Policies/Library Code Policy

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Revision as of 21:15, 9 December 2006 by Dhaumann (talk | contribs) (svn r610179: remove wrong statement.)

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.

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.

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 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 */
    private:
        KFooPrivate * const d;
};

In the .cpp file: 

class KFooPrivate
{
    public:
        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.

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.

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 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 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);
        /* [...] */
};
Q_DECLARE_OPERATORS_FOR_FLAGS(KApplication::MakeStandardCaptionOptions)

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.

Properites

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: 

#include <kfoobase.h>
class KBar;
class KFoo : public KFooBase
{
    public:
        /* [...] */             
        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 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).


Documentation

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 Mart 2006

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