Policies/Library Code Policy

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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.

As an introduction, you should read the document Designing Qt-Style C++ APIs.

For kdelibs, it is recommended to follow the Kdelibs Coding Style.


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: KUrl instead of KURL and isNssEnabled() instead of isNSSEnabled()

Accessors should usually be const.

This example shows some possible functions names public:

   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 _EXPORT macro related to the library they are in. Private classes should be declared in the .cpp file, or in a _p.h file.


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 Binary Compatibility Issues With C++.

By convention, the private class will be named the same as the public class, with Private appended to the name. class KFooPrivate; class KFoo { public:

   /* public members */


   KFooPrivate * const d;

}; In the .cpp file: class KFooPrivate {

       int someInteger;


KFoo::KFoo() : d(new KFooPrivate) {

   /* ... */


KFoo::~KFoo() {

   delete d;

} Notice that the member d is const to avoid modifying it by mistake.

If you are implementing an implicitly shared class, you should consider using QSharedData and 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.

Shared D-Pointers

If your class hierarchy is large and/or deep, you may want to try the concept of shared d-pointers. You'll be trading the added complexity for a smaller memory footprint in the main object (there will be only one "d" variable in it). Other advantages include:

  • direct access to the private data of the whole hierarchy (in other words, the Private classes are in fact "protected", not "private")
  • access to the parent's d-pointer methods

The latter advantage is especially useful if your class has moved the code from the main class to the Private class. If that's the case, you should be calling the Private methods instead: since they are not exported, they will create simpler relocations in the final library (or none at all). By simply calling the Private method instead of the public one, you contribute to a faster load-time of your library.

To implement a "shared d-pointer", you need to:

  1. define a protected variable (d_ptr) in the least derived class of your hierarchy
  2. in each class of the hierarchy, define a private function called d_func() that reinterpret_casts that d_ptr to the current class's Private class
  3. use Q_D(Foo) at the beginning of the functions to have access to a variable "d"
  4. the private classes derive from one another just like the public hierarchy; they also have virtual destructors
  5. add one extra, protected constructor that takes the private class as a parameter
  6. in each constructor for all derived classes, call the parent's constructor that takes the d pointer as a parameter

The following example illustrates this (it derives from QObject to illustrate parameter-passing in the constructor; it is not necessary): class KFooBasePrivate; class KFooBase : public QObject { public:

   KFooBase(QObject *parent);
   int someInteger() const;


   KFooBasePrivate * const d_ptr;
   KFooBase(KFooBasePrivate &dd, QObject *parent);


   inline KFooBasePrivate *d_func() const { return d_ptr; }


class KFooDerivedPrivate; class KFooDerived : public KFooBase { public:

   KFooDerived(QObject *parent);
   int sumOfIntegers() const;


   KFooDerived(KFooDerivedPrivate &dd, QObject *parent);


   inline KFooDerivedPrivate *d_func() const
   { return reinterpret_cast<KFooDerivedPrivate *>(d_ptr); }

}; In the .cpp file: /*** KFooBase ***/ class KFooBasePrivate { public:

   virtual ~KFooBasePrivate() { }
   int someInteger;


KFooBase::KFooBase(QObject *parent)

   : QObject(parent), d_ptr(new KFooBasePrivate)

{ }

KFooBase::KFooBase(KFooBasePrivate &dd, QObject *parent)

   : QObject(parent), d_ptr(&dd)

{ }

KFooBase::~KFooBase() {

   delete d_ptr;


int KFooBase::someInteger() const {

   return d->someInteger;


/*** KFooDerived ***/

class KFooDerivedPrivate: public KFooBasePrivate { public:

   int anotherInteger;


KFooDerived::KFooDerived(QObject *parent)

   : KFooBase(*new KFooDerivedPrivate, parent)

{ }

KFooDerived::KFooDerived(KFooDerivedPrivate &dd, QObject *parent)

   : KFooBase(dd, parent)

{ }

KFooDerived::~KFooDerived() {

   /* no need to delete the d-pointer! */


int KFooDerived::sumOfIntegers() const {

   return d->someInteger + d->anotherInteger;


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: QT_NO_CAST_FROM_ASCII, QT_NO_CAST_TO_ASCII, QT_NO_KEYWORD. So don't forget QLatin1String.
  • No C casts in the header. Use static_cast if types are known. Use qobject_cast instead of dynamic_cast if types are QObject based. dynamic_cast is not only slower, but is also unreliable across shared libraries.
  • In general, check your code for common mistakes.

These recommendations are also true for code that are not in headers.


Try to avoid meaningless boolean parameters in functions. Example of a bad boolean argument: static QString KApplication::makeStdCaption( const QString &caption,

                                            bool withAppName,
                                            bool modified);

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 QFlags. If the options only apply to one function, call the enum FunctionNameOption and the QFlags typedef FunctionNameOptions. 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 { public:

   /* [...] */
   enum StandardCaptionOption {
        * Indicates to include the application name
       WithApplicationName = 0x01,
        * Note in the caption that there is unsaved data
       Modified = 0x02
    * 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 StandardCaptionOptions& options = WithApplicationName);
   /* [...] */

}; Q_DECLARE_OPERATORS_FOR_FLAGS(KApplication::StandardCaptionOptions)

Const References

Each object parameter that is not a basic type (int, float, bool, enum, or pointers) should be passed by reference-to-const. 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 Q_SIGNALS and Q_SLOTS instead of signals and slots. They are syntactically equivalent and should be used to avoid conflicts with boost signals, and with python's use of "slots" in its headers.


Consider using Q_PROPERTY for properties. The reason is that properties (especially thoses marked SCRIPTABLE) will be accessible through the javascript interface.

If you follow the propname / setPropname naming sheme, moc sets a special flag for the QMetaProperty.

Explicit Constructors

For each constructor (other than the copy constructor), check if you should make the constructor explicit 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:

  1. 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.


static QString foo; // wrong - object might not be constructed static QString bar("hello"); // as above static int foo = myInitializer(); // myInitializer() might not be called


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 Q_GLOBAL_STATIC to create global static objects which will be initialized the first time you use them.

Signal and Slot Normalization

Since QObject::connect uses a string-based comparison of the function signature, it requires some normalization to take place. It does that automatically for you, but it takes some CPU time, so, if it doesn't hurt your code's readability, normalize manually your SIGNAL and SLOT entries.

For example, you may have the following code: QObject::connect(this, SIGNAL( newValue(const QString&,

                                       const MyNamespace::Type&) ),
                other, SLOT( value(const QString &) ));

It would be preferrable to write as follows: QObject::connect(this, SIGNAL(newValue(QString,Type)),

                other, SLOT(value(QString)));

Note the absence of namespace markers, extra whitespace and the reduction of pass-by-reference-to-const parameters to simple pass-by-value ones. The normalization may involve other transformations, but these are the most common ones. To be sure what the proper normalization is, read the .moc file generated for the class.

Note: If you are unsure about the normalization, don't do it. Let QObject do it for you (the performance penalty is negligible in most cases).


Every class and method should be well documented. Read the 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 Licensing Policy, kdelibs code must be licensed under the LGPL, BSD, or X11 license.

Author: Olivier Goffart March 2006

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