<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_includes"><tt>#include</tt> 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><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>
+ <li><a href="#ci_class_struct">Use of class/struct Keywords</a></li>
</ol></li>
</ol></li>
<li><a href="#styleissues">Style Issues</a>
<li><a href="#hl_dontinclude">#include as Little as Possible</a></li>
<li><a href="#hl_privateheaders">Keep "internal" Headers
Private</a></li>
+ <li><a href="#ll_iostream"><tt>#include <iostream></tt> is
+ <em>forbidden</em></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>
+ <li><a href="#ll_assert">Assert Liberally</a></li>
+ <li><a href="#ll_ns_std">Do not use 'using namespace std'</a></li>
+ <li><a href="#ll_virtual_anch">Provide a virtual method anchor for
+ classes in headers</a></li>
+ <li><a href="#ll_preincrement">Prefer Preincrement</a></li>
+ <li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></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_author">
+ <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
+ <a href="mailto:void@nondot.org">Bill Wendling</a></p>
+</div>
+
<!-- *********************************************************************** -->
<div class="doc_section">
<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
documentation is very useful:</p>
-<ol>
-<li><h4>File Headers</h4>
+<b>File Headers</b>
-<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
+<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 Subversion. Most source trees will probably have a standard
file header format. The standard format for the LLVM source tree looks like
this:</p>
+<div class="doc_code">
<pre>
//===-- llvm/Instruction.h - Instruction class definition -------*- C++ -*-===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file was developed by <whoever started the file> and is distributed under
+// the University of Illinois Open Source License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
//
// This file contains the declaration of the Instruction class, which is the
// base class for all of the VM instructions.
//
//===----------------------------------------------------------------------===//
</pre>
+</div>
-<p>A few things to note about this particular format. The "<tt>-*- C++
+<p>A few things to note about this particular format: The 'developed by' line
+should be the name of the person or organization who initially contributed the
+file. 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>
+is a C++ file, not a C file (Emacs assumes .h files are C files by default).
+Note that this tag is not necessary in .cpp files. The name of the file is also
+on the first line, along with a very short description of the purpose of the
+file. This is important when printing out code and flipping though lots of
+pages.</p>
+
+<p>The next section in the file is a concise note that defines the license that
+the file is released under. This makes it perfectly clear what terms the source
+code can be distributed under.</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>
+<b>Class overviews</b>
-<p>Classes are one fundemental part of a good object oriented design. As such,
+<p>Classes are one fundamental part of a good object oriented design. As such,
a class definition should have a comment block that explains what the class is
used for... if it's not obvious. If it's so completely obvious your grandma
could figure it out, it's probably safe to leave it out. Naming classes
-something sane goes a long ways towards avoiding writing documentation. :)</p>
+something sane goes a long ways towards avoiding writing documentation.</p>
-</li>
-<li><h4>Method information</h4>
+<b>Method information</b>
<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
<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>
<!-- _______________________________________________________________________ -->
<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>
+ comments.</li>
+ <li>When writing a header file that may be <tt>#include</tt>d 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>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="scf_includes">#include Style</a>
+ <a name="scf_includes"><tt>#include</tt> 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>
+href="#hl_dontinclude">minimal</a> list of <tt>#include</tt>s required by the
+file should be listed. We prefer these <tt>#include</tt>s 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><tt>llvm/*</tt></li>
+ <li><tt>llvm/Analysis/*</tt></li>
+ <li><tt>llvm/Assembly/*</tt></li>
+ <li><tt>llvm/Bytecode/*</tt></li>
+ <li><tt>llvm/CodeGen/*</tt></li>
<li>...</li>
- <li>Support/*</li>
- <li>Config/*</li>
- <li>System #includes</li>
+ <li><tt>Support/*</tt></li>
+ <li><tt>Config/*</tt></li>
+ <li>System <tt>#includes</tt></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>
+which implement an interface defined by a .h file. This <tt>#include</tt>
+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>
syntax of the code slightly. For example, an warning that annoys me occurs when
I write code like this:</p>
+<div class="doc_code">
<pre>
- if (V = getValue()) {
- ..
- }
+if (V = getValue()) {
+ ...
+}
</pre>
+</div>
<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>
+<div class="doc_code">
<pre>
- if ((V = getValue())) {
- ..
- }
+if ((V = getValue())) {
+ ...
+}
</pre>
+</div>
<p>...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can
be fixed by massaging the code appropriately.</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_subsubsection">
+<a name="ci_class_struct">Use of <tt>class</tt> and <tt>struct</tt> Keywords</a>
+</div>
+<div class="doc_text">
+
+<p>In C++, the <tt>class</tt> and <tt>struct</tt> keywords can be used almost
+interchangeably. The only difference is when they are used to declare a class:
+<tt>class</tt> makes all members private by default while <tt>struct</tt> makes
+all members public by default.</p>
+
+<p>Unfortunately, not all compilers follow the rules and some will generate
+different symbols based on whether <tt>class</tt> or <tt>struct</tt> was used to
+declare the symbol. This can lead to problems at link time.</p>
+
+<p>So, the rule for LLVM is to always use the <tt>class</tt> keyword, unless
+<b>all</b> members are public, in which case <tt>struct</tt> is allowed.</p>
+
+</div>
+
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="styleissues">Style Issues</a>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="hl_dontinclude">#include as Little as Possible</a>
+ <a name="hl_dontinclude"><tt>#include</tt> as Little as Possible</a>
</div>
<div class="doc_text">
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>
+to inherit from it. In these cases go ahead and <tt>#include</tt> 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
+<b>must</b> include all of the header files that you are using -- you can
+include them 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
<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>
+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
</div>
-<!-- ======================================================================= -->
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="ll_iostream"><tt>#include <iostream></tt> is forbidden</a>
+</div>
+
+<div class="doc_text">
+
+<p>The use of <tt>#include <iostream></tt> in library files is
+hereby <b><em>forbidden</em></b>. The primary reason for doing this is to
+support clients using LLVM libraries as part of larger systems. In particular,
+we statically link LLVM into some dynamic libraries. Even if LLVM isn't used,
+the static c'tors are run whenever an application start up that uses the dynamic
+library. There are two problems with this:</p>
+
+<ol>
+ <li>The time to run the static c'tors impacts startup time of
+ applications—a critical time for gui apps.</li>
+ <li>The static c'tors cause the app to pull many extra pages of memory off the
+ disk: both the code for the static c'tors in each .o file and the small
+ amount of data that gets touched. In addition, touched/dirty pages put
+ more pressure on the VM system on low-memory machines.</li>
+</ol>
+
+<div style="align: center">
+<table>
+ <tbody>
+ <tr>
+ <th>Old Way</th>
+ <th>New Way</th>
+ </tr>
+ <tr>
+ <td align="left"><pre>#include <iostream></pre></td>
+ <td align="left"><pre>#include "llvm/Support/Streams.h"</pre></td>
+ </tr>
+ <tr>
+ <td align="left"><pre>DEBUG(std::cerr << ...);
+DEBUG(dump(std::cerr));</pre></td>
+ <td align="left"><pre>DOUT << ...;
+dump(DOUT);</pre></td>
+ </tr>
+ <tr>
+ <td align="left"><pre>std::cerr << "Hello world\n";</pre></td>
+ <td align="left"><pre>llvm::cerr << "Hello world\n";</pre></td>
+ </tr>
+ <tr>
+ <td align="left"><pre>std::cout << "Hello world\n";</pre></td>
+ <td align="left"><pre>llvm::cout << "Hello world\n";</pre></td>
+ </tr>
+ <tr>
+ <td align="left"><pre>std::cin >> Var;</pre></td>
+ <td align="left"><pre>llvm::cin >> Var;</pre></td>
+ </tr>
+ <tr>
+ <td align="left"><pre>std::ostream</pre></td>
+ <td align="left"><pre>llvm::OStream</pre></td>
+ </tr>
+ <tr>
+ <td align="left"><pre>std::istream</pre></td>
+ <td align="left"><pre>llvm::IStream</pre></td>
+ </tr>
+ <tr>
+ <td align="left"><pre>std::stringstream</pre></td>
+ <td align="left"><pre>llvm::StringStream</pre></td>
+ </tr>
+ <tr>
+ <td align="left"><pre>void print(std::ostream &Out);
+// ...
+print(std::cerr);</pre></td>
+ <td align="left"><pre>void print(std::ostream &Out);
+void print(std::ostream *Out) { if (Out) print(*Out) }
+// ...
+print(llvm::cerr);</pre>
+
<div class="doc_text">
+<i>N.B.</i> The second <tt>print</tt> method is called by the <tt>print</tt>
+expression. It prevents the execution of the first <tt>print</tt> method if the
+stream is <tt>cnull</tt>.</div></td>
+ </tbody>
+</table>
+</div>
+
+</div>
+
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
<a name="micro">The Low Level Issues</a>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="hl_assert">Assert Liberally</a>
+ <a name="ll_assert">Assert Liberally</a>
</div>
<div class="doc_text">
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>
+<div class="doc_code">
<pre>
- inline Value *getOperand(unsigned i) {
- assert(i < Operands.size() && "getOperand() out of range!");
- return Operands[i];
- }
+inline Value *getOperand(unsigned i) {
+ assert(i < Operands.size() && "getOperand() out of range!");
+ return Operands[i];
+}
</pre>
+</div>
<p>Here are some examples:</p>
+<div class="doc_code">
<pre>
- assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
+assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
- assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
+assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
- assert(idx < getNumSuccessors() && "Successor # out of range!");
+assert(idx < getNumSuccessors() && "Successor # out of range!");
- assert(V1.getType() == V2.getType() && "Constant types must be identical!");
+assert(V1.getType() == V2.getType() && "Constant types must be identical!");
- assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
+assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
</pre>
+</div>
<p>You get the idea...</p>
</div>
-
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="hl_preincrement">Prefer Preincrement</a>
+ <a name="ll_ns_std">Do not use '<tt>using namespace std</tt>'</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>
+<p>In LLVM, we prefer to explicitly prefix all identifiers from the standard
+namespace with an "<tt>std::</tt>" prefix, rather than rely on
+"<tt>using namespace std;</tt>".</p>
+
+<p> In header files, adding a '<tt>using namespace XXX</tt>' directive pollutes
+the namespace of any source file that includes the header. This is clearly a
+bad thing.</p>
+
+<p>In implementation files (e.g. .cpp files), the rule is more of a stylistic
+rule, but is still important. Basically, using explicit namespace prefixes
+makes the code <b>clearer</b>, because it is immediately obvious what facilities
+are being used and where they are coming from, and <b>more portable</b>, because
+namespace clashes cannot occur between LLVM code and other namespaces. The
+portability rule is important because different standard library implementations
+expose different symbols (potentially ones they shouldn't), and future revisions
+to the C++ standard will add more symbols to the <tt>std</tt> namespace. As
+such, we never use '<tt>using namespace std;</tt>' in LLVM.</p>
+
+<p>The exception to the general rule (i.e. it's not an exception for
+the <tt>std</tt> namespace) is for implementation files. For example, all of
+the code in the LLVM project implements code that lives in the 'llvm' namespace.
+As such, it is ok, and actually clearer, for the .cpp files to have a '<tt>using
+namespace llvm</tt>' directive at their top, after the <tt>#include</tt>s. The
+general form of this rule is that any .cpp file that implements code in any
+namespace may use that namespace (and its parents'), but should not use any
+others.</p>
</div>
-
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="hl_avoidendl">Avoid endl</a>
+ <a name="ll_virtual_anch">Provide a virtual method anchor for classes
+ in headers</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 << endl;
- cout << "\n" << 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>
+<p>If a class is defined in a header file and has a v-table (either it has
+virtual methods or it derives from classes with virtual methods), it must
+always have at least one out-of-line virtual method in the class. Without
+this, the compiler will copy the vtable and RTTI into every .o file that
+#includes the header, bloating .o file sizes and increasing link times.
+</p>
</div>
+
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
- <a name="hl_exploitcpp">Exploit C++ to its Fullest</a>
+ <a name="ll_preincrement">Prefer Preincrement</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>Hard fast rule: Preincrement (<tt>++X</tt>) may be no slower than
+postincrement (<tt>X++</tt>) and could very well be a lot faster than it. Use
+preincrementation whenever possible.</p>
-<p>For these reasons, come to know and love the contents of your local
-<algorithm> header file. Know about <functional> and what it can do
-for you. C++ is just a tool that wants you to master it. :)</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_subsection">
- <a name="iterators">Writing Iterators</a>
+<!-- _______________________________________________________________________ -->
+<div class="doc_subsubsection">
+ <a name="ll_avoidendl">Avoid <tt>std::endl</tt></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>
+<p>The <tt>std::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>
+<div class="doc_code">
<pre>
-From: Ross Smith <ross.s@ihug.co.nz>
-Newsgroups: comp.lang.c++.moderated
-Subject: Writing iterators (was: Re: Non-template functions that take iterators)
-Date: 28 Jun 2001 12:07:10 -0400
-
-Andre Majorel wrote:
-> Any pointers handy on "writing STL-compatible iterators for
-> dummies ?"
-
-I'll give it a try...
-
-The usual situation requiring user-defined iterators is that you have
-a type that bears some resemblance to an STL container, and you want
-to provide iterators so it can be used with STL algorithms. You need
-to ask three questions:
-
-First, is this simply a wrapper for an underlying collection of
-objects that's held somewhere as a real STL container, or is it a
-"virtual container" for which iteration is (under the hood) more
-complicated than simply incrementing some underlying iterator (or
-pointer or index or whatever)? In the former case you can frequently
-get away with making your container's iterators simply typedefs for
-those of the underlying container; your begin() function would call
-member_container.begin(), and so on.
-
-Second, do you only need read-only iterators, or do you need separate
-read-only (const) and read-write (non-const) iterators?
-
-Third, which kind of iterator (input, output, forward, bidirectional,
-or random access) is appropriate? If you're familiar with the
-properties of the iterator types (if not, visit
-<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 <iterator>
-
- class container {
- public:
- typedef something_or_other value_type;
- class const_iterator:
- public std::iterator<std::forward_iterator_tag, value_type> {
- friend class container;
- public:
- const value_type& operator*() const;
- const value_type* operator->() const;
- const_iterator& operator++();
- const_iterator operator++(int);
- friend bool operator==(const_iterator lhs,
- const_iterator rhs);
- friend bool operator!=(const_iterator lhs,
- const_iterator rhs);
- private:
- //...
- };
- //...
- };
-
-An iterator should always be derived from an instantiation of the
-std::iterator template. The iterator's life cycle functions
-(constructors, destructor, and assignment operator) aren't declared
-here; in most cases the compiler-generated ones are sufficient. The
-container needs to be a friend of the iterator so that the container's
-begin() and end() functions can fill in the iterator's private members
-with the appropriate values.
-
-<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&
- container::const_iterator::operator*() const {
- // find the element and return a reference to it
- }
-
- const container::value_type*
- container::const_iterator::operator->() const {
- return &**this;
- }
-
-If there's an underlying real container, operator*() can just return a
-reference to the appropriate element. If there's no actual container
-and the elements need to be generated on the fly -- what I think of as
-a "virtual container" -- things get a bit more complicated; you'll
-probably need to give the iterator a value_type member object, and
-fill it in when you need to. This might be done as part of the
-increment operator (below), or if the operation is nontrivial, you
-might choose the "lazy" approach and only generate the actual value
-when one of the dereferencing operators is called.
-
-The operator->() function is just boilerplate around a call to
-operator*().
-
- container::const_iterator&
- container::const_iterator::operator++() {
- // the incrementing logic goes here
- return *this;
- }
-
- container::const_iterator
- container::const_iterator::operator++(int) {
- const_iterator old(*this);
- ++*this;
- return old;
- }
-
-Again, the incrementing logic will usually be trivial if there's a
-real container involved, more complicated if you're working with a
-virtual container. In particular, watch out for what happens when you
-increment past the last valid item -- this needs to produce an
-iterator that will compare equal to container.end(), and making this
-work is often nontrivial for virtual containers.
-
-The post-increment function is just boilerplate again (and
-incidentally makes it obvious why all the experts recommend using
-pre-increment wherever possible).
-
- bool operator==(container::const_iterator lhs,
- container::const_iterator rhs) {
- // equality comparison goes here
- }
-
- bool operator!=(container::const_iterator lhs,
- container::const_iterator rhs) {
- return !(lhs == rhs);
- }
-
-For a real container, the equality comparison will usually just
-compare the underlying iterators (or pointers or indices or whatever).
-The semantics of comparisons for virtual container iterators are often
-tricky. Remember that iterator comparison only needs to be defined for
-iterators into the same container, so you can often simplify things by
-taking for granted that lhs and rhs both point into the same container
-object. Again, the second function is just boilerplate.
-
-It's a matter of taste whether iterator arguments are passed by value
-or reference; I've shown tham passed by value to reduce clutter, but
-if the iterator contains several data members, passing by reference
-may be better.
-
-That convers the const-iterator-only situation. When we need separate
-const and mutable iterators, one small complication is added beyond
-the simple addition of a second class.
-
- class container {
- public:
- typedef something_or_other value_type;
- class const_iterator;
- class iterator:
- public std::iterator<std::forward_iterator_tag, value_type> {
- friend class container;
- friend class container::const_iterator;
- public:
- value_type& operator*() const;
- value_type* operator->() const;
- iterator& operator++();
- iterator operator++(int);
- friend bool operator==(iterator lhs, iterator rhs);
- friend bool operator!=(iterator lhs, iterator rhs);
- private:
- //...
- };
- class const_iterator:
- public std::iterator<std::forward_iterator_tag, value_type> {
- friend class container;
- public:
- const_iterator();
- const_iterator(const iterator& i);
- const value_type& operator*() const;
- const value_type* operator->() const;
- const_iterator& operator++();
- const_iterator operator++(int);
- friend bool operator==(const_iterator lhs,
- const_iterator rhs);
- friend bool operator!=(const_iterator lhs,
- const_iterator rhs);
- private:
- //...
- };
- //...
- };
-
-There needs to be a conversion from iterator to const_iterator (so
-that mixed-type operations, such as comparison between an iterator and
-a const_iterator, will work). This is done here by giving
-const_iterator a conversion constructor from iterator (equivalently,
-we could have given iterator an operator const_iterator()), which
-requires const_iterator to be a friend of iterator, so it can copy its
-data members. (It also requires the addition of an explicit default
-constructor to const_iterator, since the existence of another
-user-defined constructor inhibits the compiler-defined one.)
-
-Bidirectional iterators add just two member functions to forward
-iterators:
-
- class iterator:
- public std::iterator<std::bidirectional_iterator_tag, value_type> {
- public:
- //...
- iterator& operator--();
- iterator operator--(int);
- //...
- };
-
-I won't detail the implementations, they're obvious variations on
-operator++().
-
-Random access iterators add several more member and friend functions:
-
- class iterator:
- public std::iterator<std::random_access_iterator_tag, value_type> {
- public:
- //...
- iterator& operator+=(difference_type rhs);
- iterator& operator-=(difference_type rhs);
- friend iterator operator+(iterator lhs, difference_type rhs);
- friend iterator operator+(difference_type lhs, iterator rhs);
- friend iterator operator-(iterator lhs, difference_type rhs);
- friend difference_type operator-(iterator lhs, iterator rhs);
- friend bool operator<(iterator lhs, iterator rhs);
- friend bool operator>(iterator lhs, iterator rhs);
- friend bool operator<=(iterator lhs, iterator rhs);
- friend bool operator>=(iterator lhs, iterator rhs);
- //...
- };
-
- container::iterator&
- container::iterator::operator+=(container::difference_type rhs) {
- // add rhs to iterator position
- return *this;
- }
-
- container::iterator&
- container::iterator::operator-=(container::difference_type rhs) {
- // subtract rhs from iterator position
- return *this;
- }
-
- container::iterator operator+(container::iterator lhs,
- container::difference_type rhs) {
- return iterator(lhs) += rhs;
- }
-
- container::iterator operator+(container::difference_type lhs,
- container::iterator rhs) {
- return iterator(rhs) += lhs;
- }
-
- container::iterator operator-(container::iterator lhs,
- container::difference_type rhs) {
- return iterator(lhs) -= rhs;
- }
-
- container::difference_type operator-(container::iterator lhs,
- container::iterator rhs) {
- // calculate distance between iterators
- }
-
- bool operator<(container::iterator lhs, container::iterator rhs) {
- // perform less-than comparison
- }
-
- bool operator>(container::iterator lhs, container::iterator rhs) {
- return rhs < lhs;
- }
-
- bool operator<=(container::iterator lhs, container::iterator rhs) {
- return !(rhs < lhs);
- }
-
- bool operator>=(container::iterator lhs, container::iterator rhs) {
- return !(lhs < rhs);
- }
-
-Four of the functions (operator+=(), operator-=(), the second
-operator-(), and operator<()) are nontrivial; the rest are
-boilerplate.
-
-One feature of the above code that some experts may disapprove of is
-the declaration of all the free functions as friends, when in fact
-only a few of them need direct access to the iterator's private data.
-I originally got into the habit of doing this simply to keep the
-declarations together; declaring some functions inside the class and
-some outside seemed awkward. Since then, though, I've been told that
-there's a subtle difference in the way name lookup works for functions
-declared inside a class (as friends) and outside, so keeping them
-together in the class is probably a good idea for practical as well as
-aesthetic reasons.
-
-I hope all this is some help to anyone who needs to write their own
-STL-like containers and iterators.
-
---
-Ross Smith <ross.s@ihug.co.nz> The Internet Group, Auckland, New Zealand
+std::cout << std::endl;
+std::cout << '\n' << std::flush;
</pre>
+</div>
+
+<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_section">
<a name="seealso">See Also</a>
<ol>
-<li><a href="http://www.aw.com/product/0,2627,0201924889,00.html">Effective
+<li><a href="http://www.aw-bc.com/catalog/academic/product/0,1144,0201310155,00.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>
+href="http://www.awlonline.com/cseng/meyerscddemo/">available as well</a>. Also
+interesting and useful are "More Effective C++" and "Effective STL" by the same
+author.</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>
+something.</p>
</div>
<!-- *********************************************************************** -->
<hr>
-
-<div class="doc_footer">
- <address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
+<address>
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+
+ <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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