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              <title>A Few Coding Standards</title>

            
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            <div class="doc_title">

            
              A Few Coding Standards

            
            </div>


            <ol>

            
              <li><a href="#introduction">Introduction</a></li>

            
              <li><a href="#mechanicalissues">Mechanical Source Issues</a>

            
                <ol>

            
                  <li><a href="#sourceformating">Source Code Formatting</a>

            
                    <ol>

            
                      <li><a href="#scf_commenting">Commenting</a></li>

            
                      <li><a href="#scf_commentformat">Comment Formatting</a></li>

            
                      <li><a href="#scf_includes">#include Style</a></li>

            
                      <li><a href="#scf_codewidth">Source Code Width</a></li>

            
                      <li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a></li>

            
                      <li><a href="#scf_indentation">Indent Code Consistently</a></li>

            
                    </ol></li>

            
                  <li><a href="#compilerissues">Compiler Issues</a>

            
                    <ol>

            
                      <li><a href="#ci_warningerrors">Treat Compiler Warnings Like

            
                          Errors</a></li>

            
                      <li><a href="#ci_cpp_features">Which C++ features can I use?</a></li>

            
                      <li><a href="#ci_portable_code">Write Portable Code</a></li>

            
                    </ol></li>

            
                </ol></li>

            
              <li><a href="#styleissues">Style Issues</a>

            
                <ol>

            
                  <li><a href="#macro">The High Level Issues</a>

            
                    <ol>

            
                      <li><a href="#hl_module">A Public Header File <b>is</b> a

            
                          Module</a></li>

            
                      <li><a href="#hl_dontinclude">#include as Little as Possible</a></li>

            
                      <li><a href="#hl_privateheaders">Keep "internal" Headers

            
                          Private</a></li>

            
                    </ol></li>

            
                  <li><a href="#micro">The Low Level Issues</a>

            
                    <ol>

            
                      <li><a href="#hl_assert">Assert Liberally</a></li>

            
                      <li><a href="#hl_preincrement">Prefer Preincrement</a></li>

            
                      <li><a href="#hl_avoidendl">Avoid endl</a></li>

            
                      <li><a href="#hl_exploitcpp">Exploit C++ to its Fullest</a></li>

            
                    </ol></li>

            
                  <li><a href="#iterators">Writing Iterators</a></li>

            
                </ol></li>

            
              <li><a href="#seealso">See Also</a></li>

            
            </ol>


            <!-- *********************************************************************** -->

            
            <div class="doc_section">

            
              <a name="introduction">Introduction</a>

            
            </div>

            
            <!-- *********************************************************************** -->


            <div class="doc_text">


            <p>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.</p>


            <p>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:</p>


            <blockquote>


            <p><b><a name="goldenrule">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.</a></b></p>


            </blockquote>


            <p>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 <a

            
            href="mailto:sabre@nondot.org">Chris</a>.</p>


            </div>


            <!-- *********************************************************************** -->

            
            <div class="doc_section">

            
              <a name="mechanicalissues">Mechanical Source Issues</a>

            
            </div>

            
            <!-- *********************************************************************** -->


            <!-- ======================================================================= -->

            
            <div class="doc_subsection">

            
              <a name="sourceformating">Source Code Formatting</a>

            
            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="scf_commenting">Commenting</a>

            
            </div>


            <div class="doc_text">


            <p>Comments are one critical part of readability and maintainability.  Everyone

            
            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:</p>


            <ol>

            
            <li><h4>File Headers</h4>


            <p>Every source file should have a header on it that

            
            describes the basic purpose of the file.  If a file does not have a header, it

            
            should not be checked into CVS.  Most source trees will probably have a standard

            
            file header format.  The standard format for the LLVM source tree looks like

            
            this:</p>


            <pre>

            
            //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//

            
            //

            
            // This file contains the declaration of the Instruction class, which is the

            
            // base class for all of the VM instructions.

            
            //

            
            //===----------------------------------------------------------------------===//

            
            </pre>


            <p>A few things to note about this particular format.  The "<tt>-*- C++

            
            -*-</tt>" 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.</p>


            <p>The main body of the description does not have to be very long in most cases.

            
            Here it's only two lines.  If an algorithm is being implemented or something

            
            tricky is going on, a reference to the paper where it is published should be

            
            included, as well as any notes or "gotchas" in the code to watch out for.</p>


            </li>


            <li><h4>Class overviews</h4>


            <p>Classes are one fundemental 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. :)</p>


            </li>


            <li><h4>Method information</h4>


            <p>Methods defined in a class (as well as any global functions) should also be

            
            documented properly.  A quick note about what it does any a description of the

            
            borderline behaviour is all that is necessary here (unless something

            
            particularly tricky or insideous is going on).  The hope is that people can

            
            figure out how to use your interfaces without reading the code itself... that is

            
            the goal metric.</p>


            <p>Good things to talk about here are what happens when something unexpected

            
            happens: does the method return null?  Abort?  Format your hard disk?</p>


            </li>

            
            </ol>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="scf_commentformat">Comment Formatting</a>

            
            </div>


            <div class="doc_text">


            <p>In general, prefer C++ style (<tt>//</tt>) comments.  They take less space,

            
            require less typing, don't have nesting problems, etc.  There are a few cases

            
            when it is useful to use C style (<tt>/* */</tt>) comments however:</p>


            <ol>

            
              <li>When writing a C code: Obviously if you are writing C code, use C style

            
                  comments.  :)</li>

            
              <li>When writing a header file that may be #included by a C source file.</li>

            
              <li>When writing a source file that is used by a tool that only accepts C

            
                  style comments.</li>

            
            </ol>


            <p>To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>.

            
            These nest properly and are better behaved in general than C style comments.</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="scf_includes">#include Style</a>

            
            </div>


            <div class="doc_text">


            <p>Immediately after the <a href="#scf_commenting">header file comment</a> (and

            
            include guards if working on a header file), the <a

            
            href="hl_dontinclude">minimal</a> list of #includes required by the file should

            
            be listed.  We prefer these #includes to be listed in this order:</p>


            <ol>

            
              <li><a href="#mmheader">Main Module header</a></li>

            
              <li><a href="#hl_privateheaders">Local/Private Headers</a></li>

            
              <li>llvm/*</li>

            
              <li>llvm/Analysis/*</li>

            
              <li>llvm/Assembly/*</li>

            
              <li>llvm/Bytecode/*</li>

            
              <li>llvm/CodeGen/*</li>

            
              <li>...</li>

            
              <li>Support/*</li>

            
              <li>Config/*</li>

            
              <li>System #includes</li>

            
            </ol>


            <p>... and each catagory should be sorted by name.</p>


            <p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file

            
            which implement an interface defined by a .h file.  This #include should always

            
            be included <b>first</b> 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.</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="scf_codewidth">Source Code Width</a>

            
            </div>


            <div class="doc_text">


            <p>Write your code to fit within 80 columns of text.  This helps those of us who

            
            like to print out code and look at your code in an xterm without resizing

            
            it.</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="scf_spacestabs">Use Spaces Instead of Tabs</a>

            
            </div>


            <div class="doc_text">


            <p>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.</p>


            <p>As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the

            
            style of existing code if your are modifying and extending it.  If you like four

            
            spaces of indentation, <b>DO NOT</b> do that in the middle of a chunk of code

            
            with two spaces of indentation.  Also, do not reindent a whole source file: it

            
            makes for incredible diffs that are absolutely worthless.</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="scf_indentation">Indent Code Consistently</a>

            
            </div>


            <div class="doc_text">


            <p>Okay, your first year of programming you were told that indentation is

            
            important.  If you didn't believe and internalize this then, now is the time.

            
            Just do it.</p>


            </div>


            <!-- ======================================================================= -->

            
            <div class="doc_subsection">

            
              <a name="compilerissues">Compiler Issues</a>

            
            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>

            
            </div>


            <div class="doc_text">


            <p>If your code has compiler warnings in it, something is wrong: you aren't

            
            casting values correctly, your have "questionable" constructs in your code, or

            
            you are doing something legitimately wrong.  Compiler warnings can cover up

            
            legitimate errors in output and make dealing with a translation unit

            
            difficult.</p>


            <p>It is not possible to prevent all warnings from all compilers, nor is it

            
            desirable.  Instead, pick a standard compiler (like <tt>gcc</tt>) that provides

            
            a good thorough set of warnings, and stick to them.  At least in the case of

            
            <tt>gcc</tt>, it is possible to work around any spurious errors by changing the

            
            syntax of the code slightly.  For example, an warning that annoys me occurs when

            
            I write code like this:</p>


            <pre>

            
              if (V = getValue()) {

            
                ..

            
              }

            
            </pre>


            <p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>

            
            operator, and that I probably mistyped it.  In most cases, I haven't, and I

            
            really don't want the spurious errors.  To fix this particular problem, I

            
            rewrite the code like this:</p>


            <pre>

            
              if ((V = getValue())) {

            
                ..

            
              }

            
            </pre>


            <p>...which shuts <tt>gcc</tt> up.  Any <tt>gcc</tt> warning that annoys you can

            
            be fixed by massaging the code appropriately.</p>


            <p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall

            
            -Winline -W -Wwrite-strings -Wno-unused</tt></p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="ci_cpp_features">Which C++ features can I use?</a>

            
            </div>


            <div class="doc_text">


            <p>Compilers are finally catching up to the C++ standard.  Most compilers

            
            implement most features, so you can use just about any features that you would

            
            like.  In the LLVM source tree, I have chosen to not use these features:</p>


            <ol>

            
            <li><p>Exceptions: Exceptions are very useful for error reporting and handling

            
            exceptional conditions.  I do not use them in LLVM because they do have an

            
            associated performance impact (by restricting restructuring of code), and parts

            
            of LLVM are designed for performance critical purposes.</p>


            <p>Just like most of the rules in this document, this isn't a hard and fast

            
            requirement.  Exceptions are used in the Parser, because it simplifies error

            
            reporting <b>significantly</b>, and the LLVM parser is not at all in the

            
            critical path.</p>

            
            </li>


            <li>RTTI: RTTI has a large cost in terms of executable size, and compilers are

            
            not yet very good at stomping out "dead" class information blocks.  Because of

            
            this, typeinfo and dynamic cast are not used.</li>

            
            </ol>


            <p>Other features, such as templates (without partial specialization) can be

            
            used freely.  The general goal is to have clear, consise, performant code... if

            
            a technique assists with that then use it.</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="ci_portable_code">Write Portable Code</a>

            
            </div>


            <div class="doc_text">


            <p>In almost all cases, it is possible and within reason to write completely

            
            portable code.  If there are cases where it isn't possible to write portable

            
            code, isolate it behind a well defined (and well documented) interface.</p>


            <p>In practice, this means that you shouldn't assume much about the host

            
            compiler, including its support for "high tech" features like partial

            
            specialization of templates.  In fact, Visual C++ 6 could be an important target

            
            for our work in the future, and we don't want to have to rewrite all of our code

            
            to support it.</p>


            </div>


            <!-- *********************************************************************** -->

            
            <div class="doc_section">

            
              <a name="styleissues">Style Issues</a>

            
            </div>

            
            <!-- *********************************************************************** -->


            <!-- ======================================================================= -->

            
            <div class="doc_subsection">

            
              <a name="macro">The High Level Issues</a>

            
            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="hl_module">A Public Header File <b>is</b> a Module</a>

            
            </div>


            <div class="doc_text">


            <p>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.</p>


            <p>Ideally, modules should be completely independent of each other, and their

            
            header files should only include the absolute minimum number of headers

            
            possible. A module is not just a class, a function, or a namespace: <a

            
            href="http://www.cuj.com/articles/2000/0002/0002c/0002c.htm">it's a collection

            
            of these</a> that defines an interface.  This interface may be several

            
            functions, classes or data structures, but the important issue is how they work

            
            together.</p>


            <p>In general, a module should be implemented with one or more <tt>.cpp</tt>

            
            files.  Each of these <tt>.cpp</tt> 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.</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="hl_dontinclude">#include as Little as Possible</a>

            
            </div>


            <div class="doc_text">


            <p><tt>#include</tt> hurts compile time performance.  Don't do it unless you

            
            have to, especially in header files.</p>


            <p>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 <tt>#include</tt>'ing

            
            speeds up compilation.</p>


            <p>It is easy to try to go too overboard on this recommendation, however.  You

            
            <b>must</b> include all of the header files that you are using, either directly

            
            or indirectly (through another header file).  To make sure that you don't

            
            accidently forget to include a header file in your module header, make sure to

            
            include your module header <b>first</b> in the implementation file (as mentioned

            
            above).  This way there won't be any hidden dependencies that you'll find out

            
            about later...</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="hl_privateheaders">Keep "internal" Headers Private</a>

            
            </div>


            <div class="doc_text">


            <p>Many modules have a complex implementation that causes them to use more than

            
            one implementation (<tt>.cpp</tt>) 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.  :)</p>


            <p>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

            
            that your private interface remains private and undisturbed by outsiders.</p>


            <p>Note however, that it's okay to put extra implementation methods a public

            
            class itself... just make them private (or protected), and all is well.</p>


            </div>


            <!-- ======================================================================= -->

            
            <div class="doc_text">

            
              <a name="micro">The Low Level Issues</a>

            
            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="hl_assert">Assert Liberally</a>

            
            </div>


            <div class="doc_text">


            <p>Use the "<tt>assert</tt>" function to its fullest.  Check all of your

            
            preconditions and assumptions, you never know when a bug (not neccesarily even

            
            yours) might be caught early by an assertion, which reduces debugging time

            
            dramatically.  The "<tt>&lt;cassert&gt;</tt>" header file is probably already

            
            included by the header files you are using, so it doesn't cost anything to use

            
            it.</p>


            <p>To further assist with debugging, make sure to put some kind of error message

            
            in the assertion statement (which is printed if the assertion is tripped). This

            
            helps the poor debugging make sense of why an assertion is being made and

            
            enforced, and hopefully what to do about it.  Here is one complete example:</p>


            <pre>

            
              inline Value *getOperand(unsigned i) {

            
                assert(i &lt; Operands.size() &amp;&amp; "getOperand() out of range!");

            
                return Operands[i];

            
              }

            
            </pre>


            <p>Here are some examples:</p>


            <pre>

            
              assert(Ty-&gt;isPointerType() &amp;&amp; "Can't allocate a non pointer type!");


              assert((Opcode == Shl || Opcode == Shr) &amp;&amp; "ShiftInst Opcode invalid!");


              assert(idx &lt; getNumSuccessors() &amp;&amp; "Successor # out of range!");


              assert(V1.getType() == V2.getType() &amp;&amp; "Constant types must be identical!");


              assert(isa&lt;PHINode&gt;(Succ-&gt;front()) &amp;&amp; "Only works on PHId BBs!");

            
            </pre>


            <p>You get the idea...</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="hl_preincrement">Prefer Preincrement</a>

            
            </div>


            <div class="doc_text">


            <p>Hard fast rule: Preincrement (++X) may be no slower than postincrement (X++)

            
            and could very well be a lot faster than it.  Use preincrementation whenever

            
            possible.</p>


            <p>The semantics of postincrement include making a copy of the value being

            
            incremented, returning it, and then preincrementing the "work value".  For

            
            primitive types, this isn't a big deal... but for iterators, it can be a huge

            
            issue (for example, some iterators contains stack and set objects in them...

            
            copying an iterator could invoke the copy ctor's of these as well).  In general,

            
            get in the habit of always using preincrement, and you won't have a problem.</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="hl_avoidendl">Avoid endl</a>

            
            </div>


            <div class="doc_text">


            <p>The <tt>endl</tt> modifier, when used with iostreams outputs a newline to the

            
            output stream specified.  In addition to doing this, however, it also flushes

            
            the output stream.  In other words, these are equivalent:</p>


            <pre>

            
              cout &lt;&lt; endl;

            
              cout &lt;&lt; "\n" &lt;&lt; flush;

            
            </pre>


            <p>Most of the time, you probably have no reason to flush the output stream, so

            
            it's better to use a literal <tt>"\n"</tt>.</p>


            </div>


            <!-- _______________________________________________________________________ -->

            
            <div class="doc_subsubsection">

            
              <a name="hl_exploitcpp">Exploit C++ to its Fullest</a>

            
            </div>


            <div class="doc_text">


            <p>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.</p>


            <p>For these reasons, come to know and love the contents of your local

            
            &lt;algorithm&gt; header file.  Know about &lt;functional&gt; and what it can do

            
            for you.  C++ is just a tool that wants you to master it. :)</p>


            </div>


            <!-- ======================================================================= -->

            
            <div class="doc_subsection">

            
              <a name="iterators">Writing Iterators</a>

            
            </div>


            <div class="doc_text">


            <p>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):</p>


            <pre>

            
            From: Ross Smith &lt;ross.s@ihug.co.nz&gt;

            
            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:

            
            &gt; Any pointers handy on "writing STL-compatible iterators for

            
            &gt; 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

            
            <a href="http://www.sgi.com/tech/stl/">http://www.sgi.com/tech/stl/</a>), 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 &lt;iterator&gt;


              class container {

            
                public:

            
                  typedef something_or_other value_type;

            
                  class const_iterator:

            
                    public std::iterator&lt;std::forward_iterator_tag, value_type&gt; {

            
                      friend class container;

            
                      public:

            
                        const value_type&amp; operator*() const;

            
                        const value_type* operator-&gt;() const;

            
                        const_iterator&amp; 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.


            <i>[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.]</i>


            There are normally only three member functions that need nontrivial

            
            implementations; the rest are just boilerplate.


              const container::value_type&amp;

            
                container::const_iterator::operator*() const {

            
                  // find the element and return a reference to it

            
                }


              const container::value_type*

            
                container::const_iterator::operator-&gt;() const {

            
                  return &amp;**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-&gt;() function is just boilerplate around a call to

            
            operator*().


              container::const_iterator&amp;

            
                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&lt;std::forward_iterator_tag, value_type&gt; {

            
                      friend class container;

            
                      friend class container::const_iterator;

            
                      public:

            
                        value_type&amp; operator*() const;

            
                        value_type* operator-&gt;() const;

            
                        iterator&amp; 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&lt;std::forward_iterator_tag, value_type&gt; {

            
                      friend class container;

            
                      public:

            
                        const_iterator();

            
                        const_iterator(const iterator&amp; i);

            
                        const value_type&amp; operator*() const;

            
                        const value_type* operator-&gt;() const;

            
                        const_iterator&amp; 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&lt;std::bidirectional_iterator_tag, value_type&gt; {

            
                  public:

            
                    //...

            
                    iterator&amp; 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&lt;std::random_access_iterator_tag, value_type&gt; {

            
                  public:

            
                    //...

            
                    iterator&amp; operator+=(difference_type rhs);

            
                    iterator&amp; 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&lt;(iterator lhs, iterator rhs);

            
                    friend bool operator&gt;(iterator lhs, iterator rhs);

            
                    friend bool operator&lt;=(iterator lhs, iterator rhs);

            
                    friend bool operator&gt;=(iterator lhs, iterator rhs);

            
                    //...

            
                };


              container::iterator&amp;

            
                container::iterator::operator+=(container::difference_type rhs) {

            
                  // add rhs to iterator position

            
                  return *this;

            
                }


              container::iterator&amp;

            
                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&lt;(container::iterator lhs, container::iterator rhs) {

            
                // perform less-than comparison

            
              }


              bool operator&gt;(container::iterator lhs, container::iterator rhs) {

            
                return rhs &lt; lhs;

            
              }


              bool operator&lt;=(container::iterator lhs, container::iterator rhs) {

            
                return !(rhs &lt; lhs);

            
              }


              bool operator&gt;=(container::iterator lhs, container::iterator rhs) {

            
                return !(lhs &lt; rhs);

            
              }


            Four of the functions (operator+=(), operator-=(), the second

            
            operator-(), and operator&lt;()) 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 &lt;ross.s@ihug.co.nz&gt; The Internet Group, Auckland, New Zealand

            
            </pre>


            </div>


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            <div class="doc_section">

            
              <a name="seealso">See Also</a>

            
            </div>

            
            <!-- *********************************************************************** -->


            <div class="doc_text">


            <p>A lot of these comments and recommendations have been culled for other

            
            sources.  Two particularly important books for our work are:</p>


            <ol>


            <li><a href="http://www.aw.com/product/0,2627,0201924889,00.html">Effective

            
            C++</a> by Scott Meyers.  There is an online version of the book (only some

            
            chapters though) <a

            
            href="http://www.awlonline.com/cseng/meyerscddemo/">available as well</a>.</li>


            <li><a href="http://cseng.aw.com/book/0,3828,0201633620,00.html">Large-Scale C++

            
            Software Design</a> by John Lakos</li>


            </ol>


            <p>If you get some free time, and you haven't read them: do so, you might learn

            
            something. :)</p>


            </div>


            <!-- *********************************************************************** -->


            <hr>


            <div class="doc_footer">

            
              <address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>

            
              <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>

            
              <br>

            
              Last modified: $Date$

            
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