path: root/doc/api/coding-style.qdoc

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/*!
    \contentspage{index.html}{Qt Creator}
    \previouspage qtcreator-ui-text.html
    \page coding-style.html
    \nextpage qtcreator-api.html

    \title Qt Creator Coding Rules

    \note This document is work in progress.

    The coding rules aim to guide Qt Creator developers, to help them write
    understandable and maintainable code, and to minimize confusion and surprises.

    As usual, rules are not set in stone. If you have a good reason to break one,
    do so. But first make sure that at least some other developers agree with you.

    To contribute to the main Qt Creator source, you should comply to the
    following rules:

    \list
        \o  The most important rule is: KISS (keep it short and simple). Always
            choose the simpler implementation option over the more complicated one.
            This makes maintenance a lot easier.
        \o  Write good C++ code. That is, readable, well commented when necessary,
            and object-oriented.
        \o  Take advantage of Qt. Do not re-invent the wheel. Think about which parts
            of your code are generic enough that they might be incorporated into
            Qt instead of Qt Creator.
        \o  Adapt the code to the existing structures in Qt Creator.
            If you have improvement ideas, discuss them with other developers
            before writing the code.
        \o  Follow the guidelines in \l{Code Constructs}, \l{Formatting}, and
            \l{Patterns and Practices}.
        \o  Document interfaces. Right now we use qdoc, but changing to doxygen
            is being considered.
        \endlist


    \section1 Submitting Code

    To submit code to Qt, you must understand the tools and mechanics as well as
    the philosophy behind Qt development. For more information about how to set up
    the development environment for working on a Qt library and how to submit code
    and documentation for inclusion to Qt, see
    \l{http://developer.qt.nokia.com/wiki/Qt_Contribution_Guidelines}
    {Guidelines for Contributions to the Qt Framework}.

    \section1 Binary and Source Compatibility

    The following list describes how the releases are numbered and defines
    \e {binary compatibility} and \e {source code compatibility} between
    releases:

    \list
        \o  Qt Creator 2.0.0 is a \e {major release}, Qt Creator 2.1.0 is a \e {minor
            release}, and Qt Creator 2.1.3 is a \e {patch release}.
        \o  \e {Backward binary compatibility} means that code linked to an
            earlier version of the library still works.
        \o  \e {Forward binary compatibility} means that code linked to a
            newer version of the library works with an older library.
        \o  \e {Source code compatibility} means that code compiles without
            modification.
        \endlist

    We do not currently guarantee API nor ABI
    (\l{http://en.wikipedia.org/wiki/Application_binary_interface}{application binary interface})
    compatibility between major releases.

    However, we try to preserve compatibility between minor and patch releases,
    as follows:

    \list
        \o  Preserve backward binary compatibility and backward source code
            compatibility in minor releases.
        \o  Preserve backward and forward binary compatibility and forward and
            backward source code compatibility in patch releases:
        \list
            \o  Do not add or remove any public API (e.g. global functions,x
                public/protected/private methods).
            \o  Do not reimplement methods (not even inlines,
                nor protected or private methods).
            \o  Check
                \l {http://developer.qt.nokia.com/wiki/Binary_Compatibility_Workarounds}{Binary Compatibility Workarounds}
                for ways to preserve binary compatibility.
        \endlist
    \endlist

    \note This is not yet mandatory.
    For more information on binary compatibility, see
    \l{http://techbase.kde.org/Policies/Binary_Compatibility_Issues_With_C++}{Binary Compatibility Issues With C++}.

    \section1 Code Constructs

    Follow the guidelines for code constructs to make the code faster and
    clearer. In addition, the guidelines allow you to take advantage of the strong
    type checking in C++.

    \list
        \o  Prefer preincrement to postincrement whenever possible.
            Preincrement is potentially faster than postincrement. Just
            think about the obvious implementations of pre/post-increment. This
            rule applies to decrement too:

            \code
            ++T;
            --U;

            -NOT-

            T++;
            U--;
            \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  You can use the Qt \c foreach loop in non-time-critical code with a Qt
            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);

                -NOT-

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

            Make the loop variable const, if possible. This might prevent
            unnecessary detaching of shared data:

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

                - NOT -

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

        \endlist

    \section1 Formatting

    \section2 Capitalizing Identifiers

    Use \l{http://en.wikipedia.org/wiki/CamelCase}{camel case} in identifiers.

    Capitalize the first word in an identifier as follows:

    \list
        \o  Class names begin with a capital letter.
        \o  Function names begin with a lower case letter.
        \o  Variable names begin with a lower case letter.
        \o  Enum names begin with a capital letter. Enum values use lower case and
            contain some part of the name of the enum type.
    \endlist

    \section2 Whitespace

    \list
        \o  Use four spaces for indentation, no tabs.
        \o  Use blank lines to group statements together where suited.
        \o  Always use only one blank line.
    \endlist

    \section3 Pointers and References

    For pointers or references, always use a single space before an asterisk (*)
    or an ampersand (&), but never after.
    Avoid C-style casts when possible:

    \code
    char *blockOfMemory = (char *)malloc(data.size());
    char *blockOfMemory = reinterpret_cast<char *>(malloc(data.size()));

    -NOT-

    char* blockOfMemory = (char* ) malloc(data.size());
    \endcode

    Of course, in this particulare case, using \c new might be an even better
    option.

    \section3  Operator Names and Parentheses

    Do not use spaces between operator names and function names. The equation
    marks (==) are a part of the function name, and therefore, spaces make the
    declaration look like an expression:

    \code
    operator==(type)

    -NOT-

    operator == (type)
    \endcode

    \section3 Function Names and Parentheses

    Do not use spaces between function names and parentheses:

    \code
    void mangle()

    -NOT-

    void mangle ()
    \endcode

    \section3 Keywords

    Always use a single space after a keyword, and before a curly brace:

    \code
    if (foo) {
    }

    -NOT-

    if(foo){
    }
    \endcode

    \section2 Braces

    As a base rule, place the left curly brace on the same line as the
    start of the statement:

    \code
    if (codec) {
    }

    -NOT-

    if (codec)
    {
    }
    \endcode

    Exception: Function implementations and class declarations always have
    the left brace in the beginning 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.
    Otherwise, omit them:

    \code
    if (address.isEmpty())
        return false;

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

    -NOT-

    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 or if it wraps:

    \code
    if (address.isEmpty()
            || !isValid()
            || !codec) {
        return false;
    }
    \endcode

    \note This could be re-written as:

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

    -NOT-

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

    \code
    if (a) {
        if (b)
            ...
        else
            ...
    }

    -NOT-

    if (a)
        if (b)
            ...
        else
            ...
    \endcode

    Use curly braces when the body of a conditional statement is empty:

    \code
    while (a) {}

    -NOT-

    while (a);
    \endcode

    \section2 Parentheses

    Use parentheses to group expressions:

    \code
    if ((a && b) || c)

    -NOT-

    if (a && b || c)
    \endcode

    \code
    (a + b) & c

    -NOT-

    a + b & c
    \endcode

    \section2 Line Breaks

    \list
        \o  Keep lines shorter than 100 characters.
        \o  Insert line breaks if necessary.
        \o  Commas go at the end of a broken line.
        \o  Operators start at the beginning of the new line.
            \code
            if (longExpression
                || otherLongExpression
                || otherOtherLongExpression) {
            }

            -NOT-

            if (longExpression ||
                otherLongExpression ||
                otherOtherLongExpression) {
            }
            \endcode
    \endlist

    \section2 Declarations

    \list
        \o  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).

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

        \o  Use \c{class} instead of \c{struct}. Some compilers mangle that
            difference into the symbol names and spit out warnings if a struct
            declaration is followed by a class definition. To avoid ongoing
            changes from one to the other we declare \c{class} the prefered way.

    \endlist

    \section3 Declaring Variables

    \list
        \o  Avoid global variables of class type to rule out initialization order problems.
            Consider using \c Q_GLOBAL_STATIC if they cannot be avoided.
        \o  Declare global string literals as
            \code
            const char aString[] = "Hello";
            \endcode
        \o  Avoid short names (such as, a, rbarr, nughdeget) whenever possible.
            Use single-character variable names only 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";
            \endcode

            \note \c{QString a = "Joe"} formally calls a copy constructor on a
            temporary that is constructed from a string literal. Therefore, it is
            potentially more expensive than direct construction by
            \c {QString a("Joe")}. However, the compiler is allowed to elide the
            copy (even if this has 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, it cannot be
            mistaken as function declaration, and it reduces the level of nested
            parantheses in more initializations.

        \o  Avoid abbreviations:

            \code
            int height;
            int width;
            char *nameOfThis;
            char *nameOfThat;

            -NOT-

            int a, b;
            char *c, *d;
            \endcode

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

    \section1 Patterns and Practices

    \section2 Passing File Names

    Qt Creator API expects file names in portable format, that is, with slashes (/)
    instead of backslashes (\\) even on Windows. To pass a file name from the user
    to the API, convert it with QDir::fromNativeSeparators first. To present a file
    name to the user, 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 (even if it is an expensive operation).

    \section2 Plugin Extension Points

    A plugin extension point is an interface that is provided by one plugin
    to be implemented by others. The plugin then retrieves all
    implementations of the interface and uses them. That is, they \e extend the
    functionality of the plugin. Typically, 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
    other plugins to implement. With the FindFilter interface, additional search
    scopes can be added, that appear in the \gui {Advanced Search} dialog. The
    Find plugin retrieves all FindFilter implementations from the global
    object pool and presents them in the dialog. The plugin forwards 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}.

    \note Do not put a singleton into the pool, and do not retrieve
    it from there. Use the singleton pattern instead.

    \section2 C++ Features

    \list
        \o  Do not use exceptions, unless you know what you do.

        \o  Do not 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  Do not use virtual inheritance, 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 \c {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).
                    For example: QString s = QString::fromUtf8("\\213\\005");
              \o  For umlauts in documentation, or other non-ASCII characters,
                  either use the qdoc \c {\unicode} command or use the relevant macro.
                  For example: \c{\uuml} for \uuml.
           \endlist

        \o  Use static keywords instead of anonymous namespaces whenever 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
            (ISO/IEC 14882, 7.1.1/6, or see various discussions about this on the gcc
            mailing lists).
    \endlist

    \section3 Null Pointers

    Using a plain zero (0) for null pointer constants is always correct and
    least effort to type.

    \code
    void *p = 0;

    -NOT-

    void *p = NULL;

    -NOT-

    void *p = '\0';

    -NOT-

    void *p = 42 - 7 * 6;
    \endcode

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

    \section2 Using QObject

    \list
        \o  Every QObject subclass must have a Q_OBJECT macro, even if it
            does not have signals or slots, if it is intended to be used
            with qobject_cast<>. See also \l{Casting}.
        \o  Normalize the arguments for signals and slots
            (see \l{http://doc.qt.nokia.com/4.7/qmetaobject.html#normalizedSignature}{QMetaObject::normalizedSignature}
            inside connect statements
            to safely make signal and slot lookup a few cycles faster.
            You can use $QTDIR/util/normalize to normalize existing code.
    \endlist

    \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 Including Headers

    \list
        \o  Use the following format to include Qt headers:
            \c{#include <QtCore/QWhatEver>}.
        \o  Arrange includes in an order that goes from specific to generic to
            ensure that the headers are self-contained. For example:
        \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>}
        \endlist
        \o  Enclose headers from other plugins in square brackets (<>) rather than
            quotation marks ("") to make it easier to spot external dependencies in
            the sources.
        \o  Add empty lines between long blocks of \e peer headers and try to
            arrange the headers in alphabetic order within a block.
    \endlist

    \section2 Casting

    \list
        \o  Avoid C casts, prefer C++ casts (\c static_cast, \c const_cast,
            \c reinterpret_cast) Both \c reinterpret_cast and
            C-style casts are dangerous, but at least \c reinterpret_cast
            will not remove the const modifier.
        \o  Do not use \c dynamic_cast, use \c {qobject_cast} for QObjects, or
            refactor your design, for example by introducing a \c {type()}
            method (see QListWidgetItem), unless you know what you do.
    \endlist

    \section2 Compiler and Platform-specific Issues

    \list
        \o  Be extremely careful when using the question mark operator.
            If the returned types are not identical, some compilers generate
            code that crashes at runtime (you will not 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 \c {const char *} is cast to a
            \c {const int *}, it will crash on machines where integers have to be
            aligned at two-byte 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 or code will be run
            (on first usage, on library load, before \c {main()} or not at all).

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

            \code
            // global scope

            -NOT-

            // Default constructor needs to be run to initialize x:
            static const QString x;

            -NOT-

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

            -NOT-

            QString z;

            -NOT-

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

            Things you can do:

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

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

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

            \code
            // Condition is always true on platforms where the
            // default is unsigned:
            if (c >= 0) {
                ...
            }
            \endcode

        \o  Avoid 64-bit enum values. The AAPCS (Procedure Call Standard
            for the ARM Architecture) embedded ABI hard codes
            all enum values to a 32-bit integer.

        \o  Do not mix const and non-const iterators. This will silently crash
            on broken compilers.
            \code
            for (Container::const_iterator it = c.constBegin(); it != c.constEnd(); ++it)

            -NOT-

            for (Container::const_iterator it = c.begin(); it != c.end(); ++it)
            \endcode
    \endlist

    \section2 Esthetics

    \list
        \o Prefer enums to define const over static const int or defines.
           Enumeration values will be replaced by the compiler at compile time,
           resulting in faster code. Defines are not namespace safe.
        \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 Inheriting from Template or Tool Classes

    Inheriting from template or tool classes has the following potential
    pitfalls:

    \list
        \o  The destructors are not virtual, which can lead to memory leaks.
        \o  The symbols are not exported (and mostly inline), which can lead to
            symbol clashes.
    \endlist

    For example, library A has class \c {Q_EXPORT X: public QList<QVariant> {};}
    and library B has class \c {Q_EXPORT Y: public QList<QVariant> {};}.
    Suddenly, QList symbols are exported from two libraries which results in a
    clash.

    \section2 Namespacing

    Read \l {http://developer.qt.nokia.com/wiki/Qt_In_Namespace}{Qt In Namespace}
    and keep in mind that all of Qt Creator is \e{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 (\c{-Wold-style-cast}). Use \c static_cast, \c const_cast
            or \c reinterpret_cast, for basic types, use the constructor form:
            \c {int(a)} instead of \c {(int)a}. For more information, see \l{Casting}.

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

       \o  Do not hide virtual methods in subclasses (\{-Woverloaded-virtual}).
           If the baseclass A has a virtual \c {int val()} and subclass B an
           overload with the same name, \c {int val(int x)}, the A \c val function
           is hidden. Use the \c 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
               };
           \endcode

        \o  Do not shadow variables (\c{-Wshadow}).

        \o  Avoid things like \c {this->x = x;} if possible.

        \o  Do not give variables the same name as functions declared in
            your class.

        \o  To improve code readability, always check whether a preprocessor
            variable is defined before probing its value (\c{-Wundef}).

            \code
              #if defined(Foo) && Foo == 0

              -NOT-

              #if Foo == 0

              -NOT-

              #if Foo - 0 == 0
           \endcode

    \endlist

    \section1 Documentation

    The documentation is generated from source and header files. You document
    for the other developers, not for yourself. In the header files, document
    interfaces. That is, what the function does, not the implementation.

    In the .cpp files, you can document the implementation if the
    implementation is not obvious.

*/