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### 15. Programming Guidelines

Close to the end of this handbook, I want to summarize several issues that programmers should watch out while coding. These are mostly C++ programming tips that relate to KDE and Qt programming especially and are taken from the KDE Developer's Center which can be found on the Internet at http://www.kde.org.

Filenames

  First of all, when creating sourcefiles, you should always use lowercase filenames. KDevelop supports this guideline already if you stick to its filename aut-suggestion. This makes it easier for other developers to remember what source files to look for when they have to debug your application.

Classnames

  The classnaming for KDE projects is recommended to be:

  - class names should begin with a prefixed K followed by the name of the class by purpose (your choice). This would be e.g. `KMyWidget` for an application specific widget.
  - the class members should always begin with lowercase letters, followed by uppercase beginnings for the next word, e.g. `myWidgetPointer()`
  - methods that return a private members value shouldn't use the `get`-prefix. You should prefer using a descriptive name for those types of classmembers. Example: `b_myboolean` is a private member. The method returning the current value would be e.g. `myBoolean()`.

File access within code

  Hardcoding any path should be avoided by using the KDE File System Standard. You only have to watch the installation path for your files by the according macros of the `Makefile.am` as described in this handbook. Within the code, you should use the methods of `KApplication` to retrieve the actual path.

Class documentation

  Another thing already mentionend is class documentation. You should stick to use `KDoc` formatting rules as they are used by all KDE developers to document their classes. You should at least add a single line to all of your classmembers for yourself to remember the prurpose and for others to reuse code. The code-reuse by the GPL makes much more sense if you know where to find an already existing solution if classes are documented. The Qt library referece is a good example of well-documented interfaces, though it doesn't use `KDoc`.

Use `new` to create widgets

  Within your implementation, you should always prefer to create widgets on the heap with `new`. The Qt library has a nice habbit to automatically delete all child widgets you created with `new`, so you don't ever have to use `delete` again in those cases. This is one of the most important practical features of the Qt library and you should make wide use of this.

Debugging

  When it comes to debugging, you should make use of the macros `KDebug` provides. Those are similar to the Qt macros, but can be retrieved by the keycode STRG+ALT+F12. See the KDE Library Reference Guide for more information about the event filtering of these Macros. You could as well use `assert()`, but should try to be consistent with your debugging code.

`const`-declarations

  Further, you should use `const` declarations for member functions that should or do not change any private member. This would be the case for all methods that only return the current value of a private member. This avoids changing the value accidently and will catch those logical errors at compile time. Now, towards initializing const members you should stick to do that together with using `static` in the declaration and initialize the value outside the constructor like this:

  > `

  >     class foo {
  >             static const int value;
  >     };

  >     const foo::value = 10;

  > `

  ANSI C++ allows to initialize the member inside the constructor but you should avoid this as a few compilers are not capable of this feature.

Virtual methods

  As explained in section User Interaction, you should stick to the access rights and the declaration by `virtual` when overwriting virtual methods. At least you shouldn't reduce the access of a virtual method from protected to private.

Forward declarations

  Class-headers should be included where you dereference any object or instance of a class in your sourcecode. That means if your class uses a member of another class, replace the #include directive with a forward declaration of the class, e.g instead of:

  > `

  >     #include <qpushbutton.h>

  >     class KMyWidget:public QWidget
  >     {

  >     private:
  >       QPushButton* ok_button;
  >     };

  > `

  you should prefer to only declaring the class `QPushButton` in the header file:

  > `

  >     class QPushButton;

  >     class KMyWidget:public QWidget
  >     {

  >     private:
  >       QPushButton* ok_button;
  >     };

  > `

  and place the include directive into the according sourcefile where e.g. the instance `ok_button` is dereferenced with any method of the class `QPushButton`. This saves compile time at any rate, especially if you're using instances of classes that your're working on. The compiler will recompile all souces that include the header file if you made any changes on the interface of the class, therefore a simple addition of a method that only returns an internal value will lead to a recompilation of all sources tha include the header file of the class.

Unused Parmeter Warnings and default arguments

  Also you should leave out formal parameters of methods that don't necessarily require the actual parameter to work. This avoids the `unused parameter` warnings of your compiler when he sees a method that retrieves a formal parameter but doesn't use it in its implementation. Usually, you will set some default arguments for several methods. Those should always be placed in the declaration of the class member instead of setting them in the member implementation.

Using `config.h`

  KDevelop projects as well as any other project that is using `autoconf` to create a `configure`-script produce a file `config.h` after executing the `configure`-script on the target machine. The values found by `configure` are listed there and can be used within the sourcecode. The directive to include the `config.h` file is:

  > `

  >     #ifdef HAVE_CONFIG_H
  >     #include <config.h>
  >     #endif

  > `

  One of the most recently used entries of `config.h` is probably the type-definition of `bool` for compilers that don't comply with the newest ANSI C++ draft.

Use `0` instead of `NULL`

  You should stickt to using `0` directly instead of `NULL` for preset values like the Qt and KDE libraries already do. This increases portablility of your applications towards different compilers that have problems with `NULL`.

Temporaries

  You should declare temporary instances always before using them. This is generally considered better than direct use. Example:

  > `

  >     // Don't:
  >     for( int i=0; i<n; i++){
  >       //do something
  >       };

  >     // Do:
  >     int i;

  >     for(i=0; i<n; i++){
  >       //do something
  >       };

  > `

  This also counts on using temporaries in function calls:

  > `

  >     // Don't:
  >     setColor( &(QColor(black)) );

  >     // Do:
  >     QColor color(black);
  >     setColor( &color );

  > `

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