptr_t is 64 bits then I think the semantics are value preserving, signed value 0x0000000080000000. If intptr_t is 32 bits then you get a signed result after the overflow.

"(long long) &var"

If the pointer is 32 bits then I think the semantics are value preserving. I think your example demonstrated that the pointer representation is treated as an unsigned representation before being converted.

I see why you want a warning when a pointer type is cast to a signed type, but I wonder if it would be more suitable in Prefast tests than built into the DDK's compiler. Do you also want a warning when an unsigned int constant is cast to a signed type, to catch your first example too? Maybe you should suggest these to the Prefast team?

re: Some C++ Gotchas

Myria — Mon, 23 Jun 2008 05:44:52 GMT

In my ~10 years of using Visual Studio, I've only encountered one previously unknown compiler bug. It was a ridiculously obscure bug involving rare compilation conditions.

In C and C++, pointers are considered signed for the purposes of casting. This means that if you cast a pointer to an integer type of size larger than a pointer, sign extension is used. This can occur with 32-bit compilation, and indeed works in this context:

int wmain()

{

void* ptr = (void*) (intptr_t) 0x80000000;

wprintf(L"%016llX", (long long) ptr);

}

This will print FFFFFFFF80000000 due to the sign extension, and is the expected answer.

However, if the value is a compile-time constant referring to the address of a global object or function, the compiler can zero-extend. This happens if the global object's address is larger than 7FFFFFFF on a 32-bit target:

int var;

long long address = (long long) &var;

If the base address of the DLL has its high bit set, the compiler will emit a zero-extended address of var. This can only occur when compiling Windows NT kernel drivers.

The compiler or linker should actually error or warn instead of allowing this. It is not possible in the 32-bit Portable Executable format to emit this type of relocation (sign-extend relocated address). However, this case is so obscure that it was deemed unworthy of being fixed.

re: Some C++ Gotchas

Christian Gross — Sun, 22 Jun 2008 18:21:23 GMT

To David Roe

I stumbled on this blog by accident (was searching something else). But I USED stress USED to be a C++ programmer. When I see blog entries like these I think, "gee I am glad I switched programming languages."

When I need to switch to low level, believe it or not I use C! I find C a whole lot more comforting and straight forward... And with this functional drive coming implementing functional constructs in C is DEAD SIMPLE...

Intellisense

Norman Diamond — Sun, 22 Jun 2008 13:41:47 GMT

The MSDN forums are still giving ASP error 500 every time I try to log in and/or post. I see other people have posted but I can't guess how they do it. I'm using IE6 in Windows XP SP3, but the errors aren't in IE, they're server error 500's in Microsoft.

Therefore I resort to posting here again.

This time the problem isn't user error. Intellisense is still the same in VC++2008 RTM as it was in VC++2005 betas.

I close Visual Studio 2008, delete the .ncb file, double-click the .sln file again, wait for Visual Studio 2008 to rebuild broken Intellisense information, and it is rebuilt broken.

If I hover the mouse cursor over an identifier, a tooltip might or might not display. Today the failure occurs most often if the identifier is a member of a derived class.

If I type code like

this->

there is an error that the left hand operator of -> is invalid. It compiles correctly, but the bug isn't in the compiler, the bug is in Intellisense.

If I type a member function name correctly and then a left parenthesis, no tooltip appears.

In the pair of combo boxes above the code window, the left-hand combo box always says global scope, even if the text cursor is in the middle of a class's code. The class name cannot be selected. The right-hand combo box is empty and nothing can be selected. Today this problem is happening only with derived classes but base classes are working.

Anyway, Intellisense gets rebroken every time it gets rebuilt. Please, is there any way to get Intellisense working again?

re: Some C++ Gotchas

Steve Nuchia — Fri, 20 Jun 2008 01:16:53 GMT

Regarding the compiler "calling the wrong function", I recently spent half a day chasing around in circles on an unrelated problem that will match the same search terms.

For functions with identical object code and at sufficiently high optimization levels, the linker will combine the bodies. When it does, the debugger, performance profiler, etc will pick one of the names more-or-less at random. It looks very much like the wrong function got called.

Caveat Debugger.

re: Some C++ Gotchas

Norman Diamond — Sun, 15 Jun 2008 11:04:25 GMT

Uh yeah, it went to bool because my parentheses were in the wrong place. Sorry to disturb some number of readers and blog owner.