1 Rules for the code in LyX
2 -------------------------
3 [updated from the C++STYLE distributed with the GNU C++ Standard]
5 The aim of this file is to serve as a guide for the developers, to aid us to
6 get clean and uniform code. This document is still incomplete.
8 We really like to have new developers joining the LyX Project. However,
9 we have had problems in the past with developers leaving the
10 project and their contributed code in a far from perfect state. Most
11 of this happened before we really became aware of these issues,
12 but still, we don't want it to happen again. So we have put together
13 some guidelines and rules for the developers.
19 These guidelines should save us a lot of work while cleaning up the code and
20 help us to have quality code. LyX has been haunted by problems coming from
21 unfinished projects by people who have left the team. Those problems will
22 hopefully disappear if the code is easy to hand over to somebody else.
24 In general, if you want to contribute to the main source, we expect at least
27 - the most important rule first: kiss (keep it simple stupid), always
28 use a simple implementation in favor of a more complicated one.
29 This eases maintenance a lot.
30 - write good C++ code: Readable, well commented and taking advantage of the
31 OO model. Follow the formatting guidelines. See Formatting.
32 - adapt the code to the structures already existing in LyX, or in the case
33 that you have better ideas, discuss them on the developer's list before
35 - take advantage of the C++ standard library. especially don't use
36 custom containers when a standard container is usable; learn to use
37 the algorithms and functors in the standard library.
38 - be aware of exceptions and write exception safe code. See Exceptions.
39 - document all variables, methods, functions, classes etc. We are
40 using the source documentation program doxygen, a program that handles
41 javadoc syntax, to document sources. You can download doxygen from :
43 http://www.stack.nl/~dimitri/doxygen/
45 - we have certain code constructs that we try to follow. See Code
52 It is implicitly understood that all patches contributed to The LyX
53 Project is under the Gnu General Public License, version 2 or later.
54 If you have a problem with that, don't contribute code.
56 Also please don't just pop up out of the blue with a huge patch (or
57 small) that changes something substantial in LyX. Always discuss your
58 ideas with the developers on the developer's mailing list.
60 When you create the patch, please use "diff -up" since we find that a
61 lot easier to read than the other diff formats. Also please do not
62 send patches that implements or fixes several different things; several
63 patches is a much better option.
65 We also require you to provide a commit message entry with every patch,
66 this describes in detail what the patch is doing.
72 We have several guidelines on code constructs, some of these exist to
73 make the code faster, others to make the code clearer. Yet others
74 exist to allow us to take advantage of the strong type checking
77 - Declaration of variables should wait as long as possible. The rule
78 is: "Don't declare it until you need it." In C++ there are a lot of
79 user defined types, and these can very often be expensive to
80 initialize. This rule connects to the next rule too.
82 - Declare the variable as const if you don't need to change it. This
83 applies to POD types like int as well as classes.
85 - Make the scope of a variable as small as possible.
87 - Make good use of namespaces. Prefer anonymous namespaces to declaring
88 "static" for file scope.
90 - Prefer preincrement to postincrement whenever possible.
91 Preincrement has potential of being faster than postincrement. Just
92 think about the obvious implementations of pre/post-increment. This
93 rule applies to decrement too.
101 - Try to minimize evaluation of the same code over and over. This is
102 aimed especially at loops.
104 Container::iterator end = large.end();
105 for (Container::iterator it = large.begin(); it != end; ++it) {
109 for (Container::iterator it = large.begin();
110 it != large.end(); ++it) {
114 - For functions and methods that return a non-POD type T, return T
115 const instead. This gives better type checking, and will give a
116 compiler warning when temporaries are used wrongly.
122 - Avoid using the default cases in switch statements unless you have
123 too. Use the correct type for the switch expression and let the
124 compiler ensure that all cases are exhausted.
132 case foo: ...; break;
133 case bar: ...; break;
134 default: ...; break; // not needed and would shadow a wrong use of Foo
141 Even if LyX currently is not using exceptions we need to be aware of
142 them. One important thing to realize is that you often do not have to
143 use throw, try or catch to be exception safe. Let's look at the
144 different types of exceptions safety: (These are taken from Herb
148 1. Basic guarantee: Even in the presence of exceptions thrown by T or
149 other exceptions, Stack objects don't leak resources.
150 Note that this also implies that the container will be
151 destructible and usable even if an exception is thrown while
152 performing some container operation. However, if an exception
153 is thrown, the container will be in a consistent, but not
154 necessarily predictable, state. Containers that support the
155 basic guarantee can work safely in some settings.
157 2. Strong guarantee: If an operation terminates because of an
158 exception, program state will remain unchanged.
159 This always implies commit-or-rollback semantics, including
160 that no references or iterators into the container be
161 invalidated if an operation fails. For example, if a Stack
162 client calls Top and then attempts a Push that fails because
163 of an exception, then the state of the Stack object must be
164 unchanged and the reference returned from the prior call to
165 Top must still be valid. For more information on these
166 guarantees, see Dave Abrahams's documentation of the SGI
167 exception-safe standard library adaption at:
169 http://www.stlport.org/doc/exception_safety.html
171 Probably the most interesting point here is that when you
172 implement the basic guarantee, the strong guarantee often
173 comes for free. For example, in our Stack implementation,
174 almost everything we did was needed to satisfy just the basic
175 guarantee -- and what's presented above very nearly satisfies
176 the strong guarantee, with little of no extra work. Not half
177 bad, considering all the trouble we went to.
179 In addition to these two guarantees, there is one more
180 guarantee that certain functions must provide in order to make
181 overall exception safety possible:
183 3. Nothrow guarantee: The function will not emit an exception under any
185 Overall exception safety isn't possible unless certain
186 functions are guaranteed not to throw. In particular, we've
187 seen that this is true for destructors; later in this
188 miniseries, we'll see that it's also needed in certain helper
189 functions, such as Swap().
192 For all cases where we might be able to write exception safe functions
193 without using try, throw or catch we should do so. In particular we
194 should look over all destructors to ensure that they are as exception
197 Later when more compiler support exceptions sufficiently well we will
198 begin using them too. One reason for this is that the C++ standard
199 library actually requires exceptions, e.g. "new" will throw
200 bad_allocation if the requested memory is not available.
206 * Only one declaration on each line.
211 This is especially important when initialization is done at the same
216 string a = "Lars", b = "Gullik"; // wrong
218 [Note that 'string a = "Lars"' is formally calling a copy constructor
219 on a temporary constructed from a string literal and therefore has the
220 potential of being more expensive then direct construction by
221 'string a("Lars")'. However the compiler is allowed to elide the copy
222 (even if it had side effects), and modern compilers typically do so.
223 Given these equal costs, LyX code favours the '=' idiom as it is in
224 line with the traditional C-style initialization, _and_ cannot be
225 mistaken as function declaration, _and_ reduces the level of nested
226 parantheses in more initializations.]
229 * Pointers and references
233 char *p = "flop"; // wrong
234 char &c = *p; // wrong
236 Some time ago we had a huge discussion on this subject and after
237 convincing argumentation from Asger this is what we decided. Also note
241 const char * p; // wrong
243 * Operator names and parentheses
246 operator == (type) // wrong
248 The == is part of the function name, separating it makes the
249 declaration look like an expression.
251 * Function names and parentheses
254 void mangle () // wrong
263 enum { one = 1, two = 2, three 3 }; // wrong
271 * Naming rules for classes
273 - Use descriptive but simple and short names. Do not abbreviate.
275 - Class names are usually capitalized, and function names lowercased.
276 Enums are named like Classes, values are usually in lower-case.
278 - Long variables are named like thisLongVariableName.
280 New types are capitalized, so this goes for typedefs, classes, structs
285 - Adapt the formatting of your code to the one used in the
286 other parts of LyX. In case there is different formatting for
287 the same construct, use the one used more often.
289 * Use existing structures
291 - Use string wherever possible. LyX will someday move to Unicode, and
292 that will be easy if everybody uses string now. Unicode strings
293 should prefer using docstring instead of UTF-8 encoded std::string.
295 - Check out the filename and path tools in filetools.h
297 - Check out the string tools in lstring.h.
299 - Use the LyXErr class to report errors and messages using
300 the lyxerr instantiation.
302 [add description of other existing structures]
307 - Use this order for the access sections of your class: public,
308 protected, private. The public section is interesting for every
309 user of the class. The private section is only of interest for the
310 implementors of the class (you). [Obviously not true since this is
311 for developers, and we do not want one developer only to be able to
312 read and understand the implementation of class internals. Lgb]
314 - Avoid declaring global objects in the declaration file of the class.
315 If the same variable is used for all objects, use a static member.
317 - Avoid global or static variables. An exception to this rule is
318 very private stuff like the math stack.
323 - If you create a new file, the top of the file should look something
328 * This file is part of LyX, the document processor.
329 * Licence details can be found in the file COPYING.
331 * \author Kaiser Sose
333 * Full author contact details are available in file CREDITS
338 - The documentation is generated from the header files.
339 - You document for the other developers, not for yourself.
340 - You should document what the function does, not the implementation.
341 - in the .cpp files you document the implementation.
342 - Single line description (///), multiple lines description (/** ... */)
343 - see the doxygen webpage referenced above
346 * NAMING RULES FOR USER-COMMANDS
348 Here's the set of rules to apply when a new command name is introduced:
350 1) Use the object.event order. That is, use `word-forward' instead of
352 2) Don't introduce an alias for an already named object. Same for events.
353 3) Forward movement or focus is called `forward' (not `right').
354 4) Backward movement or focus is called `backward' (not `left').
355 5) Upward movement of focus is called `up'.
356 6) Downward movement is called `down'.
357 7) The begin of an object is called `begin' (not `start').
358 8) The end of an object is called `end'.
361 *************************************************************
363 How to create class interfaces.
364 (a.k.a How Non-Member Functions Improve Encapsulation)
365 ======================================================
367 I recently read an article by Scott Meyers in C/C++ User's
368 Journal (Vol.18,No.2), where he makes a strong case on how non-member
369 functions makes classes more encapsulated, not less. Just skipping
370 to the core of this provides us with the following algorithm for
371 deciding what kind of function to add to a class interface:
373 - We need to add a function f to the class C's API.
375 if (f needs to be virtual)
376 make f a member function of C;
377 else if (f is operator>> or operator<<) {
378 make f a non-member function;
379 if (f needs access to non-public members of C)
380 make f a friend of C;
381 } else if (f needs type conversions on its left-most argument) {
382 make f a non-member function;
383 if (f needs access to non-public members of C)
384 make f a friend of C;
385 } else if (f can be implemented via C's public interface)
386 make f a non-member function;
388 make f a member function of C;
390 (I'll fill in more from Scott Meyers article when time allows.)
395 [ExC++] Sutter, Herb. Exceptional C++: 47 engineering puzzles,
396 programming problems, and solutions. ISBN 0-201-61562-2