qt-creator/doc/api/coding-style.qdoc

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/*!

\contentspage{index.html}{Qt Creator}
\page coding-style.html

\title Qt Creator Coding Rules

THIS IS PRELIMINARY.

\section1 Introduction

The aim of this section is to serve as a guide for the developers, to aid us
to build understandable and maintainable code, to create less confusion and
surprises when working on Qt Creator.

As usual: Rules are not set in stone. If there's a good reason to break one,
do it, preferably after making sure that there are others agreeing.

This document is incomplete.

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

\list 1
\o The most important rule first: KISS (keep it simple ...): always
   use a simple implementation in favor of a more complicated one.
   This eases maintenance a lot.
\o Write good C++ code: Readable, well commented when necessary,
   and taking advantage of the OO model. Follow the \l{Formatting} guidelines.
   There are also certain \l{Code Constructs} that we try to follow.
\o Adapt the code to the structures already existing in Qt Creator, or in
   the case that you have better ideas, discuss them with other developers
   before writing the code.
\o Follow the \l{Patterns and practices} that we use in Qt Creator.
\o Take advantage of Qt. Don't re-invent the wheel. Think about what parts
   of your code are generic enough that they might be incorporated into
   Qt proper.
\o Document interfaces. Right now we use qdoc, but changing to doxygen
   is being considered.
\endlist


\section1 Submitting Code

See http://qt.gitorious.org/qt/pages/QtContributionGuidelines

\section1 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++.

\list 1
\o 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.

\o Make the scope of a variable as small as possible.

\o 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.

\code
	++T;
	--U;
	-NOT-
	T++; // not used in Qt Creator
	U--; // not used in Qt Creator
\endcode

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

\code

	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) {
		...;
	}
\endcode


\o Using Qt's foreach is ok in non-time critical code when using a QTL
   container. It is a nice way to keep line noise down and to give the
   loop variable a proper name:

\code
        foreach (QWidget *widget, container)
            doSomething(widget);

        -VS-

	Container::iterator end = container.end();
	for (Container::iterator it = container.begin(); it != end; ++it)
            doSomething(*it);
\endcode

    If the loop variable can be made const, do so. This can prevent
    unnecessary detaching of shared data in some cases. So:

\code
        foreach (const QString &name, someListOfNames)
            doSomething(name);

        - NOT -

        foreach (QString name, someListOfNames)
            doSomething(name);
\endcode


\section1 Formatting

\section2 Naming rules

 \list
  \o Class names are capitalized, and function names lowercased.
  \o Variables and functions start with a lower case letter.
  \o Enums are named like Classes, values are in loweri case, containing
      some part of the name of the enum type.
 \endlist


\section2 Declaration of variables

 \list
  \o Each consecutive word in a variable's name starts with a capital letter.

  \o Avoid short (e.g., a, rbarr, nughdeget) names whenever possible.
     Single character variable names are only okay for counters and
     temporaries, where the purpose of the variable is obvious.

  \o Declare each variable on a separate line.
        \code
            QString a = "Joe";
            QString b = "Foo";
            -NOT-
            QString a = "Joe", b = "Foo"; // not used in Qt Creator
        \endcode
	[Note that 'QString a = "Joe"' 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
	'QString a("Joe")'. 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, Qt Creator 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.]

  \o Avoid abbreviations

    // Wrong
    int a, b;
    char *c, *d;

    // Correct
    int height;
    int width;
    char *nameOfThis;
    char *nameOfThat;

  \o Wait with declaring a variable until it is needed. This is especially
     important when initialization is done at the same time.
 \endlist



\section2  Pointers and references

\code
	char *p = "flop";
	char &c = *p;
	-NOT-
	char* p = "flop"; // not used in Qt Creator
	char & c = *p;     // not used in Qt Creator
\endcode

  Also note that we will have:
\code
	const char *p;
	-NOT-
	char const * p; // not used in Qt Creator
\endcode


  Using a plain 0 for Null pointer constants is always correct and
  least effort to type. So:
\code
	void *p = 0;
	-NOT-
	void *p = NULL; // not used in Qt Creator
	-NOT-
	void *p = '\0'; // not used in Qt Creator
	-NOT-
	void *p = 42 - 7 * 6; // also not used in Qt Creator
\endcode
  Note: As an exception, imported third party code as well as code
  interfacing the "native" APIs (src/support/os_*) can use NULL.


\section2  Operator names and parentheses
\code
	operator==(type)
	-NOT-
	operator == (type)  // not used in Qt Creator
\endcode

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


\section2 Function names and parentheses
\code
	void mangle()
	-NOT-
	void mangle ()  // not used in Qt Creator
\endcode



\section2 Indentation
  Four spaces, no tabs.



\section2 Whitespace
 \list
  \o Use blank lines to group statements together where suited.
  \o Always use only one blank line.
  \o Always use a single space after a keyword, and before a curly brace.
 \endlist

\code
    // Wrong
    if(foo){
    }

    // Correct
    if (foo) {
    }
\endcode

  For pointers or references, always use a single space before '*' or '&',
  but never after.
  Avoid C-style casts when possible.
\code
    // Wrong
    char* blockOfMemory = (char* ) malloc(data.size());

    // Correct
    char *blockOfMemory = (char *)malloc(data.size());
    char *blockOfMemory = reinterpret_cast<char *>(malloc(data.size()));
\endcode
  Of course, in this particulare case, using \c new might be an even better
  option.

Braces
  As a base rule, the left curly brace goes on the same line as the
  start of the statement:
\code
    // Wrong
    if (codec)
    {
    }

    // Correct
    if (codec) {
    }
\endcode

  Exception: Function implementations and class declarations always have
  the left brace on the start of a line:
\code
    static void foo(int g)
    {
        qDebug("foo: %i", g);
    }

    class Moo
    {
    };
\endcode

  Use curly braces when the body of a conditional statement contains more
  than one line, and also if a single line statement is somewhat complex.
\code
    // Wrong
    if (address.isEmpty()) {
        return false;
    }

    for (int i = 0; i < 10; ++i) {
        qDebug("%i", i);
    }

    // Correct
    if (address.isEmpty())
        return false;

    for (int i = 0; i < 10; ++i)
        qDebug("%i", i);
\endcode

  Exception 1: Use braces also if the parent statement covers several
  lines / wraps
\code
    // Correct
    if (address.isEmpty()
            || !isValid()
            || !codec)
        return false;
\endcode

  Note: This could be re-written as:

\code
    // Als correct
    if (address.isEmpty())
        return false;

    if (!isValid())
        return false;

    if (!codec)
        return false;
\endcode

  Exception 2: Use braces also in if-then-else blocks where either the
  if-code or the else-code covers several lines
\code
    // Wrong
    if (address.isEmpty())
        --it;
    else {
        qDebug("%s", qPrintable(address));
        ++it;
    }

    // Correct
    if (address.isEmpty()) {
        --it;
    } else {
        qDebug("%s", qPrintable(address));
        ++it;
    }

    // Wrong
    if (a)
        if (b)
            ...
        else
            ...

    // Correct
    if (a) {
        if (b)
            ...
        else
            ...
    }
\endcode

  Use curly braces when the body of a conditional statement is empty
\code
    // Wrong
    while (a);

    // Correct
    while (a) {}
\endcode


\section2 Parentheses
  Use parentheses to group expressions:
\code
    // Wrong
    if (a && b || c)

    // Correct
    if ((a && b) || c)

    // Wrong
    a + b & c

    // Correct
    (a + b) & c
\endcode


\section2 Line breaks
  Keep lines shorter than 100 characters; insert line breaks if necessary.
  Commas go at the end of a broken line; operators start at the beginning
  of the new line. The operator is at the end of the line to avoid having
  to scroll if your editor is too narrow.
\code
    // Wrong
    if (longExpression +
        otherLongExpression +
        otherOtherLongExpression) {
    }

    // Correct
    if (longExpression
        + otherLongExpression
        + otherOtherLongExpression) {
    }
\endcode


\section2 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.


\section2 API/ABI stability
  We currently do not gurantee any API nor ABI compatibility between releases.


\section2 File headers

  If you create a new file, the top of the file should include a
  header comment equal to the one found in other source files of Qt Creator.

\section2 Include order

  Always go from less general to more general. In a typical implementation
  file that would look like
\code
  #include "myownheader.h"
  ...
  #include "other_headers_from_my_own_plugin.h"
  ...
  #include <other_plugin/headers_from_other_plugin.h>
  ...
  #include <QtCore/QSomeCoreClass>
  ...
  #include <QtGui/QSomeGuiClass>
  ...
  #include <some_system_C++_header>
  ...
  #include <some_system_C_header>
\endcode
  This order ensures that the headers are self-contained.

  Using <...> instead of "..." for headers from other plugins helps
  spotting plugin-external dependencies in the sources.

  Using empty lines between blocks of "peer" headers are encouraged.

\section2 Documentation

  The documentation is generated from source and header files.
  You document for the other developers, not for yourself.
  In the header you should document interfaces, i.e.  what the function does,
   not the implementation.
  In the .cpp files you document the implementation if the implementation
  in non-obvious.

\section1 Patterns and practices

\section2 Passing file names

Qt Creator API expects file names in "portable" format, i.e. with '/'
instead of '\\' even on Windows. If you want to pass a file name to
the API that comes from the user, you need to convert it with
QDir::fromNativeSeparators first. Whenever you present a file name to the user,
you need to convert it back to native format with QDir::toNativeSeparators.

When comparing file names consider using FileManager::fixFileName which
makes sure that paths are clean and absolute and also takes Windows'
case-insensitivity into account (but is an expensive operation).

\section2 Plugin extension points

A plugin extension point is an interface that is provided by a plugin
to be implemented by others. The plugin then retrieves all
implementations of the interface and uses them, they "extend" the
functionality of the plugin. A common pattern is that the
implementations of the interface are put into the global object pool
during plugin initialization, and the plugin retrieves them from the
object pool at the end of plugin initialization.

For example: The Find plugin provides the FindFilter interface for
others to implement.  With the FindFilter interface, additional search
scopes can be added, that appear in the Advanced Search dialog. The
Find plugin retrieves all FindFilter implementations (from the global
object pool, see below) and presents them in the dialog, forwarding the
actual search request to the correct FindFilter implementation, which
then performs the search.

\section2 Using the global object pool

You can add objects to the global object pool via
\l{ExtensionSystem::PluginManager::addObject()}, and retrieve objects
of a specific type again via
\l{ExtensionSystem::PluginManager::getObjects()}.  This should mostly
be used for implementations of \l{Plugin extension points}.

Cases where you should not use it are:
\list
\o Singletons. Don't put a singleton into the pool, and don't retrieve
   it from there.  Use the singleton pattern instead.
\endlist


\section2 C++ features

  \list
    \o Don’t use exceptions _unless you know what you do_.
    \o Don’t use RTTI (Run-Time Type Information; that is, the typeinfo
       struct, the dynamic_cast or the typeid operators, including throwing
       exceptions) _unless you know what you do_.
    \o Use templates wisely, not just because you can.

     Hint: Use the compile autotest to see whether a C++ feature is supported
     by all compilers in the test farm.

    \o All code is ascii only (7-bit characters only, run man ascii if unsure)
      \list
          \o Rationale: We have too many locales inhouse and an unhealthy
             mix of UTF-8 and latin1 systems. Usually, characters > 127 can
             be broken without you even knowing by clicking SAVE in your
             favourite editor.
          \o For strings: Use \nnn (where nnn is the octal representation
             of whatever locale you want your string in) or \xnn (where nn
             is hexadecimal).
             Example: QString s = QString::fromUtf8("\213\005");
          \o For umlauts in documentation, or other non-ASCII characters,
             either use qdoc’s \unicode command or use the relevant macro;
             e.g. \uuml for ü
       \endlist

    \o Avoid the use of anonymous namespaces in favor of the static keyword
       if possible. A name localized to the compilation unit with static is
       guaranteed to have internal linkage. For names declared in anonymous
       namespaces the C++ standard unfortunately mandates external linkage.
       (7.1.1/6, or see various discussions about this on the gcc mailing
       lists)

   \endlist


\section2 QObject usage

  \list

    \o Every QObject subclass must have a Q_OBJECT macro, even if it
       doesn’t have signals or slots, if it is intended to be used
       with qobject_cast<>.

    \o Normalize the arguments for signals + slots
       (see QMetaObject::normalizedSignature) inside connect statements
       to safe a few cycles faster signal/slot lookups.
       You can use $QTDIR/util/normalize to normalize existing code.

   \endlist


\section2 Including headers

   \list
     \o Prefer use this form to include Qt headers:
        \c{#include <QtCore/QWhatEver>}.
     \o Order includes from most-specific to least-specific, i.e.
        \list
           \o \c{#include "myclass.h"}
           \o \c{#include "otherclassinplugin.h"}
           \o \c{#include <otherplugin/someclass.h>"}
           \o \c{#include <QtModule/QtClass>"}
           \o \c{#include <stdthing>"}
           \o \c{#include <system.h>"}
         \endif
         Add empty line between long blocks and try to keep
         alphabetical order within a block.
   \endlist

\section2 Casting

   \list
    \o Avoid C casts, prefer C++ casts (static_cast, const_cast,
       reinterpret_cast) Rationale: Both reinterpret_cast and
       C-style casts are dangerous, but at least reinterpret_cast
       won’t remove the const modifier.
    \o Don’t use dynamic_cast, use qobject_cast for QObjects or
       refactor your design, for example by introducing a type()
       method (see QListWidgetItem) _unless you know what you do_.
   \endlist


\section Compiler/Platform specific issues

   \list
     \o Be extremely careful when using the question mark operator.
         If the returned types aren’t identical, some compilers generate
         code that crashes at runtime (you won’t even get a compiler warning).
        \code
        QString s;
        // crash at runtime - QString vs. const char *
        return condition ? s : "nothing";
        \endcode

    \o Be extremely careful about alignment.

       Whenever a pointer is cast such that the required alignment of
       the target is increased, the resulting code might crash at runtime
       on some architectures. For example, if a const char * is cast to an
       const int *, it’ll crash on machines where integers have to be
       aligned at a two- or four-byte boundaries.

       Use a union to force the compiler to align variables correctly.
       In the example below, you can be sure that all instances of
       AlignHelper are aligned at integer-boundaries.
        \code
            union AlignHelper
            {
                char c;
                int i;
            };
        \endcode

    \o Anything that has a constructor or needs to run code to be
       initialized cannot be used as global object in library code,
       since it is undefined when that constructor/code will be run
       (on first usage, on library load, before main() or not at all).

       Even if the execution time of the initializer is defined for
       shared libraries, you’ll get into trouble when moving that code
       in a plugin or if the library is compiled statically:

        \code
          // global scope
          // Wrong - default constructor needs to be run to initialize x:
          static const QString x;

          // Wrong - constructor that takes a const char * has to be run  :
          static const QString y = "Hello";

          // Super wrong:
          QString z;

          // Wrong, call time of foo() undefined, might not be called at all:
          static const int i = foo();
        \endcode

    Things you can do:

      \code
        // global scope
        // Works - no constructor must be run, x set at compile time:
        static const char x[] = "someText";

        // Works - y will be set at compile time:
        static int y = 7;

        // Works - will be initialized statically, no code being run.
        static MyStruct s = {1, 2, 3};

        // Pointers to objects are ok, no code needed to be run to
        // initialize ptr:
        static QString *ptr = 0;

        // Use Q_GLOBAL_STATIC to create static global objects instead:

        Q_STATIC_GLOBAL(QString, s)

        void foo()
        {
            s()->append("moo");
        }
      \endcode

    Note: Static objects in function scope are no problem, the constructor
    will be run the first time the function is
    entered. The code is not reentrant, though.


  \o A char is signed or unsigned dependent on the architecture. Use signed
     char or uchar if you explicitely want a signed/unsigned char.
     The following code will break on PPC, for example:

     \code
      char c = 'A';
      // WRONG - condition is always true on platforms where the
      // default is unsigned:
      if (c < 0) { ... }
     \endcode

  \o Try to avoid 64-bit enum values. The aapcs embedded ABI hard codes
     all enum values to a 32-bit integer.
 \endlist


\section2 Aestethics

  \list

    \o Prefer enums to define constants over static const int or defines.
       enum values will be replaced by the compiler at compile time,
       resulting in faster code defines are not namespace safe (and look ugly).
    \o Prefer verbose argument names in headers.
       Qt Creator will show the argument names in their completion-box.
       It will look better in the documentation.

   \endlist


\section2 Things to avoid

   \list
     \o Do not inherit from template/tool classes _unless you know what
        you do_. Possible pitfalls: The destructors are not virtual,
        leading to potential memory leaks. The symbols are not exported
        (and mostly inline), leading to interesting symbol clashes.
        Example: Library A has class Q_EXPORT X: public QList<QVariant> {};
        and library B has class Q_EXPORT Y: public QList<QVariant> {};.
        Suddenly, QList’s symbols are exported from two libraries – /clash/.
      \o Don’t mix const and non-const iterators. This will silently crash
         on broken compilers.

        \code
        // Wrong:
      for (Container::const_iterator it = c.begin(); it != c.end(); ++it)
        // Correct:
      for (Container::const_iterator it = c.constBegin(); it != c.constEnd(); ++it)
        \endcode
    \endlist

\section2 Binary and Source Compatibility:

  \list
    \o Definitions:
        \list
          \o Qt 4.0.0 is a major release, Qt 4.1.0 is a minor release,
             Qt 4.1.1 is a patch release.
          \o Backward binary compatibility: Code linked to an earlier
             version of the library keeps working.
          \o Forward binary compatibility: Code linked to a newer
             version of the library works with an older library.
          \o Source code compatibility: Code compiles without modification.
        \endlist

    \o TRY TO keep backward binary compatibility and backward source code
        compatibility in minor releases.
        NOTE: This is not yet mandatory.

    \o TRY TO Keep backward and forward binary compatibility and forward
        and backward source code compatibility in patch releases
        \list
          \o Don’t add/remove any public API (e.g. global functions,x
              public/protected/private methods)
          \o Don’t reimplement methods (not even inlines,
             nor protected/private methods)
          \o Check BinaryCompatibilityWorkarounds for ways to keep b/c
        \endlist
        NOTE: This is not yet mandatory!

    \o Some information on binary compatibility:
        http://techbase.kde.org/Policies/Binary_Compatibility_Issues_With_C++

   \endlist


\section2 Namespacing

    Read QtInNamespace and keep in mind that all of Qt Creator is
    “namespace aware” code.


\section2 Conventions for public header files

    Our public header files have to survive the strict settings of
    some of our users. All installed headers have to follow these rules:

    \list

      \o No C style casts (-Wold-style-cast). Use static_cast, const_cast
         or reinterpret_cast, for basic types, use the constructor form:
         int(a) instead of (int)a. See chapter “Casting” for more info.

      \o No float comparisons (-Wfloat-equal). Use qFuzzyCompare to compare
         values with a delta. Use qIsNull to check whether a float is
         binary 0, instead of comparing it to 0.0, or, prefered, move
         such code into an implementaion fole.

      \o Don’t hide virtual methods in subclasses (-Woverloaded-virtual).
         If the baseclass A has a virtual int val() and subclass B an
         overload with the same name, int val(int x), A’s val function
         is hidden. Use the using keyword to make it visible again, and
         add the following silly workaround for broken compilers:
        \code
            class B: public A
            {
            #ifdef Q_NO_USING_KEYWORD
            inline int val() { return A::val(); }
            #else
            using A::val;
            #endif
            };
        \code

      \o Don’t shadow variables (-Wshadow). Avoid things like this->x = x;

      \o Don’t give variables the same name as functions declared in
        yours class.

      \o Always check whether a preprocessor variable is defined before
        probing its value (-Wundef).

        \code
          // Wrong:
          #if Foo == 0

          // Correct:
          #if defined(Foo) && Foo == 0

          // Wrong. We don't want to do "clever" things like that.
          // Use the one above instead, for better readability.
          #if Foo - 0 == 0

        \endcode

    \endlist

*/