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/****************************************************************************
**
** This file is part of Qt Creator
**
** Copyright (c) 2011 Nokia Corporation and/or its subsidiary(-ies).
**
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** No Commercial Usage
**
** This file contains pre-release code and may not be distributed.
** You may use this file in accordance with the terms and conditions
** contained in the Technology Preview License Agreement accompanying
** this package.
**
** GNU Free Documentation License
**
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of this
** file.
**
** If you have questions regarding the use of this file, please contact
** Nokia at qt-info@nokia.com.
**
****************************************************************************/
/*!
\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
*/
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