How to shoot yourself in the foot with const (or the lack thereof)

AngryRichard — Wed, 24 May 2006 21:00:00 GMT

It has long puzzled me why various Win32 functions take non-const string parameters, when clearly they have no business manipulating the string. Take, for instance, GetNamedSecurityInfo:

DWORD GetNamedSecurityInfo(
  LPTSTR pObjectName,
  SE_OBJECT_TYPE ObjectType,
  SECURITY_INFORMATION SecurityInfo,
  PSID* ppsidOwner,
  PSID* ppsidGroup,
  PACL* ppDacl,
  PACL* ppSacl,
  PSECURITY_DESCRIPTOR* ppSecurityDescriptor
);

I can't imagine why this function should ever need to write to pObjectName. In fact, the SDK documentation explicitly lists it as an [in] parameter. Also note that pObjectName is an LPTSTR. This is important to the shooting of the foot.

A tidy, const-aware developer might want to do something like this:

const LPWSTR SomeFile = L"\\somefile.txt";

[...]

GetNamedSecurityInfo(Somefile, ...)

Of course, this will fail to compile, because SomeFile is const. But casting solves this problem, right?

GetNamedSecurityInfo((LPTSTR)Somefile, ...)

Yay, this compiles. Why, it even appears to return the correct data.

Fast forward a year or two, when suddenly the code stops working on Vista. What could be wrong? Why, in the process of casting away const, we've also turned an LPWSTR into an LPTSTR. In this case, TCHAR resolved to char, so the code above was actually returning a security descriptor for "\\". Well, that is until Vista came along.

So while I hang my head in shame for being lame enough to cast between incompatible types, I will also caution the rest of you to consider whether your string parameters really need to be non-const, and avoid making your clients play casting games to use your API. And yes, using const_cast<> might have prevented this issue, but I didn't know about it at the time. You, having read this far, have no excuse.

C'mon, all your Friends are Doing it

AngryRichard — Thu, 27 May 2004 22:15:00 GMT

A bunch of the guys on the team I work for have been starting up blogs. I started feeling left out,
which made me very angry.

It appears all blogs start with "Hi, I'm a developer who does X and I'm going to talk about Y and maybe Z."
It's all part of Microsoft's new image -- we're transparent now.

Transparency's good, right?

Go down to your local German car dealer, or, if you own a newer German car, go out to your driveway. Shiny, pretty, isn't it? Open the hood. Look at that -- a big, fat, intake pipe that goes into a big box that says "BMW" in a 4" Century Gothic font. Cool. Now pop that plastic thing off the top. Go ahead, I dare you. Not so pretty now. Look at all those wires and tubes. Look at all that stuff you could cut yourself on. That's why that plastic thing is there; it makes owning all that power a little less scary.

Some of us spend all our time under the shiny plastic thing, hands full of wires and tubes and spark plugs. That'd be me and some of my less 'transparent' friends, working on the instrumentation and data collection engine in the profiler.

Of course, with great power, comes great potential for disaster. Some day you'll have your turn with the profiler. Trust me, something's always too slow. If it works, great. If you find yourself upside down in a ditch, tell us, we want to know where we need to cover up the sharp edges.

Angry Richard's WebLog : Useless rants

How to shoot yourself in the foot with const (or the lack thereof)

C'mon, all your Friends are Doing it