Posts Tagged ‘c++

22
May
13

Default assignment operators and const values

It’s been so long! Update: I’ve been working primarily in C++ for the past four months or so. Hooray! I have so many drafts. I’ve really got to just start posting stuff even when it’s not (gasp) perfectly complete. Today, we’ve got another thing that I didn’t actually learn today, and it’s pretty basic, but I encountered a surprisingly bad suggestion from Microsoft’s documentation, and it seemed worth commenting on. You C++ gurus can probably skip this one.

You may recall, lo these many months ago, I posted about warnings-as-errors and why you should enable it. Well, I’m in the position of taking a large existing project and trying to turn that functionality on, and let me tell you, even with a lot of the work done for me — I was not the first, or even the second, engineer to tackle this — it’s no picnic. Reminder: turn on warnings-as-errors from the start of your project.

Anyway, one of the issues that has come up is the following:

c4512: The compiler cannot generate an assignment operator for the given class. No assignment operator was created.

For those of you not in the know, the C++ compiler generates a default copy constructor and assignment operator for you, unless you’ve specified them yourself. This warning simply means that, for some reason, the default assignment operator cannot be generated. Note that if some part of the code were actually attempting to use this operator, the compiler would throw an error, rather than a warning. Microsoft’s documentation on the warning helpfully points out that if you have any const members in your class, you will get this warning because a const member is not modifiable and thus assignment to it will not work. It continues to add that your options for dealing with the warning are the following:

  • Explicitly define an assignment operator for the class.

  • Remove const or the reference operator from the data item in the class.

  • Use the #pragma warning statement to suppress the warning.

The engineer who looked at this before me went for both the first and the second option. The second option is the wrong option, and frankly, Microsoft should be ashamed for even suggesting it. And while the third option is appropriate for some warnings, in this case, completely ignoring the error is almost certainly never the right answer. Even the first option isn’t quite on target, or at least ignores a possible solution.

I haven’t talked about const much in this blog, but I’m a huge fan. If a member variable is declared const, any programmer using that API can assume that once the data is set in the constructor, it will not change. Removing const-ness should not be done lightly, especially when (as in this case) the member is public. Removing this type modifier opens up that member to modification by any other code. Scott Meyers would tell you to “use const whenever possible” because it is such a strong contract, and I’m inclined to agree with him.

As I mentioned before, if any code were actually attempting to use the assignment operator, you would get an actual error — the code could not be generated, so anything attempting to use that code would not compile. So we don’t need an assignment operator. We just need for the compiler not to generate one. The fix for this is easy — declare it private in the header file and don’t define it. You can do this yourself (there are a million examples on the internet), or you can just make your class inherit from boost::noncopyable, which will also declare the copy constructor private, if you happen to be using the Boost libraries already.

This is a really great practice even if you’re not getting this warning. (I believe it’s one of the first item’s in Effective C++.) If you don’t expect your class to be copied, go ahead and declare the copy constructor and the assignment operator private.

14
Nov
12

C++ Basics: Static

I’ve been working on this long, boring post about the differences in equality comparison in Perl and Python and the fundamental differences of the two languages, but you know what? I’m sick of Perl and Python! And that post was boring! So let’s talk about C++ instead.

I’m in the unenviable position of not having touched a compiler (for work) in about a year. How do you live, Maitland?, you ask, Without C++, what gives your life meaning? Those are really wonderful questions for another blog, but the fact is, while I’ve learned tons in the last year about Perl and automation (and debugging and debugging and debugging), my C++ knowledge has been getting fuzzier and fuzzier. It’s extremely unnerving. Just yesterday, I opened up one of my favorite books and I freaked out because I saw my second favorite* C++ keyword, static, and, frankly, I wasn’t sure I could remember all of its usage. Thank god I have a blog so I can write it all down.

The static keyword can be the slightest bit tricky because it has several applications, and different implications in different situations. Some people will tell you that it behaves differently or that it means something different in these different situations, but I feel like this point of view overcomplicates the issue. From my point of view, static really just means one thing: it means that for the duration of the program, there will be only one of the described object or function. Once the thing is defined, it’s defined for the lifetime of the program. In fact, static objects don’t even allocate memory from the stack or the heap — they have their own separate memory segment, creatively called static memory.

Static Objects. Static non-member objects are probably the simplest usage of static. They look like this:

static int myInt = 42;

Once this variable is initialized, it’s there for the lifetime of the program. So if, for example, you put this variable in a header file, you can change that object in your functions or class members, and the change will persist. The example above uses an integer, but the concept is the same for user-defined types as well. Static objects are scoped in the same way that any other object is. If the object is declared in a header, it is scoped by the translation unit (i.e., the source and headers required to compile a given source file) that includes it. If it’s declared inside a method or function, like any local variable, it cannot be referenced outside of that method or function.

Static Members. A static member is much like a static object except for the fact that it belongs to a class. A static member is shared by all instances of a class. The biggest implication of this is that you can modify the member completely independently from an instance of the class. For example,

class MyClass
{
public:
  static int m_myInt;
}

MyClass::m_myInt = 0;

is totally valid code! Furthermore, any instances of MyClass can modify that member, and that change will persist until some other instance (or external code) modifies that member. Obviously, making a static non-const member public is fairly dangerous (as is making any member variable public), because you have no way of controlling who can modify it, but a non-public member can be very useful when you want to keep a reference count of something, for example. And when you combine static and const, you have a great way to store immutable data that may want to be scoped and defined at compile time.

One thing worth pointing out about static members is that, unless they are const and integral, you cannot initialize them within the class declaration. Logically, this makes sense, when you consider that you could construct the class multiple times, and, if the static member were initialized within the declaration, it would theoretically be re-initialized every time the class was instantiated. Because a static member is defined for all instances of a class, re-defining it in every instance would violate C++’s one definition rule. (C++ does make an exception for static const integral members, but if I’m being honest, I don’t entirely understand the rationale except “it makes life easier,” which, in fairness, is a good thing.)

Static Methods. Static methods (not to be confused with static functions!) have a lot in common with static members. Like static members, they can be referenced without an instance of the class. The natural implication of this is that static methods cannot reference any non-static members of the class. Because they don’t require state, static methods are especially good when you have software that uses callbacks. They allow you to group functionality with your class even when that functionality does not operate on the class’s state directly.

Static LinkageOkay, here’s where it gets a little tricky. Remember how I said that static really only means one thing? I kinda lied.

So, a long time ago, in a galaxy far, far away, there was a little programming language called C. And it defined static as a keyword, and it was slightly really a lot different from how C++ defines it. If you define (not declare) a method or function as static, the compiler, knowing that it has to be backwards compatible with C, treats this as static linkage. This means that your function or method cannot be referenced outside of the file it’s defined in. I’m sure there are good reasons to do this — certainly, it allows you to write helper functions that you don’t intend to be used outside of a certain file — but I can’t say I’ve ever used it myself.

I feel like I must be forgetting something, but I suppose that’s to be expected, given how rusty I am. Hell, I haven’t even talked about extern or referenced Meyer’s excellent point about how the intialization order of non-local static objects is totally undefined, meaning you should probably avoid that and use a local static variable inside a function instead. But this is getting lengthy, so I think I’ll close it off here. Next time, memory segments?

* My favorite being, of course, const.

01
Aug
11

Inlining and #includes

I’m on build coverage tonight, meaning I’m working until 2AM. Not the most glamorous aspect of the game dev industry, but when you’re trying to hit beta, making sure we have solid builds for QA every day is pretty important. So at least I’m important. Right?

So, the incremental build failed on me in an interesting way this evening. A header failed to compile because there was a class method defined (inlined) in it that used a function that wasn’t declared in any of the included headers. This, in and of itself, isn’t a particularly interesting failure. In fact, it’s a fairly typical, easy-to-catch-on-your-own-machine failure, one that you can usually shoot your fellow devs with nerf darts for causing. What made this particular instance interesting is that the check-in that broke the build wasn’t the one that broke the code.

Of course, it’s fairly common for a perfectly valid piece of code to expose a flaw in another piece of code, but I just couldn’t seem to figure out how the original code, which clearly called a function that wasn’t declared in any of its included headers, had ever compiled. My first thought was that maybe before this check-in, the header had included another header that had included the necessary header, but when I checked, none of the included headers had changed. It turns out the issue was that the new code changed the method from private to public.

Previously, the method had been private, meaning that it was guaranteed not to be used by any other translation units, so, even though the header was included by other files, that particular method was expanded. In the source file for the defined class, the header include was preceded by an include for another header which did declare the needed function. Once the method was made public, that method became expandable in other translation units, ones that may not have had the missing header in their includes.

This fix for this particular issue is incredibly simple — add the necessary header to the includes. But what’s the moral here?

  1. Always include all of the necessary headers for your code, and don’t rely on compilation to tell you whether you’ve done so. If you’re worried about build times or violating the one definition rule, use include guards. Actually, always use include guards. It’s a tiny amount of effort and extremely low-risk for a potentially epic payoff. (If you name your guards after the namespace and class that they guard, your chance of naming collisions is slim.)
  2. Please, for the love of all that is good, compile your code before you check it in.



About

My name is Maitland Lederer, and I’m a video game developer. I learn stuff you probably already knew and have opinions you've probably already heard. I figured it might be a good idea for me to start writing down the stuff I've learned so I don't have to relearn it. It's not, like, great wisdom or anything. It's just things I happened to learn, usually today.

Header photo by D Sharon Pruitt, used under a Creative Commons License.