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This document describes some of the recommended conventions that should be applied in the KDE libraries (not applications). Respecting these guidelines helps create a consistant API and also may help ease maintainence of the libraries later. While these conventions are not mandatory, they are important guidelines, and should be respected unless you have a good reason to disregard them.
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As an introduction, you should read the document [http://doc.trolltech.com/qq/qq13-apis.html Designing Qt-Style C++ APIs].
== Naming Conventions ==
In KDE, we basically follow the same naming conventions as Qt.
Class names starts with a capital K. The rest is in camel case. Function names starts with a lower case, but the first letter of each successive word is capitalized.
Unless dealing with central libraries (kdecore, kdeui), classes should be in the library namespace. In that case, it is the namespace which starts with K and the classes inside may not start with it. New libraries should choose their namespace.
The prefix 'set' is used for setters, but the prefix ''''get'''' is not used for accessors. Accessors are simply named with the name of the property they access. The exception is for accessors of a boolean which may start with the prefix ''''is''''.
Acronyms are lowercased too. Example:
<code>KUrl</code> instead of <code>KURL</code> and <code>isNssEnabled()</code> instead of <code>isNSSEnabled()</code>
Accessors should usually be <code>const</code>.
This example shows some possible functions names
    void setColor(const QColor& c);
    QColor color() const;
    void setDirty(bool b);
    bool isDirty() const;
private Q_SLOTS:
    void slotParentChanged();
Make one public class for every .h file. Add the <code>_EXPORT</code> macro related to the library they are in.
Private classes should be declared in the .cpp file, or in a _p.h file.
== D-Pointers ==
In order to more easily maintain binary compatibility, there shouldn't be private members in a public class. For more information about binary compatibility, read [http://developer.kde.org/documentation/other/binarycompatibility.html|Binary Compatibility Issues With C++].
By convention, the private class will be named the same as the public class, with <code>Private</code> appended to the name.
class KFooPrivate;
class KFoo
        /* public members */
        KFooPrivate * const d;
In the .cpp file:
class KFooPrivate
        int someInteger;
KFoo::KFoo() : d(new KFooPrivate)
    /* ... */
    delete d;
Notice that the member d is <code>const</code> to avoid modifying it by mistake. The private class must not be a nested class or it will be exported too if you added the <code>_EXPORT</code> keyword to the parent class.
If you are implementing an implicitly shared class, you should consider using {{qt|QSharedData}} and {{qt|QSharedDataPointer}} for d. If you don't use them, then use QAtomic for reference counting. Don't try to implement your own refcounting with integers.
Sometimes, complex code may be moved to a member method of the Private class itself. Doing this may give the compiler an extra register to optimize the code, since you won't be using "d" all the time.
== Inline Code ==
For binary compatibility reasons, try to avoid inline code in headers. Specifically no inline constructor or destructor.
If ever you add inline code please note the following:
* Installed headers should compile with the following preprocessor defines: <code>QT_NO_CAST_FROM_ASCII</code>, <code>QT_NO_CAST_TO_ASCII</code>, <code>QT_NO_KEYWORD</code>. So don't forget {{qt|QLatin1String}}.
* No C casts in the header. Use <code>static_cast</code> if types are known. Use <code>qobject_cast</code> instead of <code>dynamic_cast</code> if types are QObject based. dynamic_cast is not only slower, but is also unreliable across shared libraries.
* In general, check your code for [http://developer.kde.org/documentation/other/mistakes.html common mistakes].
These recommendations are also true for code that are not in headers.
== Flags ==
Try to avoid meaningless boolean parameters in functions. Example of a bad boolean argument:
static QString KApplication::makeStdCaption(const QString &userCaption,
                                            bool withAppName = true,
                                            bool modified = false);
Because when you read code that uses the above function, you can't easily know the significance of the parameters
window->setCaption(KApplication::makeStdCaption("Document Foo", true, true));
The solution is to use {{qt|QFlags}}. If the options only apply to one function, call the <code>enum FunctionNameOption</code> and the QFlags typedef <code>FunctionNameOptions</code>. Do that even if there is only one option, this will allow you to add more options later and keep the binary compatibility.
So a better API would be:
class KApplication
        /* [...] */
        enum MakeStandardCaptionOption {
              * Indicates that the method shall include the application name
            WithApplicationName = 0x01,
              * If set, a 'modified' sign is included in the returned string.
            Modified = 0x02
        Q_DECLARE_FLAGS(MakeStandardCaptionOptions, MakeStandardCaptionOption)
          * Builds a caption using a standard layout.
          * @param userCaption The caption string you want to display
          * @param options a set of flags from MakeStandartCaptionOption
        static QString makeStandardCaption(const QString& userCaption,
            const MakeStandardCaptionOptions& options = WithApplicationName);
        /* [...] */
== Const References ==
Each object parameter that is not a basic type (int, float, bool, enum,  or pointers) should be passed by constant reference. This is faster, because it is not required to do a copy of the object. Do that even for object that are already implicitly shared, like QString:
QString myMethod(const QString& foo, const QPixmap& bar, int number);
== Signals and Slots ==
In the libraries, use <code>Q_SIGNALS</code> and <code>Q_SLOTS</code> instead of <code>signals</code> and <code>slots</code>. They are syntactically equivalent and should be used to avoid conflicts with boost signals, and with python's use of "slots" in its headers.
== Properites ==
Consider using <code>Q_PROPERTY</code> for properties. The reason is that properties (especially thoses marked <code>SCRIPTABLE</code>) will be accessible through the javascript interface.
If you follow the propname / setPropname naming sheme, moc sets a special flag for the {{qt|QMetaProperty}}.
== Explicit Constructors ==
For each constructor, check if you should make the constructor <code>explicit</code> in order to minimize wrong use of the constructor.
Basically, each constructor that may take only one argument should be marked explicit unless the whole point of the constructor is to allow implicit casting.
== Avoid including other headers in headers ==
Try to reduce as much as possible the number of includes in header files. This will generally help reduce the compilation time, especially for developers when just one header has been modified. It may also avoid errors that can be caused by conflicts between headers.
If an object in the class is only used by pointer or by reference, it is not required to include the header for that object. Instead, just add a forward declaration before the class.
In this example, the class KFoo uses KBar by reference, so we do not need to include KBar's header:
#include <kfoobase.h>
class KBar;
class KFoo : public KFooBase
        /* [...] */           
        void myMethod(const KBar& bar);
== Static Objects ==
Global static objects in libraries should be avoided. You never know when the constructor will be run or if it will be run at all.
; Wrong
static QString foo; // wrong - object might not be constructed
static QString bar("hello"); // as above
static int foo = myInitializer(); // myInitializer() might not be called
; Correct
static const int i = 42;
static const int ii[3] = {1, 2, 3};
static const char myString[] = "hello";
static const MyStruct s = {3, 4.4, "hello"};
You can use <code>Q_GLOBAL_STATIC</code> to create global static objects which will be initialized the first time you use them.
== Documentation ==
Every class and method should be well documented. Read the [[Policies/Library Documentation Policy|KDE Library Documentation Policy]] for the guidelines to follow when documenting your code.
Also don't forget the license headers and copyrights in each file. As stated in the [[Policies/Licensing Policy|Licensing Policy]], kdelibs code must be licensed under the LGPL, BSD, or X11 license.
Author: [mailto:[email protected] Olivier Goffart] Mart 2006

Latest revision as of 18:17, 10 March 2016

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This page was last edited on 10 March 2016, at 18:17. Content is available under Creative Commons License SA 4.0 unless otherwise noted.