Archive for May, 2013

GotW #91

Posted in C++11, GotW on May 31, 2013 by Crazy Eddie

I can’t seem to post to Herb Sutter’s blog, my comments just never appear and ones I’ve made are gone.  Maybe he has me blocked for some mysterious reason.  So I thought I’d put my answer to the problems here.

Anyone that wants to write solid C++ should be reading his blog.  I suggest subscribing.

So, my answer to GotW 91 is:

#1 – Some extra locking and reference counting will take place. A new small object will be created on the stack. Not much will happen actually. The locking is the greatest concern.

#2 – I don’t believe it matters until threads are introduced. If you call this function via a thread spawning though you want to be sure that you do so by value. Doing it by reference can create a race condition in which you reduce the reference count while the pointer is being used or copied by the other side. When that happens it could be that you’re the last one holding the pointer and then the other side gets to have it’s resource yanked out from under it. This is bad voodoo.

#3 has made me want to come up with a set of smart pointer types that secure and ensure correct ownership semantics. In this case you’d take a pointer (by value) of the kind that specifies how you’ll use it. If only within scope then you’d specify that with type. It’s a pretty tough problem though…at least for me.

The answer to the question is one of three (given the types in the question):

1. Use reference – when in doubt, use a reference rather than pointer. If you don’t need a pointer, don’t use one.
2. Use unique_ptr by value – You intend to steal ownership of the pointer and you want this ownership transfered away from client code immediately.
3. The parameter could be null but you don’t want to steal it.  Use a raw pointer (for now at least…if I ever solve the above idea then you may have a smart pointer to use instead)


A case against naming conventions based on type

Posted in Uncategorized with tags on May 17, 2013 by Crazy Eddie

Many places I have worked have naming conventions that are, in my opinion, poorly conceived.  Naming conventions really should be limited to deciding between camel case, pascal case, and simply, “Give it a name that explains what it is supposed to be for.”  Unfortunately a lot of places go much further and do things like putting ‘m_’ before member variables, having different naming conventions for different classes of entities (Pascal for classes, underscore for functions, camel for variables, _t after typedefs, etc…).  These kinds of conventions miss the point of generic and object oriented programming and thus miss the point of C++ in a great way.

The issue is that what kind of thing an entity is doesn’t really matter so long as a certain interface is obeyed.  Take for example putting the “_t” postfix on typedefs, which many developers and firms do.  What happens now when someone comes along and decides that this type would be better implemented as a class of its own rather than a typedef of something else.  Since typedefs in C++ are weak this is not a surprising or all to rare thing to do.  Now in order to keep from breaking clients everywhere we need to follow the convention for naming classes and then add a typedef so that the “_t” name exists.  This is silly.

Another example comes up with regard to functors.  Functors are classes thats’ instances look and behave like functions that may have state.  If you’ve specified that variables must be named one way, and functions another, then in fact you’ve got a problem here because conceptually this variable is both a variable and a function!  If you replace a function with a functor someplace then again you are in the unfortunate position of having to violate naming conventions or do something silly like make a function to call the functor just so both names exist.

The C++ standard library has a pretty good C++ naming convention.  It uses one convention for almost everything with the caveats that template parameters are named differently (and this is important because of dependent name issues) and so are macros.  Both of these things require special consideration when using so it’s worthwhile to make them stand out.

In C++ types are strong at the time of compilation, but in the source they are interchangeable to a great degree.  So long as something obeys an interface, calling code shouldn’t have to change when you completely change the type some entity is.  Naming conventions that force you to change names based on type hinder this ideal.