Comments are one critical part of readability and maintainability. Everyone -knows they should comment, so should you. :) Although we all should probably +knows they should comment, so should you. Although we all should probably comment our code more than we do, there are a few very critical places that documentation is very useful:
@@ -142,10 +144,11 @@ this:A few things to note about this particular format: The "-*- C++ -*-" string on the first line is there to tell Emacs that the source file -is a C++ file, not a C file (Emacs assumes .h files are C files by default [Note -that tag this is not necessary in .cpp files]). The name of the file is also on -the first line, along with a very short description of the purpose of the file. -This is important when printing out code and flipping though lots of pages.
+is a C++ file, not a C file (Emacs assumes .h files are C files by default). +Note that this tag is not necessary in .cpp files. The name of the file is also +on the first line, along with a very short description of the purpose of the +file. This is important when printing out code and flipping though lots of +pages.The next section in the file is a concise note that defines the license that the file is released under. This makes it perfectly clear what terms the source @@ -158,11 +161,11 @@ included, as well as any notes or "gotchas" in the code to watch out for.
Class overviews -Classes are one fundemental part of a good object oriented design. As such, +
Classes are one fundamental part of a good object oriented design. As such, a class definition should have a comment block that explains what the class is used for... if it's not obvious. If it's so completely obvious your grandma could figure it out, it's probably safe to leave it out. Naming classes -something sane goes a long ways towards avoiding writing documentation. :)
+something sane goes a long ways towards avoiding writing documentation. Method information @@ -192,8 +195,9 @@ when it is useful to use C style (/* */) comments however:... and each catagory should be sorted by name.
The "Main Module Header" file applies to .cpp file -which implement an interface defined by a .h file. This #include should always -be included first regardless of where it lives on the file system. By -including a header file first in the .cpp files that implement the interfaces, -we ensure that the header does not have any hidden dependencies which are not -explicitly #included in the header, but should be. It is also a form of -documentation in the .cpp file to indicate where the interfaces it implements -are defined.
+which implement an interface defined by a .h file. This #include +should always be included first regardless of where it lives on the file +system. By including a header file first in the .cpp files that implement the +interfaces, we ensure that the header does not have any hidden dependencies +which are not explicitly #included in the header, but should be. It is also a +form of documentation in the .cpp file to indicate where the interfaces it +implements are defined.In C++, the class and struct keywords can be used almost +interchangeably. The only difference is when they are used to declare a class: +class makes all members private by default while struct makes +all members public by default.
+ +Unfortunately, not all compilers follow the rules and some will generate +different symbols based on whether class or struct was used to +declare the symbol. This can lead to problems at link time.
+ +So, the rule for LLVM is to always use the class keyword, unless +all members are public, in which case struct is allowed.
+ +But wait, sometimes you need to have the definition of a class to use it, or -to inherit from it. In these cases go ahead and #include that header file. Be -aware however that there are many cases where you don't need to have the full -definition of a class. If you are using a pointer or reference to a class, you -don't need the header file. If you are simply returning a class instance from a -prototyped function or method, you don't need it. In fact, for most cases, you -simply don't need the definition of a class... and not #include'ing -speeds up compilation.
+to inherit from it. In these cases go ahead and #include that header +file. Be aware however that there are many cases where you don't need to have +the full definition of a class. If you are using a pointer or reference to a +class, you don't need the header file. If you are simply returning a class +instance from a prototyped function or method, you don't need it. In fact, for +most cases, you simply don't need the definition of a class... and not +#include'ing speeds up compilation.It is easy to try to go too overboard on this recommendation, however. You must include all of the header files that you are using, either directly @@ -447,7 +471,7 @@ about later...
Many modules have a complex implementation that causes them to use more than one implementation (.cpp) file. It is often tempting to put the internal communication interface (helper classes, extra functions, etc) in the -public module header file. Don't do this. :)
+public module header file. Don't do this.If you really need to do something like this, put a private header file in the same directory as the source files, and include it locally. This ensures @@ -466,7 +490,7 @@ class itself... just make them private (or protected), and all is well.
In LLVM, we prefer to explicitly prefix all identifiers from the standard +namespace with an "std::" prefix, rather than rely on "using namespace std;". +
+ +In header files, adding a 'using namespace XXX' directive pollutes the +namespace of any source file that includes the header. This is clearly a bad +thing.
+ +In implementation files (e.g. .cpp files) the rule is more of a stylistic +rule, but is still important. Basically, using explicit namespace prefixes +makes +the code more clear - because it is immediately obvious what facilities +are being used and where they are coming from - and more portable - +because namespace clashes cannot occur between LLVM code and other namespaces. +The portability rule is important because different standard library +implementations expose different symbols (potentially ones they shouldn't) and +future revisions to the C++ standard will add more symbols to the std +namespace. As such, we never 'using namespace std;' in LLVM.
+ +The exception to the general rule (i.e. it's not an exception for the std +namespace) is for implementation files. For example, all of the code in the +LLVM project implements code that lives in the 'llvm' namespace. As such, it +is ok, and actually more clear, for the .cpp files to have a 'using namespace +llvm' directive at their top, after the #includes. The general form of this +rule is that any .cpp file that implements code in any namespace may use that +namespace (and its parents), but should not use any others.
+ +If a class is defined in a header file and has a v-table (either it has +virtual methods or it derives from classes with virtual methods), it must +always have at least one out-of-line virtual method in the class. Without +this, the compiler will copy the vtable and RTTI into every .o file that +#includes the header, bloating .o file sizes and increasing link times. +
+ +C++ is a powerful language. With a firm grasp on its capabilities, you can -make write effective, consise, readable and maintainable code all at the same -time. By staying consistent, you reduce the amount of special cases that need -to be remembered. Reducing the total number of lines of code you write is a -good way to avoid documentation, and avoid giving bugs a place to hide.
- -For these reasons, come to know and love the contents of your local -<algorithm> header file. Know about <functional> and what it can do -for you. C++ is just a tool that wants you to master it. :)
- -If you get some free time, and you haven't read them: do so, you might learn -something. :)
+something.