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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
<html><head><title>A Few Coding Standards</title></head> "http://www.w3.org/TR/html4/strict.dtd">
<body bgcolor=white> <html>
<head>
<link rel="stylesheet" href="llvm.css" type="text/css">
<title>A Few Coding Standards</title>
</head>
<body>
<table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0> <div class="doc_title">
<tr><td>&nbsp; <font size=+5 color="#EEEEFF" face="Georgia,Palatino,Times,Roman"><b>A Few Coding Standards</b></font></td> A Few Coding Standards
</tr></table> </div>
<ol> <ol>
<li><a href="#introduction">Introduction</a> <li><a href="#introduction">Introduction</a></li>
<li><a href="#mechanicalissues">Mechanical Source Issues</a> <li><a href="#mechanicalissues">Mechanical Source Issues</a>
<ol> <ol>
<li><a href="#sourceformating">Source Code Formatting</a> <li><a href="#sourceformating">Source Code Formatting</a>
<ol> <ol>
<li><a href="#scf_commenting">Commenting</a> <li><a href="#scf_commenting">Commenting</a></li>
<li><a href="#scf_commentformat">Comment Formatting</a> <li><a href="#scf_commentformat">Comment Formatting</a></li>
<li><a href="#scf_includes">#include Style</a> <li><a href="#scf_includes">#include Style</a></li>
<li><a href="#scf_codewidth">Source Code Width</a> <li><a href="#scf_codewidth">Source Code Width</a></li>
<li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a> <li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a></li>
<li><a href="#scf_indentation">Indent Code Consistently</a> <li><a href="#scf_indentation">Indent Code Consistently</a></li>
</ol> </ol></li>
<li><a href="#compilerissues">Compiler Issues</a> <li><a href="#compilerissues">Compiler Issues</a>
<ol> <ol>
<li><a href="#ci_warningerrors">Treat Compiler Warnings Like Errors</a> <li><a href="#ci_warningerrors">Treat Compiler Warnings Like
<li><a href="#ci_cpp_features">Which C++ features can I use?</a> Errors</a></li>
<li><a href="#ci_portable_code">Write Portable Code</a> <li><a href="#ci_cpp_features">Which C++ features can I use?</a></li>
</ol> <li><a href="#ci_portable_code">Write Portable Code</a></li>
</ol> </ol></li>
</ol></li>
<li><a href="#styleissues">Style Issues</a> <li><a href="#styleissues">Style Issues</a>
<ol> <ol>
<li><a href="#macro">The High Level Issues</a> <li><a href="#macro">The High Level Issues</a>
<ol> <ol>
<li><a href="#hl_module">A Public Header File <b>is</b> a Module</a> <li><a href="#hl_module">A Public Header File <b>is</b> a
<li><a href="#hl_dontinclude">#include as Little as Possible</a> Module</a></li>
<li><a href="#hl_privateheaders">Keep "internal" Headers Private</a> <li><a href="#hl_dontinclude">#include as Little as Possible</a></li>
</ol> <li><a href="#hl_privateheaders">Keep "internal" Headers
Private</a></li>
</ol></li>
<li><a href="#micro">The Low Level Issues</a> <li><a href="#micro">The Low Level Issues</a>
<ol> <ol>
<li><a href="#hl_assert">Assert Liberally</a> <li><a href="#hl_assert">Assert Liberally</a></li>
<li><a href="#hl_preincrement">Prefer Preincrement</a> <li><a href="#hl_preincrement">Prefer Preincrement</a></li>
<li><a href="#hl_avoidendl">Avoid endl</a> <li><a href="#hl_avoidendl">Avoid endl</a></li>
<li><a href="#hl_exploitcpp">Exploit C++ to its Fullest</a> <li><a href="#hl_exploitcpp">Exploit C++ to its Fullest</a></li>
</ol> </ol></li>
<li><a href="#iterators">Writing Iterators</a> <li><a href="#iterators">Writing Iterators</a></li>
</ol> </ol></li>
<li><a href="#seealso">See Also</a> <li><a href="#seealso">See Also</a></li>
</ol><p> </ol>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0><tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b> <div class="doc_section">
<a name="introduction">Introduction <a name="introduction">Introduction</a>
</b></font></td></tr></table><ul> </div>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
This document attempts to describe a few coding standards that are being used in <div class="doc_text">
the LLVM source tree. Although no coding standards should be regarded as
<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 absolute requirements to be followed in all instances, coding standards can be
useful.<p> useful.</p>
This document intentionally does not prescribe fixed standards for religious <p>This document intentionally does not prescribe fixed standards for religious
issues such as brace placement and space usage. For issues like this, follow issues such as brace placement and space usage. For issues like this, follow
the golden rule: the golden rule:</p>
<a name="goldenrule"> <blockquote>
<blockquote><b>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.</b></blockquote>
The ultimate goal of these guidelines is the increase readability and <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 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> be included, please mail them to <a
href="mailto:sabre@nondot.org">Chris</a>.</p>
</div>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0><tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b> <div class="doc_section">
<a name="mechanicalissues">Mechanical Source Issues <a name="mechanicalissues">Mechanical Source Issues</a>
</b></font></td></tr></table><ul> </div>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
<!-- ======================================================================= --> <!-- ======================================================================= -->
</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0><tr><td>&nbsp;</td><td width="100%">&nbsp; <font color="#EEEEFF" face="Georgia,Palatino"><b> <div class="doc_subsection">
<a name="sourceformating">Source Code Formatting <a name="sourceformating">Source Code Formatting</a>
</b></font></td></tr></table><ul> </div>
<!-- _______________________________________________________________________ --> <!-- _______________________________________________________________________ -->
</ul><a name="scf_commenting"><h4><hr size=0>Commenting</h4><ul> <div class="doc_subsubsection">
<a name="scf_commenting">Commenting</a>
</div>
Comments are one critical part of readability and maintainability. Everyone <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 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 comment our code more than we do, there are a few very critical places that
documentation is very useful:<p> documentation is very useful:</p>
<ol> <ol>
<h4><li>File Headers</h4> <li><h4>File Headers</h4>
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. <p>Every source file should have a header on it that
Most source trees will probably have a standard file header format. The describes the basic purpose of the file. If a file does not have a header, it
standard format for the LLVM source tree looks like this:<p> 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> <pre>
//===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===// //===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
@ -109,399 +130,487 @@ standard format for the LLVM source tree looks like this:<p>
//===----------------------------------------------------------------------===// //===----------------------------------------------------------------------===//
</pre> </pre>
A few things to note about this particular format. The "<tt>-*- C++ -*-</tt>" <p>A few things to note about this particular format. The "<tt>-*- C++
string on the first line is there to tell Emacs that the source file is a C++ -*-</tt>" string on the first line is there to tell Emacs that the source file
file, not a C file (Emacs assumes .h files are C files by default [Note that tag is a C++ file, not a C file (Emacs assumes .h files are C files by default [Note
this is not necessary in .cpp files]). The name of the file is also on the that tag this is not necessary in .cpp files]). The name of the file is also on
first line, along with a very short description of the purpose of the file. 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> This is important when printing out code and flipping though lots of pages.</p>
The main body of the description does not have to be very long in most cases. <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 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 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> included, as well as any notes or "gotchas" in the code to watch out for.</p>
</li>
<h4><li>Class overviews</h4> <li><h4>Class overviews</h4>
Classes are one fundemental part of a good object oriented design. As such, a <p>Classes are one fundemental part of a good object oriented design. As such,
class definition should have a comment block that explains what the class is 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 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 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> something sane goes a long ways towards avoiding writing documentation. :)</p>
</li>
<h4><li>Method information</h4> <li><h4>Method information</h4>
Methods defined in a class (as well as any global functions) should also be <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 documented properly. A quick note about what it does any a description of the
borderline behaviour is all that is necessary here (unless something borderline behaviour is all that is necessary here (unless something
particularly tricky or insideous is going on). The hope is that people can 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 figure out how to use your interfaces without reading the code itself... that is
the goal metric.<p> the goal metric.</p>
Good things to talk about here are what happens when something unexpected <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> happens: does the method return null? Abort? Format your hard disk?</p>
</li>
</ol> </ol>
</div>
<!-- _______________________________________________________________________ --> <!-- _______________________________________________________________________ -->
</ul><a name="scf_commentformat"><h4><hr size=0>Comment Formatting</h4><ul> <div class="doc_subsubsection">
<a name="scf_commentformat">Comment Formatting</a>
</div>
In general, prefer C++ style (<tt>//</tt>) comments. They take less space, <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 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> when it is useful to use C style (<tt>/* */</tt>) comments however:</p>
<ol> <ol>
<li>When writing a C code: Obviously if you are writing C code, use C style <li>When writing a C code: Obviously if you are writing C code, use C style
comments. :) comments. :)</li>
<li>When writing a header file that may be #included by a C source file. <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 <li>When writing a source file that is used by a tool that only accepts C
comments. style comments.</li>
</ol><p> </ol>
To comment out a large block of code, use <tt>#if 0</tt> and <tt>#endif</tt>. <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> These nest properly and are better behaved in general than C style comments.</p>
</div>
<!-- _______________________________________________________________________ --> <!-- _______________________________________________________________________ -->
</ul><a name="scf_includes"><h4><hr size=0>#include Style</h4><ul> <div class="doc_subsubsection">
<a name="scf_includes">#include Style</a>
</div>
Immediately after the <a href="#scf_commenting">header file comment</a> (and <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 include guards if working on a header file), the <a
href="hl_dontinclude">minimal</a> list of #includes required by the file should 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> be listed. We prefer these #includes to be listed in this order:</p>
<ol> <ol>
<li><a href="#mmheader">Main Module header</a> <li><a href="#mmheader">Main Module header</a></li>
<li><a href="#hl_privateheaders">Local/Private Headers</a> <li><a href="#hl_privateheaders">Local/Private Headers</a></li>
<li>llvm/* <li>llvm/*</li>
<li>llvm/Analysis/* <li>llvm/Analysis/*</li>
<li>llvm/Assembly/* <li>llvm/Assembly/*</li>
<li>llvm/Bytecode/* <li>llvm/Bytecode/*</li>
<li>llvm/CodeGen/* <li>llvm/CodeGen/*</li>
<li>... <li>...</li>
<li>Support/* <li>Support/*</li>
<li>Config/* <li>Config/*</li>
<li>System #includes <li>System #includes</li>
</ol> </ol>
... and each catagory should be sorted by name.<p> <p>... and each catagory should be sorted by name.</p>
<a name="mmheader">The "Main Module Header" file applies to .cpp file which <p><a name="mmheader">The "Main Module Header"</a> file applies to .cpp file
implement an interface defined by a .h file. This #include should always be which implement an interface defined by a .h file. This #include should always
included <b>first</b> regardless of where it lives on the file system. By 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, 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 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 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 documentation in the .cpp file to indicate where the interfaces it implements
are defined.<p> are defined.</p>
</div>
<!-- _______________________________________________________________________ --> <!-- _______________________________________________________________________ -->
</ul><a name="scf_codewidth"><h4><hr size=0>Source Code Width</h4><ul> <div class="doc_subsubsection">
<a name="scf_codewidth">Source Code Width</a>
</div>
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. <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>
<!-- _______________________________________________________________________ --> <!-- _______________________________________________________________________ -->
</ul><a name="scf_spacestabs"><h4><hr size=0>Use Spaces Instead of Tabs</h4><ul> <div class="doc_subsubsection">
<a name="scf_spacestabs">Use Spaces Instead of Tabs</a>
</div>
In all cases, prefer spaces to tabs in source files. People have different <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 prefered indentation levels, and different styles of indentation that they
like... this is fine. What isn't is that different editors/viewers expand tabs 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 out to different tab stops. This can cause your code to look completely
unreadable, and it is not worth dealing with.<p> unreadable, and it is not worth dealing with.</p>
As always, follow the <a href="#goldenrule">Golden Rule</a> above: follow the <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 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 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 with two spaces of indentation. Also, do not reindent a whole source file: it
makes for incredible diffs that are absolutely worthless.<p> makes for incredible diffs that are absolutely worthless.</p>
</div>
<!-- _______________________________________________________________________ --> <!-- _______________________________________________________________________ -->
</ul><a name="scf_indentation"><h4><hr size=0>Indent Code Consistently</h4><ul> <div class="doc_subsubsection">
<a name="scf_indentation">Indent Code Consistently</a>
</div>
Okay, your first year of programming you were told that indentation is <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. important. If you didn't believe and internalize this then, now is the time.
Just do it.<p> Just do it.</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0><tr><td>&nbsp;</td><td width="100%">&nbsp; <font color="#EEEEFF" face="Georgia,Palatino"><b> <div class="doc_subsection">
<a name="compilerissues">Compiler Issues <a name="compilerissues">Compiler Issues</a>
</b></font></td></tr></table><ul> </div>
<!-- _______________________________________________________________________ --> <!-- _______________________________________________________________________ -->
</ul><a name="ci_warningerrors"><h4><hr size=0>Treat Compiler Warnings Like Errors</h4><ul> <div class="doc_subsubsection">
<a name="ci_warningerrors">Treat Compiler Warnings Like Errors</a>
</div>
If your code has compiler warnings in it, something is wrong: you aren't casting <div class="doc_text">
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>
It is not possible to prevent all warnings from all compilers, nor is it <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 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 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 <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 syntax of the code slightly. For example, an warning that annoys me occurs when
I write code like this:<p> I write code like this:</p>
<pre> <pre>
if (V = getValue()) { if (V = getValue()) {
.. ..
} }
</pre><p> </pre>
<tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt> operator, <p><tt>gcc</tt> will warn me that I probably want to use the <tt>==</tt>
and that I probably mistyped it. In most cases, I haven't, and I really don't operator, and that I probably mistyped it. In most cases, I haven't, and I
want the spurious errors. To fix this particular problem, I rewrite the code really don't want the spurious errors. To fix this particular problem, I
like this:<p> rewrite the code like this:</p>
<pre> <pre>
if ((V = getValue())) { if ((V = getValue())) {
.. ..
} }
</pre><p> </pre>
...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can be <p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
fixed by massaging the code appropriately.<p> be fixed by massaging the code appropriately.</p>
These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall -Winline <p>These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall
-W -Wwrite-strings -Wno-unused</tt><p> -Winline -W -Wwrite-strings -Wno-unused</tt></p>
</div>
<!-- _______________________________________________________________________ --> <!-- _______________________________________________________________________ -->
</ul><a name="ci_cpp_features"><h4><hr size=0>Which C++ features can I use?</h4><ul> <div class="doc_subsubsection">
<a name="ci_cpp_features">Which C++ features can I use?</a>
</div>
Compilers are finally catching up to the C++ standard. Most compilers implement <div class="doc_text">
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> <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> <ol>
<li>Exceptions: Exceptions are very useful for error reporting and handling <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 exceptional conditions. I do not use them in LLVM because they do have an
associated performance impact (by restricting restructuring of code), and parts associated performance impact (by restricting restructuring of code), and parts
of LLVM are designed for performance critical purposes.<p> of LLVM are designed for performance critical purposes.</p>
Just like most of the rules in this document, this isn't a hard and fast <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 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 reporting <b>significantly</b>, and the LLVM parser is not at all in the
critical path.<p> critical path.</p>
</li>
<li>RTTI: RTTI has a large cost in terms of executable size, and compilers are <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 not yet very good at stomping out "dead" class information blocks. Because of
this, typeinfo and dynamic cast are not used. this, typeinfo and dynamic cast are not used.</li>
</ol><p> </ol>
Other features, such as templates (without partial specialization) can be used <p>Other features, such as templates (without partial specialization) can be
freely. The general goal is to have clear, consise, performant code... if a used freely. The general goal is to have clear, consise, performant code... if
technique assists with that then use it.<p> a technique assists with that then use it.</p>
</div>
<!-- _______________________________________________________________________ --> <!-- _______________________________________________________________________ -->
</ul><a name="ci_portable_code"><h4><hr size=0>Write Portable Code</h4><ul> <div class="doc_subsubsection">
<a name="ci_portable_code">Write Portable Code</a>
</div>
In almost all cases, it is possible and within reason to write completely <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 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> code, isolate it behind a well defined (and well documented) interface.</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>
<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>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0><tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b> <div class="doc_section">
<a name="styleissues">Style Issues <a name="styleissues">Style Issues</a>
</b></font></td></tr></table><ul> </div>
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</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0><tr><td>&nbsp;</td><td width="100%">&nbsp; <font color="#EEEEFF" face="Georgia,Palatino"><b> <div class="doc_subsection">
<a name="macro">The High Level Issues <a name="macro">The High Level Issues</a>
</b></font></td></tr></table><ul> </div>
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</ul><a name="hl_module"><h4><hr size=0>A Public Header File <b>is</b> a Module</h4><ul> <div class="doc_subsubsection">
<a name="hl_module">A Public Header File <b>is</b> a Module</a>
</div>
C++ doesn't do too well in the modularity department. There is no real <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 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 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 source tree, they live in the top level "include" directory), you are defining a
module of functionality.<p> module of functionality.</p>
Ideally, modules should be completely independent of each other, and their <p>Ideally, modules should be completely independent of each other, and their
header files should only include the absolute minimum number of headers 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 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 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 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 functions, classes or data structures, but the important issue is how they work
together.<p> together.</p>
In general, a module should be implemented with one or more <tt>.cpp</tt> files. <p>In general, a module should be implemented with one or more <tt>.cpp</tt>
Each of these <tt>.cpp</tt> files should include the header that defines their files. Each of these <tt>.cpp</tt> files should include the header that defines
interface first. This ensure that all of the dependences of the module header their interface first. This ensure that all of the dependences of the module
have been properly added to the module header itself, and are not implicit. header have been properly added to the module header itself, and are not
System headers should be included after user headers for a translation unit.<p> implicit. System headers should be included after user headers for a
translation unit.</p>
</div>
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</ul><a name="hl_dontinclude"><h4><hr size=0>#include as Little as Possible</h4><ul> <div class="doc_subsubsection">
<a name="hl_dontinclude">#include as Little as Possible</a>
</div>
<tt>#include</tt> hurts compile time performance. Don't do it unless you have <div class="doc_text">
to, especially in header files.<p>
But wait, sometimes you need to have the definition of a class to use it, or to <p><tt>#include</tt> hurts compile time performance. Don't do it unless you
inherit from it. In these cases go ahead and #include that header file. Be 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 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 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 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 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 simply don't need the definition of a class... and not <tt>#include</tt>'ing
speeds up compilation.<p> speeds up compilation.</p>
It is easy to try to go too overboard on this recommendation, however. You <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 <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 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 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 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 above). This way there won't be any hidden dependencies that you'll find out
about later...<p> about later...</p>
</div>
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</ul><a name="hl_privateheaders"><h4><hr size=0>Keep "internal" Headers Private</h4><ul> <div class="doc_subsubsection">
<a name="hl_privateheaders">Keep "internal" Headers Private</a>
</div>
Many modules have a complex implementation that causes them to use more than one <div class="doc_text">
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>
If you really need to do something like this, put a private header file in the <p>Many modules have a complex implementation that causes them to use more than
same directory as the source files, and include it locally. This ensures that one implementation (<tt>.cpp</tt>) file. It is often tempting to put the
your private interface remains private and undisturbed by outsiders.<p> internal communication interface (helper classes, extra functions, etc) in the
public module header file. Don't do this. :)</p>
Note however, that it's okay to put extra implementation methods a public class <p>If you really need to do something like this, put a private header file in
itself... just make them private (or protected), and all is well.<p> 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>
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</ul><table width="100%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0><tr><td>&nbsp;</td><td width="100%">&nbsp; <font color="#EEEEFF" face="Georgia,Palatino"><b> <div class="doc_text">
<a name="micro">The Low Level Issues <a name="micro">The Low Level Issues</a>
</b></font></td></tr></table><ul> </div>
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</ul><a name="hl_assert"><h4><hr size=0>Assert Liberally</h4><ul> <div class="doc_subsubsection">
<a name="hl_assert">Assert Liberally</a>
</div>
Use the "<tt>assert</tt>" function to its fullest. Check all of your <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 preconditions and assumptions, you never know when a bug (not neccesarily even
yours) might be caught early by an assertion, which reduces debugging time yours) might be caught early by an assertion, which reduces debugging time
dramatically. The "<tt>&lt;cassert&gt;</tt>" header file is probably already 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 included by the header files you are using, so it doesn't cost anything to use
it.<p> it.</p>
To further assist with debugging, make sure to put some kind of error message in <p>To further assist with debugging, make sure to put some kind of error message
the assertion statement (which is printed if the assertion is tripped). This 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 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> enforced, and hopefully what to do about it. Here is one complete example:</p>
<pre> <pre>
inline Value *getOperand(unsigned i) { inline Value *getOperand(unsigned i) {
assert(i &lt; Operands.size() && "getOperand() out of range!"); assert(i &lt; Operands.size() &amp;&amp; "getOperand() out of range!");
return Operands[i]; return Operands[i];
} }
</pre> </pre>
Here are some examples: <p>Here are some examples:</p>
<pre> <pre>
assert(Ty-&gt;isPointerType() && "Can't allocate a non pointer type!"); assert(Ty-&gt;isPointerType() &amp;&amp; "Can't allocate a non pointer type!");
assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!"); assert((Opcode == Shl || Opcode == Shr) &amp;&amp; "ShiftInst Opcode invalid!");
assert(idx &lt; getNumSuccessors() && "Successor # out of range!"); assert(idx &lt; getNumSuccessors() &amp;&amp; "Successor # out of range!");
assert(V1.getType() == V2.getType() && "Constant types must be identical!"); assert(V1.getType() == V2.getType() &amp;&amp; "Constant types must be identical!");
assert(isa&lt;PHINode&gt;(Succ-&gt;front()) && "Only works on PHId BBs!"); assert(isa&lt;PHINode&gt;(Succ-&gt;front()) &amp;&amp; "Only works on PHId BBs!");
</pre><p> </pre>
You get the idea...<p> <p>You get the idea...</p>
</div>
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</ul><a name="hl_preincrement"><h4><hr size=0>Prefer Preincrement</h4><ul> <div class="doc_subsubsection">
<a name="hl_preincrement">Prefer Preincrement</a>
</div>
Hard fast rule: Preincrement (++X) may be no slower than postincrement (X++) and <div class="doc_text">
could very well be a lot faster than it. Use preincrementation whenever
possible.<p>
The semantics of postincrement include making a copy of the value being <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 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 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... 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, 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> get in the habit of always using preincrement, and you won't have a problem.</p>
</div>
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</ul><a name="hl_avoidendl"><h4><hr size=0>Avoid endl</h4><ul> <div class="doc_subsubsection">
<a name="hl_avoidendl">Avoid endl</a>
</div>
The <tt>endl</tt> modifier, when used with iostreams outputs a newline to the <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 output stream specified. In addition to doing this, however, it also flushes
the output stream. In other words, these are equivalent:<p> the output stream. In other words, these are equivalent:</p>
<pre> <pre>
cout << endl; cout &lt;&lt; endl;
cout << "\n" << flush; cout &lt;&lt; "\n" &lt;&lt; flush;
</pre> </pre>
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> <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>
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</ul><a name="hl_exploitcpp"><h4><hr size=0>Exploit C++ to its Fullest</h4><ul> <div class="doc_subsubsection">
<a name="hl_exploitcpp">Exploit C++ to its Fullest</a>
</div>
C++ is a powerful language. With a firm grasp on its capabilities, you can make <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. 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 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 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> to avoid documentation, and avoid giving bugs a place to hide.</p>
For these reasons, come to know and love the contents of your local <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 &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> for you. C++ is just a tool that wants you to master it. :)</p>
</div>
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<a name="iterators">Writing Iterators <a name="iterators">Writing Iterators</a>
</b></font></td></tr></table><ul> </div>
Here's a pretty good summary of how to write your own data structure iterators <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 in a way that is compatible with the STL, and with a lot of other code out there
(slightly edited by Chris):<p> (slightly edited by Chris):</p>
<pre> <pre>
From: Ross Smith <ross.s@ihug.co.nz> From: Ross Smith &lt;ross.s@ihug.co.nz&gt;
Newsgroups: comp.lang.c++.moderated Newsgroups: comp.lang.c++.moderated
Subject: Writing iterators (was: Re: Non-template functions that take iterators) Subject: Writing iterators (was: Re: Non-template functions that take iterators)
Date: 28 Jun 2001 12:07:10 -0400 Date: 28 Jun 2001 12:07:10 -0400
Andre Majorel wrote: Andre Majorel wrote:
> Any pointers handy on "writing STL-compatible iterators for &gt; Any pointers handy on "writing STL-compatible iterators for
> dummies ?" &gt; dummies ?"
I'll give it a try... I'll give it a try...
@ -550,7 +659,7 @@ you actually implement it. Typically it looks something like this
friend class container; friend class container;
public: public:
const value_type&amp; operator*() const; const value_type&amp; operator*() const;
const value_type* operator->() const; const value_type* operator-&gt;() const;
const_iterator&amp; operator++(); const_iterator&amp; operator++();
const_iterator operator++(int); const_iterator operator++(int);
friend bool operator==(const_iterator lhs, friend bool operator==(const_iterator lhs,
@ -585,7 +694,7 @@ implementations; the rest are just boilerplate.
} }
const container::value_type* const container::value_type*
container::const_iterator::operator->() const { container::const_iterator::operator-&gt;() const {
return &amp;**this; return &amp;**this;
} }
@ -599,7 +708,7 @@ increment operator (below), or if the operation is nontrivial, you
might choose the "lazy" approach and only generate the actual value might choose the "lazy" approach and only generate the actual value
when one of the dereferencing operators is called. when one of the dereferencing operators is called.
The operator->() function is just boilerplate around a call to The operator-&gt;() function is just boilerplate around a call to
operator*(). operator*().
container::const_iterator&amp; container::const_iterator&amp;
@ -663,7 +772,7 @@ the simple addition of a second class.
friend class container::const_iterator; friend class container::const_iterator;
public: public:
value_type&amp; operator*() const; value_type&amp; operator*() const;
value_type* operator->() const; value_type* operator-&gt;() const;
iterator&amp; operator++(); iterator&amp; operator++();
iterator operator++(int); iterator operator++(int);
friend bool operator==(iterator lhs, iterator rhs); friend bool operator==(iterator lhs, iterator rhs);
@ -678,7 +787,7 @@ the simple addition of a second class.
const_iterator(); const_iterator();
const_iterator(const iterator&amp; i); const_iterator(const iterator&amp; i);
const value_type&amp; operator*() const; const value_type&amp; operator*() const;
const value_type* operator->() const; const value_type* operator-&gt;() const;
const_iterator&amp; operator++(); const_iterator&amp; operator++();
const_iterator operator++(int); const_iterator operator++(int);
friend bool operator==(const_iterator lhs, friend bool operator==(const_iterator lhs,
@ -805,35 +914,44 @@ STL-like containers and iterators.
Ross Smith &lt;ross.s@ihug.co.nz&gt; The Internet Group, Auckland, New Zealand Ross Smith &lt;ross.s@ihug.co.nz&gt; The Internet Group, Auckland, New Zealand
</pre> </pre>
</div>
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<a name="seealso">See Also <a name="seealso">See Also</a>
</b></font></td></tr></table><ul> </div>
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A lot of these comments and recommendations have been culled for other sources. <div class="doc_text">
Two particularly important books for our work are:<p>
<p>A lot of these comments and recommendations have been culled for other
sources. Two particularly important books for our work are:</p>
<ol> <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><a href="http://cseng.aw.com/book/0,3828,0201633620,00.html">Large-Scale C++ Software Design</a> by John Lakos
</ol><p>
If you get some free time, and you haven't read them: do so, you might learn <li><a href="http://www.aw.com/product/0,2627,0201924889,00.html">Effective
something. :) 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>
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</ul>
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<hr> <hr>
<font size=-1>
<address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address> <div class="doc_footer">
<!-- Created: Tue Jan 23 15:19:28 CST 2001 --> <address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
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