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2<html><head><title>A Few Coding Standards</title></head>
3<body bgcolor=white>
4
5<table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
6<tr><td>&nbsp; <font size=+5 color="#EEEEFF" face="Georgia,Palatino,Times,Roman"><b>A Few Coding Standards</b></font></td>
7</tr></table>
8
9<ol>
10 <li><a href="#introduction">Introduction</a>
Chris Lattner7ae36bb2001-07-23 20:40:41 +000011 <li><a href="#mechanicalissues">Mechanical Source Issues</a>
Chris Lattnerd80c43c2001-07-09 03:27:08 +000012 <ol>
13 <li><a href="#sourceformating">Source Code Formatting</a>
14 <ol>
15 <li><a href="#scf_commenting">Commenting</a>
16 <li><a href="#scf_commentformat">Comment Formatting</a>
17 <li><a href="#scf_codewidth">Source Code Width</a>
18 <li><a href="#scf_spacestabs">Use Spaces Instead of Tabs</a>
19 <li><a href="#scf_indentation">Indent Code Consistently</a>
20 </ol>
21 <li><a href="#compilerissues">Compiler Issues</a>
22 <ol>
23 <li><a href="#ci_warningerrors">Treat Compiler Warnings Like Errors</a>
24 <li><a href="#ci_cpp_features">Which C++ features can I use?</a>
25 <li><a href="#ci_portable_code">Write Portable Code</a>
26 </ol>
27 </ol>
28 <li><a href="#styleissues">Style Issues</a>
29 <ol>
30 <li><a href="#macro">The High Level Issues</a>
31 <ol>
32 <li><a href="#hl_module">A Public Header File <b>is</b> a Module</a>
33 <li><a href="#hl_dontinclude">#include as Little as Possible</a>
34 <li><a href="#hl_privateheaders">Keep "internal" Headers Private</a>
35 </ol>
36 <li><a href="#micro">The Low Level Issues</a>
37 <ol>
38 <li><a href="#hl_assert">Assert Liberally</a>
39 <li><a href="#hl_preincrement">Prefer Preincrement</a>
40 <li><a href="#hl_exploitcpp">Exploit C++ to its Fullest</a>
41 </ol>
42 <li><a href="#iterators">Writing Iterators</a>
43 </ol>
44 <li><a href="#seealso">See Also</a>
45</ol><p>
46
47
48<!-- *********************************************************************** -->
49</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>
50<a name="introduction">Introduction
51</b></font></td></tr></table><ul>
52<!-- *********************************************************************** -->
53
54This 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>
55
56This 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:
57
58<a name="goldenrule">
59<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>
60
61The 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>
62
63
64<!-- *********************************************************************** -->
65</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>
66<a name="mechanicalissues">Mechanical Source Issues
67</b></font></td></tr></table><ul>
68<!-- *********************************************************************** -->
69
70<!-- ======================================================================= -->
71</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>
72<a name="sourceformating">Source Code Formatting
73</b></font></td></tr></table><ul>
74
75
76<!-- _______________________________________________________________________ -->
77</ul><a name="scf_commenting"><h4><hr size=0>Commenting</h4><ul>
78
79Comments 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>
80
81<ol>
82<h4><li>File Headers</h4>
83Every 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>
84
85<pre>
86//===-- llvm/Instruction.h - Instruction class definition --------*- C++ -*--=//
87//
88// This file contains the declaration of the Instruction class, which is the
89// base class for all of the VM instructions.
90//
91//===----------------------------------------------------------------------===//
92</pre>
93
94A 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 neccesary 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>
95
96The 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>
97
98
99<h4><li>Class overviews</h4>
100
101Classes 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>
102
103
104<h4><li>Method information</h4>
105
106Methods 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 neccesary 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>
107
108Good things to talk about here are what happens when something unexpected happens: does the method return null? Abort? Format your hard disk?<p>
109</ol>
110
111
112<!-- _______________________________________________________________________ -->
113</ul><a name="scf_commentformat"><h4><hr size=0>Comment Formatting</h4><ul>
114
115In 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>
116
117<ol>
118<li>When writing a C code: Obviously if you are writing C code, use C style comments. :)
119<li>When writing a header file that may be #included by a C source file.
120<li>When writing a source file that is used by a tool that only accepts C style comments.
121</ol><p>
122
123To 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>
124
125
126<!-- _______________________________________________________________________ -->
127</ul><a name="scf_codewidth"><h4><hr size=0>Source Code Width</h4><ul>
128
129Write 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.
130
131
132<!-- _______________________________________________________________________ -->
133</ul><a name="scf_spacestabs"><h4><hr size=0>Use Spaces Instead of Tabs</h4><ul>
134
135In 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>
136
137As 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 make for incredible diffs that are absolutely worthless.<p>
138
139
140<!-- _______________________________________________________________________ -->
141</ul><a name="scf_indentation"><h4><hr size=0>Indent Code Consistently</h4><ul>
142
143Okay, 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>
144
145
146
147
148<!-- ======================================================================= -->
149</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>
150<a name="compilerissues">Compiler Issues
151</b></font></td></tr></table><ul>
152
153
154<!-- _______________________________________________________________________ -->
155</ul><a name="ci_warningerrors"><h4><hr size=0>Treat Compiler Warnings Like Errors</h4><ul>
156
157If 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>
158
159It 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>
160
161<pre>
162 if (V = getValue()) {
163 ..
164 }
165</pre><p>
166
167<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>
168
169<pre>
170 if ((V = getValue())) {
171 ..
172 }
173</pre><p>
174
175...which shuts <tt>gcc</tt> up. Any <tt>gcc</tt> warning that annoys you can be fixed by massaging the code appropriately.<p>
176
177These are the <tt>gcc</tt> warnings that I prefer to enable: <tt>-Wall -Winline -W -Wwrite-strings -Wno-unused</tt><p>
178
179
180<!-- _______________________________________________________________________ -->
181</ul><a name="ci_cpp_features"><h4><hr size=0>Which C++ features can I use?</h4><ul>
182
183Compilers 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>
184
185<ol>
186<li>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>
187
188Just 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>
189
190<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.
191</ol><p>
192
193Other 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>
194
195
196<!-- _______________________________________________________________________ -->
197</ul><a name="ci_portable_code"><h4><hr size=0>Write Portable Code</h4><ul>
198
199In 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>
200
201In 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>
202
203
204
205<!-- *********************************************************************** -->
206</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>
207<a name="styleissues">Style Issues
208</b></font></td></tr></table><ul>
209<!-- *********************************************************************** -->
210
211
212<!-- ======================================================================= -->
213</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>
214<a name="macro">The High Level Issues
215</b></font></td></tr></table><ul>
216
217
218<!-- _______________________________________________________________________ -->
219</ul><a name="hl_module"><h4><hr size=0>A Public Header File <b>is</b> a Module</h4><ul>
220
221C++ 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>
222
223Modules should be completely independent of each other, except for their dependence. 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 issues is how they work together.<p>
224
225One example of this is the <tt>llvm/include/llvm/CFG.h</tt> file. It defines a collection of global functions, template classes, and member functions that are syntactically unrelated to each other. Semantically, however, they all provide useful functionality for operating on a CFG, and so they are bound together.<p>
226
227In 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>
228
229
230<!-- _______________________________________________________________________ -->
231</ul><a name="hl_dontinclude"><h4><hr size=0>#include as Little as Possible</h4><ul>
232
233<tt>#include</tt> hurts compile time performance. Don't do it unless you have to, especially in header files.<p>
234
235But 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>
236
237It 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>
238
239
240<!-- _______________________________________________________________________ -->
241</ul><a name="hl_privateheaders"><h4><hr size=0>Keep "internal" Headers Private</h4><ul>
242
243Many 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>
244
245If 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>
246
247Note 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>
248
249
250<!-- ======================================================================= -->
251</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>
252<a name="micro">The Low Level Issues
253</b></font></td></tr></table><ul>
254
255
256<!-- _______________________________________________________________________ -->
257</ul><a name="hl_assert"><h4><hr size=0>Assert Liberally</h4><ul>
258
259Use 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>assert.h</tt>" header file is probably already included by the header files you are using, so it doesn't cost anything to use it.<p>
260
261To 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>
262
263<pre>
264 inline Value *getOperand(unsigned i) {
265 assert(i < Operands.size() && "getOperand() out of range!");
266 return Operands[i];
267 }
268</pre>
269
270Here are some examples:
271
272<pre>
273 assert(Ty->isPointerType() && "Can't allocate a non pointer type!");
274
275 assert((Opcode == Shl || Opcode == Shr) && "ShiftInst Opcode invalid!");
276
277 assert(idx < getNumSuccessors() && "Successor # out of range!");
278
279 assert(V1.getType() == V2.getType() && "Constant types must be identical!");
280
281 assert(Succ->front()->isPHINode() && "Only works on PHId BBs!");
282</pre><p>
283
284You get the idea...<p>
285
286
287<!-- _______________________________________________________________________ -->
288</ul><a name="hl_preincrement"><h4><hr size=0>Prefer Preincrement</h4><ul>
289
290Hard 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>
291
292The 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>
293
294
295<!-- _______________________________________________________________________ -->
296</ul><a name="hl_exploitcpp"><h4><hr size=0>Exploit C++ to its Fullest</h4><ul>
297
298C++ 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>
299
300For 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>
301
302
303
304<!-- ======================================================================= -->
305</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>
306<a name="iterators">Writing Iterators
307</b></font></td></tr></table><ul>
308
309Here'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>
310
311<pre>
312From: Ross Smith <ross.s@ihug.co.nz>
313Newsgroups: comp.lang.c++.moderated
314Subject: Writing iterators (was: Re: Non-template functions that take iterators)
315Date: 28 Jun 2001 12:07:10 -0400
316
317Andre Majorel wrote:
318> Any pointers handy on "writing STL-compatible iterators for
319> dummies ?"
320
321I'll give it a try...
322
323The usual situation requiring user-defined iterators is that you have
324a type that bears some resemblance to an STL container, and you want
325to provide iterators so it can be used with STL algorithms. You need
326to ask three questions:
327
328First, is this simply a wrapper for an underlying collection of
329objects that's held somewhere as a real STL container, or is it a
330"virtual container" for which iteration is (under the hood) more
331complicated than simply incrementing some underlying iterator (or
332pointer or index or whatever)? In the former case you can frequently
333get away with making your container's iterators simply typedefs for
334those of the underlying container; your begin() function would call
335member_container.begin(), and so on.
336
337Second, do you only need read-only iterators, or do you need separate
338read-only (const) and read-write (non-const) iterators?
339
340Third, which kind of iterator (input, output, forward, bidirectional,
341or random access) is appropriate? If you're familiar with the
342properties of the iterator types (if not, visit
343<a href="http://www.sgi.com/tech/stl/">http://www.sgi.com/tech/stl/</a>), the appropriate choice should be
344obvious from the semantics of the container.
345
346I'll start with forward iterators, as the simplest case that's likely
347to come up in normal code. Input and output iterators have some odd
348properties and rarely need to be implemented in user code; I'll leave
349them out of discussion. Bidirectional and random access iterators are
350covered below.
351
352The exact behaviour of a forward iterator is spelled out in the
353Standard in terms of a set of expressions with specified behaviour,
354rather than a set of member functions, which leaves some leeway in how
355you actually implement it. Typically it looks something like this
356(I'll start with the const-iterator-only situation):
357
358 #include <iterator>
359
360 class container {
361 public:
362 typedef something_or_other value_type;
363 class const_iterator:
364 public std::iterator<std::forward_iterator_tag, value_type> {
365 friend class container;
366 public:
367 const value_type& operator*() const;
368 const value_type* operator->() const;
369 const_iterator& operator++();
370 const_iterator operator++(int);
371 friend bool operator==(const_iterator lhs,
372 const_iterator rhs);
373 friend bool operator!=(const_iterator lhs,
374 const_iterator rhs);
375 private:
376 //...
377 };
378 //...
379 };
380
381An iterator should always be derived from an instantiation of the
382std::iterator template. The iterator's life cycle functions
383(constructors, destructor, and assignment operator) aren't declared
384here; in most cases the compiler-generated ones are sufficient. The
385container needs to be a friend of the iterator so that the container's
386begin() and end() functions can fill in the iterator's private members
387with the appropriate values.
388
389<i>[Chris's Note: I prefer to not make my iterators friends. Instead, two
390ctor's are provided for the iterator class: one to start at the end of the
391container, and one at the beginning. Typically this is done by providing
392two constructors with different signatures.]</i>
393
394There are normally only three member functions that need nontrivial
395implementations; the rest are just boilerplate.
396
397 const container::value_type&
398 container::const_iterator::operator*() const {
399 // find the element and return a reference to it
400 }
401
402 const container::value_type*
403 container::const_iterator::operator->() const {
404 return &**this;
405 }
406
407If there's an underlying real container, operator*() can just return a
408reference to the appropriate element. If there's no actual container
409and the elements need to be generated on the fly -- what I think of as
410a "virtual container" -- things get a bit more complicated; you'll
411probably need to give the iterator a value_type member object, and
412fill it in when you need to. This might be done as part of the
413increment operator (below), or if the operation is nontrivial, you
414might choose the "lazy" approach and only generate the actual value
415when one of the dereferencing operators is called.
416
417The operator->() function is just boilerplate around a call to
418operator*().
419
420 container::const_iterator&
421 container::const_iterator::operator++() {
422 // the incrementing logic goes here
423 return *this;
424 }
425
426 container::const_iterator
427 container::const_iterator::operator++(int) {
428 const_iterator old(*this);
429 ++*this;
430 return old;
431 }
432
433Again, the incrementing logic will usually be trivial if there's a
434real container involved, more complicated if you're working with a
435virtual container. In particular, watch out for what happens when you
436increment past the last valid item -- this needs to produce an
437iterator that will compare equal to container.end(), and making this
438work is often nontrivial for virtual containers.
439
440The post-increment function is just boilerplate again (and
441incidentally makes it obvious why all the experts recommend using
442pre-increment wherever possible).
443
444 bool operator==(container::const_iterator lhs,
445 container::const_iterator rhs) {
446 // equality comparison goes here
447 }
448
449 bool operator!=(container::const_iterator lhs,
450 container::const_iterator rhs) {
451 return !(lhs == rhs);
452 }
453
454For a real container, the equality comparison will usually just
455compare the underlying iterators (or pointers or indices or whatever).
456The semantics of comparisons for virtual container iterators are often
457tricky. Remember that iterator comparison only needs to be defined for
458iterators into the same container, so you can often simplify things by
459taking for granted that lhs and rhs both point into the same container
460object. Again, the second function is just boilerplate.
461
462It's a matter of taste whether iterator arguments are passed by value
463or reference; I've shown tham passed by value to reduce clutter, but
464if the iterator contains several data members, passing by reference
465may be better.
466
467That convers the const-iterator-only situation. When we need separate
468const and mutable iterators, one small complication is added beyond
469the simple addition of a second class.
470
471 class container {
472 public:
473 typedef something_or_other value_type;
474 class const_iterator;
475 class iterator:
476 public std::iterator<std::forward_iterator_tag, value_type> {
477 friend class container;
478 friend class container::const_iterator;
479 public:
480 value_type& operator*() const;
481 value_type* operator->() const;
482 iterator& operator++();
483 iterator operator++(int);
484 friend bool operator==(iterator lhs, iterator rhs);
485 friend bool operator!=(iterator lhs, iterator rhs);
486 private:
487 //...
488 };
489 class const_iterator:
490 public std::iterator<std::forward_iterator_tag, value_type> {
491 friend class container;
492 public:
493 const_iterator();
494 const_iterator(const iterator& i);
495 const value_type& operator*() const;
496 const value_type* operator->() const;
497 const_iterator& operator++();
498 const_iterator operator++(int);
499 friend bool operator==(const_iterator lhs,
500 const_iterator rhs);
501 friend bool operator!=(const_iterator lhs,
502 const_iterator rhs);
503 private:
504 //...
505 };
506 //...
507 };
508
509There needs to be a conversion from iterator to const_iterator (so
510that mixed-type operations, such as comparison between an iterator and
511a const_iterator, will work). This is done here by giving
512const_iterator a conversion constructor from iterator (equivalently,
513we could have given iterator an operator const_iterator()), which
514requires const_iterator to be a friend of iterator, so it can copy its
515data members. (It also requires the addition of an explicit default
516constructor to const_iterator, since the existence of another
517user-defined constructor inhibits the compiler-defined one.)
518
519Bidirectional iterators add just two member functions to forward
520iterators:
521
522 class iterator:
523 public std::iterator<std::bidirectional_iterator_tag, value_type> {
524 public:
525 //...
526 iterator& operator--();
527 iterator operator--(int);
528 //...
529 };
530
531I won't detail the implementations, they're obvious variations on
532operator++().
533
534Random access iterators add several more member and friend functions:
535
536 class iterator:
537 public std::iterator<std::random_access_iterator_tag, value_type> {
538 public:
539 //...
540 iterator& operator+=(difference_type rhs);
541 iterator& operator-=(difference_type rhs);
542 friend iterator operator+(iterator lhs, difference_type rhs);
543 friend iterator operator+(difference_type lhs, iterator rhs);
544 friend iterator operator-(iterator lhs, difference_type rhs);
545 friend difference_type operator-(iterator lhs, iterator rhs);
546 friend bool operator<(iterator lhs, iterator rhs);
547 friend bool operator>(iterator lhs, iterator rhs);
548 friend bool operator<=(iterator lhs, iterator rhs);
549 friend bool operator>=(iterator lhs, iterator rhs);
550 //...
551 };
552
553 container::iterator&
554 container::iterator::operator+=(container::difference_type rhs) {
555 // add rhs to iterator position
556 return *this;
557 }
558
559 container::iterator&
560 container::iterator::operator-=(container::difference_type rhs) {
561 // subtract rhs from iterator position
562 return *this;
563 }
564
565 container::iterator operator+(container::iterator lhs,
566 container::difference_type rhs) {
567 return iterator(lhs) += rhs;
568 }
569
570 container::iterator operator+(container::difference_type lhs,
571 container::iterator rhs) {
572 return iterator(rhs) += lhs;
573 }
574
575 container::iterator operator-(container::iterator lhs,
576 container::difference_type rhs) {
577 return iterator(lhs) -= rhs;
578 }
579
580 container::difference_type operator-(container::iterator lhs,
581 container::iterator rhs) {
582 // calculate distance between iterators
583 }
584
585 bool operator<(container::iterator lhs, container::iterator rhs) {
586 // perform less-than comparison
587 }
588
589 bool operator>(container::iterator lhs, container::iterator rhs) {
590 return rhs < lhs;
591 }
592
593 bool operator<=(container::iterator lhs, container::iterator rhs) {
594 return !(rhs < lhs);
595 }
596
597 bool operator>=(container::iterator lhs, container::iterator rhs) {
598 return !(lhs < rhs);
599 }
600
601Four of the functions (operator+=(), operator-=(), the second
602operator-(), and operator<()) are nontrivial; the rest are
603boilerplate.
604
605One feature of the above code that some experts may disapprove of is
606the declaration of all the free functions as friends, when in fact
607only a few of them need direct access to the iterator's private data.
608I originally got into the habit of doing this simply to keep the
609declarations together; declaring some functions inside the class and
610some outside seemed awkward. Since then, though, I've been told that
611there's a subtle difference in the way name lookup works for functions
612declared inside a class (as friends) and outside, so keeping them
613together in the class is probably a good idea for practical as well as
614aesthetic reasons.
615
616I hope all this is some help to anyone who needs to write their own
617STL-like containers and iterators.
618
619--
620Ross Smith <ross.s@ihug.co.nz> The Internet Group, Auckland, New Zealand
621</pre>
622
623
624<!-- *********************************************************************** -->
625</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>
626<a name="seealso">See Also
627</b></font></td></tr></table><ul>
628<!-- *********************************************************************** -->
629
630A lot of these comments and recommendations have been culled for other sources. Two particularly important books for our work are:<p>
631
632<ol>
633<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>.
634<li><a href="http://cseng.aw.com/book/0,3828,0201633620,00.html">Large-Scale C++ Software Design</a> by John Lakos
635</ol><p>
636
637If you get some free time, and you haven't read them: do so, you might learn something. :)
638
639
640<!-- *********************************************************************** -->
641</ul>
642<!-- *********************************************************************** -->
643
644<hr>
645<font size=-1>
646<address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
647<!-- Created: Tue Jan 23 15:19:28 CST 2001 -->
648<!-- hhmts start -->
Chris Lattner7ae36bb2001-07-23 20:40:41 +0000649Last modified: Mon Jul 23 15:40:22 CDT 2001
Chris Lattnerd80c43c2001-07-09 03:27:08 +0000650<!-- hhmts end -->
651</font>
652</body></html>