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[lyx.git] / development / coding / Rules

Rules for the code in LyX
-------------------------
[updated from the C++STYLE distributed with the GNU C++ Standard]

The aim of this file is to serve as a guide for the developers, to aid
us to get clean and uniform code. This document is incomplete.

We really like to have new developers joining the LyX Project. However,
we have had problems in the past with developers leaving the
project and their contributed code in a far from perfect state. Most
of this happened before we really became aware of these issues,
but still, we don't want it to happen again. So we have put together
some guidelines and rules for the developers.


General
-------

These guidelines should save us a lot of work while cleaning up the code and
help us to have quality code. LyX has been haunted by problems coming from
unfinished projects by people who have left the team. Those problems will
hopefully disappear if the code is easy to hand over to somebody else.

In general, if you want to contribute to the main source, we expect at least
that you:

- the most important rule first: kiss (keep it simple stupid), always
  use a simple implementation in favor of a more complicated one.
  This eases maintenance a lot.
- write good C++ code: Readable, well commented and taking advantage of the
  OO model. Follow the formatting guidelines. See Formatting.
- adapt the code to the structures already existing in LyX, or in the case
  that you have better ideas, discuss them on the developer's list before
  writing the code.
- take advantage of the C++ standard library. especially don't use
  custom containers when a standard container is usable; learn to use
  the algorithms and functors in the standard library.
- be aware of exceptions and write exception safe code. See Exceptions.
- document all variables, methods, functions, classes etc. We are
  using the source documentation program doxygen, a program that handles
  javadoc syntax, to document sources. You can download doxygen from :

  http://www.stack.nl/~dimitri/doxygen/

- we have certain code constructs that we try to follow. See Code
  Constructs.


Submitting Code
---------------

It is implicitly understood that all patches contributed to The LyX
Project is under the Gnu General Public License, version 2 or later.
If you have a problem with that, don't contribute code.

Also please don't just pop up out of the blue with a huge patch (or
small) that changes something substantial in LyX. Always discuss your
ideas with the developers on the developer's mailing list.

When you create the patch, please use "diff -up" since we find that a
lot easier to read than the other diff formats. Also please do not
send patches that implements or fixes several different things; several
patches is a much better option.

We also require you to provide a commit message entry with every patch,
this describes in detail what the patch is doing.


Code Constructs
---------------

We have several guidelines on code constructs, some of these exist to
make the code faster, others to make the code clearer. Yet others
exist to allow us to take advantage of the strong type checking
in C++.

- Declaration of variables should wait as long as possible. The rule
  is: "Don't declare it until you need it." In C++ there are a lot of
  user defined types, and these can very often be expensive to
  initialize. This rule connects to the next rule too.

- Declare the variable as const if you don't need to change it. This
  applies to POD types like int as well as classes.

- Make the scope of a variable as small as possible.

- Make good use of namespaces. Prefer anonymous namespaces to declaring
  "static" for file scope.

- Prefer preincrement to postincrement whenever possible.
  Preincrement has potential of being faster than postincrement. Just
  think about the obvious implementations of pre/post-increment. This
  rule applies to decrement too.

        ++T;
        --U;
        -NOT-
        T++; // not used in LyX
        U--; // not used in LyX

- Try to minimize evaluation of the same code over and over. This is
  aimed especially at loops.

        Container::iterator end = large.end();
        for (Container::iterator it = large.begin(); it != end; ++it) {
                ...;
        }
        -NOT-
        for (Container::iterator it = large.begin();
             it != large.end(); ++it) {
                ...;
        }

- For functions and methods that return a non-POD type T, return T
  const instead. This gives better type checking, and will give a
  compiler warning when temporaries are used wrongly.

        T const add(...);
        -NOT-
        T add(...);

- Avoid using the default cases in switch statements unless you have
  too. Use the correct type for the switch expression and let the
  compiler ensure that all cases are exhausted.

        enum Foo {
                FOO_BAR1,
                FOO_BAR2
        };
        Foo f = ...;
        switch (f) {
        case FOO_BAR1: ...; break;
        case FOO_BAR2: ...; break;
        default: ...; break; // not needed and would shadow a wrong use of Foo
        }


Exceptions
----------

Be aware of the presence of exceptions. One important thing to realize
is that you often do not have to use throw, try or catch to be exception
safe. Let's look at the different types of exceptions safety: (These are
taken from Herb Sutter's book[ExC++]

"
1. Basic guarantee: Even in the presence of exceptions thrown by T or
        other exceptions, Stack objects don't leak resources.
        Note that this also implies that the container will be
        destructible and usable even if an exception is thrown while
        performing some container operation. However, if an exception
        is thrown, the container will be in a consistent, but not
        necessarily predictable, state. Containers that support the
        basic guarantee can work safely in some settings.

2. Strong guarantee: If an operation terminates because of an
        exception, program state will remain unchanged.
        This always implies commit-or-rollback semantics, including
        that no references or iterators into the container be
        invalidated if an operation fails. For example, if a Stack
        client calls Top and then attempts a Push that fails because
        of an exception, then the state of the Stack object must be
        unchanged and the reference returned from the prior call to
        Top must still be valid. For more information on these
        guarantees, see Dave Abrahams's documentation of the SGI
        exception-safe standard library adaption at:

        http://www.stlport.org/doc/exception_safety.html

        Probably the most interesting point here is that when you
        implement the basic guarantee, the strong guarantee often
        comes for free. For example, in our Stack implementation,
        almost everything we did was needed to satisfy just the basic
        guarantee -- and what's presented above very nearly satisfies
        the strong guarantee, with little of no extra work. Not half
        bad, considering all the trouble we went to.

        In addition to these two guarantees, there is one more
        guarantee that certain functions must provide in order to make
        overall exception safety possible:

3. Nothrow guarantee: The function will not emit an exception under any
        circumstances.
        Overall exception safety isn't possible unless certain
        functions are guaranteed not to throw. In particular, we've
        seen that this is true for destructors; later in this
        miniseries, we'll see that it's also needed in certain helper
        functions, such as Swap().
"

For all cases where we might be able to write exception safe functions
without using try, throw or catch we should do so. In particular we
should look over all destructors to ensure that they are as exception
safe as possible.


Formatting
----------

* Only one declaration on each line.
        int a;
        int b;
        -NOT-
        int a, b; // not used in LyX
  This is especially important when initialization is done at the same
  time:
        string a = "Lars";
        string b = "Gullik";
        -NOT-
        string a = "Lars", b = "Gullik"; // not used in LyX

        [Note that 'string a = "Lars"' is formally calling a copy constructor 
        on a temporary constructed from a string literal and therefore has the
        potential of being more expensive then direct construction by
        'string a("Lars")'. However the compiler is allowed to elide the copy
        (even if it had side effects), and modern compilers typically do so.
        Given these equal costs, LyX code favours the '=' idiom as it is in
        line with the traditional C-style initialization, _and_ cannot be
        mistaken as function declaration, _and_ reduces the level of nested
        parantheses in more initializations.]
        

* Pointers and references
        char * p = "flop";
        char & c = *p;
        -NOT-
        char *p = "flop"; // not used in LyX
        char &c = *p;     // not used in LyX

  Some time ago we had a huge discussion on this subject and after
  convincing argumentation from Asger this is what we decided. Also note
  that we will have:
        char const * p;
        -NOT-
        const char * p; // not used in LyX

* Operator names and parentheses
        operator==(type)
        -NOT-
        operator == (type)  // not used in LyX

  The == is part of the function name, separating it makes the
  declaration look like an expression.

* Function names and parentheses
        void mangle()
        -NOT-
        void mangle ()  // not used in LyX

* Enumerators
        enum Foo {
                FOO_ONE = 1,
                FOO_TWO = 2,
                FOO_THREE = 3
        };
        -NOT-
        enum { one = 1, two = 2, three 3 }; // not used in LyX
        -NOT-
        enum {
                One = 1,
                Two = 2,
                Three = 3
        };

* Null pointers

        Using a plain 0 is always correct and least effort to type. So:

        void * p = 0;
        -NOT-
        void * p = NULL; // not used in LyX
        -NOT-
        void * p = '\0'; // not used in LyX
        -NOT-
        void * p = 42 - 7 * 6; // not used in LyX

        Note: As an exception, imported third party code as well as code
        interfacing the "native" APIs (src/support/os_*) can use NULL.

* Naming rules for classes

  - Use descriptive but simple and short names. Do not abbreviate.

  - Class names are usually capitalized, and function names lowercased.
    Enums are named like Classes, values are usually in lower-case.

  - Long variables are named like thisLongVariableName.

  New types are capitalized, so this goes for typedefs, classes, structs
  and enums.

* Formatting

  - Adapt the formatting of your code to the one used in the
    other parts of LyX. In case there is different formatting for
    the same construct, use the one used more often.

* Use existing structures

  - Use string wherever possible. LyX will someday move to Unicode, and
    that will be easy if everybody uses string now. Unicode strings 
    should prefer using docstring instead of UTF-8 encoded std::string.

  - Check out the filename and path tools in filetools.h

  - Check out the string tools in lstring.h.

  - Use the LyXErr class to report errors and messages using
    the lyxerr instantiation.

  [add description of other existing structures]


* Declarations

  - Use this order for the access sections of your class: public,
    protected, private. The public section is interesting for every
    user of the class. The private section is only of interest for the
    implementors of the class (you). [Obviously not true since this is
    for developers, and we do not want one developer only to be able to
    read and understand the implementation of class internals. Lgb]

  - Avoid declaring global objects in the declaration file of the class.
    If the same variable is used for all objects, use a static member.

  - Avoid global or static variables.


* File headers

  - If you create a new file, the top of the file should look something
    like this :

  /**
   * \file NewFile.cpp
   * This file is part of LyX, the document processor.
   * Licence details can be found in the file COPYING.
   *
   * \author Kaiser Sose
   *
   * Full author contact details are available in file CREDITS
   */

* Documentation

  - The documentation is generated from the header files.
  - You document for the other developers, not for yourself.
  - You should document what the function does, not the implementation.
  - in the .cpp files you document the implementation.
  - Single line description (///), multiple lines description (/** ... */)
  - see the doxygen webpage referenced above


* NAMING RULES FOR USER-COMMANDS

  Here's the set of rules to apply when a new command name is introduced:

  1) Use the object.event order. That is, use `word-forward' instead of
     `forward-word'.
  2) Don't introduce an alias for an already named object. Same for events.
  3) Forward movement or focus is called `forward' (not `right').
  4) Backward movement or focus is called `backward' (not `left').
  5) Upward movement of focus is called `up'.
  6) Downward movement is called `down'.
  7) The begin of an object is called `begin' (not `start').
  8) The end of an object is called `end'.


 *************************************************************

 How to create class interfaces.
 (a.k.a How Non-Member Functions Improve Encapsulation)
 ======================================================

        I recently read an article by Scott Meyers in C/C++ User's
Journal (Vol.18,No.2), where he makes a strong case on how non-member
functions makes classes more encapsulated, not less. Just skipping
to the core of this provides us with the following algorithm for
deciding what kind of function to add to a class interface:

        - We need to add a function f to the class C's API.

        if (f needs to be virtual)
                make f a member function of C;
        else if (f is operator>> or operator<<) {
                make f a non-member function;
                if (f needs access to non-public members of C)
                        make f a friend of C;
        } else if (f needs type conversions on its left-most argument) {
                make f a non-member function;
                if (f needs access to non-public members of C)
                        make f a friend of C;
        } else if (f can be implemented via C's public interface)
                make f a non-member function;
        else
                make f a member function of C;

(I'll fill in more from Scott Meyers article when time allows.)

References
----------

[ExC++] Sutter, Herb. Exceptional C++: 47 engineering puzzles,
        programming problems, and solutions. ISBN 0-201-61562-2