1. Introduction
2. Mechanical Source Issues
   1. Source Code Formatting
      1. Commenting
      2. Comment Formatting
      3. Source Code Width
      4. Use Spaces Instead of Tabs
      5. Indent Code Consistently
   2. Compiler Issues
      1. Treat Compiler Warnings Like Errors
      2. Which C++ features can I use?
      3. Write Portable Code
3. Style Issues
   1. The High Level Issues
      1. A Public Header File is a Module
      2. #include as Little as Possible
      3. Keep "internal" Headers Private
   2. The Low Level Issues
      1. Assert Liberally
      2. Prefer Preincrement
      3. Exploit C++ to its Fullest
   3. Writing Iterators
4. See Also

Introduction

This document attempts to describe a few coding standards that are being used in the LLVM source tree. Although no coding standards should be regarded as absolute requirements to be followed in all instances, coding standards can be useful.

This document intentionally does not prescribe fixed standards for religious issues such as brace placement and space usage. For issues like this, follow the golden rule:

If you are adding a significant body of source to a project, feel free to use whatever style you are most comfortable with. If you are extending, enhancing, or bug fixing already implemented code, use the style that is already being used so that the source is uniform and easy to follow.

The ultimate goal of these guidelines is the increase readability and maintainability of our common source base. If you have suggestions for topics to be included, please mail them to Chris.

In all cases, prefer spaces to tabs in source files. People have different prefered indentation levels, and different styles of indentation that they like... this is fine. What isn't is that different editors/viewers expand tabs out to different tab stops. This can cause your code to look completely unreadable, and it is not worth dealing with.

As always, follow the Golden Rule above: follow the style of existing code if your are modifying and extending it. If you like four spaces of indentation, DO NOT do that in the middle of a chunk of code with two spaces of indentation. Also, do not reindent a whole source file: it make for incredible diffs that are absolutely worthless.

C++ doesn't do too well in the modularity department. There is no real encapsulation or data hiding (unless you use expensive protocol classes), but it is what we have to work with. When you write a public header file (in the LLVM source tree, they live in the top level "include" directory), you are defining a module of functionality.

Modules should be completely independent of each other, except for their dependence. A module is not just a class, a function, or a namespace: it's a collection of these that defines an interface. This interface may be several functions, classes or data structures, but the important issue is how they work together.

In general, a module should be implemented with one or more .cpp files. Each of these .cpp files should include the header that defines their interface first. This ensure that all of the dependences of the module header have been properly added to the module header itself, and are not implicit. System headers should be included after user headers for a translation unit.

Here's a pretty good summary of how to write your own data structure iterators in a way that is compatible with the STL, and with a lot of other code out there (slightly edited by Chris):

From: Ross Smith 
Newsgroups: comp.lang.c++.moderated
Subject: Writing iterators (was: Re: Non-template functions that take iterators)
Date: 28 Jun 2001 12:07:10 -0400

Andre Majorel wrote:
> Any pointers handy on "writing STL-compatible iterators for
> dummies ?"

I'll give it a try...

The usual situation requiring user-defined iterators is that you have
a type that bears some resemblance to an STL container, and you want
to provide iterators so it can be used with STL algorithms. You need
to ask three questions:

First, is this simply a wrapper for an underlying collection of
objects that's held somewhere as a real STL container, or is it a
"virtual container" for which iteration is (under the hood) more
complicated than simply incrementing some underlying iterator (or
pointer or index or whatever)? In the former case you can frequently
get away with making your container's iterators simply typedefs for
those of the underlying container; your begin() function would call
member_container.begin(), and so on.

Second, do you only need read-only iterators, or do you need separate
read-only (const) and read-write (non-const) iterators?

Third, which kind of iterator (input, output, forward, bidirectional,
or random access) is appropriate? If you're familiar with the
properties of the iterator types (if not, visit
http://www.sgi.com/tech/stl/), the appropriate choice should be
obvious from the semantics of the container.

I'll start with forward iterators, as the simplest case that's likely
to come up in normal code. Input and output iterators have some odd
properties and rarely need to be implemented in user code; I'll leave
them out of discussion. Bidirectional and random access iterators are
covered below.

The exact behaviour of a forward iterator is spelled out in the
Standard in terms of a set of expressions with specified behaviour,
rather than a set of member functions, which leaves some leeway in how
you actually implement it. Typically it looks something like this
(I'll start with the const-iterator-only situation):

  #include 

  class container {
    public:
      typedef something_or_other value_type;
      class const_iterator:
        public std::iterator {
          friend class container;
          public:
            const value_type& operator*() const;
            const value_type* operator->() const;
            const_iterator& operator++();
            const_iterator operator++(int);
            friend bool operator==(const_iterator lhs,
                                   const_iterator rhs);
            friend bool operator!=(const_iterator lhs,
                                   const_iterator rhs);
          private:
            //...
        };
      //...
  };

An iterator should always be derived from an instantiation of the
std::iterator template. The iterator's life cycle functions
(constructors, destructor, and assignment operator) aren't declared
here; in most cases the compiler-generated ones are sufficient. The
container needs to be a friend of the iterator so that the container's
begin() and end() functions can fill in the iterator's private members
with the appropriate values.

[Chris's Note: I prefer to not make my iterators friends.  Instead, two
ctor's are provided for the iterator class: one to start at the end of the
container, and one at the beginning.  Typically this is done by providing
two constructors with different signatures.]

There are normally only three member functions that need nontrivial
implementations; the rest are just boilerplate.

  const container::value_type&
    container::const_iterator::operator*() const {
      // find the element and return a reference to it
    }

  const container::value_type*
    container::const_iterator::operator->() const {
      return &**this;
    }

If there's an underlying real container, operator*() can just return a
reference to the appropriate element. If there's no actual container
and the elements need to be generated on the fly -- what I think of as
a "virtual container" -- things get a bit more complicated; you'll
probably need to give the iterator a value_type member object, and
fill it in when you need to. This might be done as part of the
increment operator (below), or if the operation is nontrivial, you
might choose the "lazy" approach and only generate the actual value
when one of the dereferencing operators is called.

The operator->() function is just boilerplate around a call to
operator*().

  container::const_iterator&
    container::const_iterator::operator++() {
      // the incrementing logic goes here
      return *this;
    }

  container::const_iterator
    container::const_iterator::operator++(int) {
      const_iterator old(*this);
      ++*this;
      return old;
    }

Again, the incrementing logic will usually be trivial if there's a
real container involved, more complicated if you're working with a
virtual container. In particular, watch out for what happens when you
increment past the last valid item -- this needs to produce an
iterator that will compare equal to container.end(), and making this
work is often nontrivial for virtual containers.

The post-increment function is just boilerplate again (and
incidentally makes it obvious why all the experts recommend using
pre-increment wherever possible).

  bool operator==(container::const_iterator lhs,
                  container::const_iterator rhs) {
    // equality comparison goes here
  }

  bool operator!=(container::const_iterator lhs,
                  container::const_iterator rhs) {
    return !(lhs == rhs);
  }

For a real container, the equality comparison will usually just
compare the underlying iterators (or pointers or indices or whatever).
The semantics of comparisons for virtual container iterators are often
tricky. Remember that iterator comparison only needs to be defined for
iterators into the same container, so you can often simplify things by
taking for granted that lhs and rhs both point into the same container
object. Again, the second function is just boilerplate.

It's a matter of taste whether iterator arguments are passed by value
or reference; I've shown tham passed by value to reduce clutter, but
if the iterator contains several data members, passing by reference
may be better.

That convers the const-iterator-only situation. When we need separate
const and mutable iterators, one small complication is added beyond
the simple addition of a second class.

  class container {
    public:
      typedef something_or_other value_type;
      class const_iterator;
      class iterator:
        public std::iterator {
          friend class container;
          friend class container::const_iterator;
          public:
            value_type& operator*() const;
            value_type* operator->() const;
            iterator& operator++();
            iterator operator++(int);
            friend bool operator==(iterator lhs, iterator rhs);
            friend bool operator!=(iterator lhs, iterator rhs);
          private:
            //...
        };
      class const_iterator:
        public std::iterator {
          friend class container;
          public:
            const_iterator();
            const_iterator(const iterator& i);
            const value_type& operator*() const;
            const value_type* operator->() const;
            const_iterator& operator++();
            const_iterator operator++(int);
            friend bool operator==(const_iterator lhs,
                                   const_iterator rhs);
            friend bool operator!=(const_iterator lhs,
                                   const_iterator rhs);
          private:
            //...
        };
      //...
  };

There needs to be a conversion from iterator to const_iterator (so
that mixed-type operations, such as comparison between an iterator and
a const_iterator, will work). This is done here by giving
const_iterator a conversion constructor from iterator (equivalently,
we could have given iterator an operator const_iterator()), which
requires const_iterator to be a friend of iterator, so it can copy its
data members. (It also requires the addition of an explicit default
constructor to const_iterator, since the existence of another
user-defined constructor inhibits the compiler-defined one.)

Bidirectional iterators add just two member functions to forward
iterators:

  class iterator:
    public std::iterator {
      public:
        //...
        iterator& operator--();
        iterator operator--(int);
        //...
    };

I won't detail the implementations, they're obvious variations on
operator++().

Random access iterators add several more member and friend functions:

  class iterator:
    public std::iterator {
      public:
        //...
        iterator& operator+=(difference_type rhs);
        iterator& operator-=(difference_type rhs);
        friend iterator operator+(iterator lhs, difference_type rhs);
        friend iterator operator+(difference_type lhs, iterator rhs);
        friend iterator operator-(iterator lhs, difference_type rhs);
        friend difference_type operator-(iterator lhs, iterator rhs);
        friend bool operator<(iterator lhs, iterator rhs);
        friend bool operator>(iterator lhs, iterator rhs);
        friend bool operator<=(iterator lhs, iterator rhs);
        friend bool operator>=(iterator lhs, iterator rhs);
        //...
    };

  container::iterator&
    container::iterator::operator+=(container::difference_type rhs) {
      // add rhs to iterator position
      return *this;
    }

  container::iterator&
    container::iterator::operator-=(container::difference_type rhs) {
      // subtract rhs from iterator position
      return *this;
    }

  container::iterator operator+(container::iterator lhs,
                                container::difference_type rhs) {
    return iterator(lhs) += rhs;
  }

  container::iterator operator+(container::difference_type lhs,
                                container::iterator rhs) {
    return iterator(rhs) += lhs;
  }

  container::iterator operator-(container::iterator lhs,
                                container::difference_type rhs) {
    return iterator(lhs) -= rhs;
  }

  container::difference_type operator-(container::iterator lhs,
                                       container::iterator rhs) {
    // calculate distance between iterators
  }

  bool operator<(container::iterator lhs, container::iterator rhs) {
    // perform less-than comparison
  }

  bool operator>(container::iterator lhs, container::iterator rhs) {
    return rhs < lhs;
  }

  bool operator<=(container::iterator lhs, container::iterator rhs) {
    return !(rhs < lhs);
  }

  bool operator>=(container::iterator lhs, container::iterator rhs) {
    return !(lhs < rhs);
  }

Four of the functions (operator+=(), operator-=(), the second
operator-(), and operator<()) are nontrivial; the rest are
boilerplate.

One feature of the above code that some experts may disapprove of is
the declaration of all the free functions as friends, when in fact
only a few of them need direct access to the iterator's private data.
I originally got into the habit of doing this simply to keep the
declarations together; declaring some functions inside the class and
some outside seemed awkward. Since then, though, I've been told that
there's a subtle difference in the way name lookup works for functions
declared inside a class (as friends) and outside, so keeping them
together in the class is probably a good idea for practical as well as
aesthetic reasons.

I hope all this is some help to anyone who needs to write their own
STL-like containers and iterators.

-- 
Ross Smith  The Internet Group, Auckland, New Zealand

A lot of these comments and recommendations have been culled for other sources. Two particularly important books for our work are:
1. Effective C++ by Scott Meyers. There is an online version of the book (only some chapters though) available as well.
2. Large-Scale C++ Software Design by John Lakos

If you get some free time, and you haven't read them: do so, you might learn something. :)