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This page is obsolete and deprecated. For Qt 6 equivalent, see [[Writing Example Documentation and Tutorials]]
 
= Writing Qt Examples =
This page is part of the [[QtWritingGuidelines | Qt Writing Guidelines]].
 
This document is about how to write an example for Qt. When you have finished writing an example and its documentation, see the [[QDocExamples | Integrating Examples]] page for information on making it work with Qt's build and documentation systems.
 
== Example Documentation ==
 
* created with command
* title should end with "Example"
 
The [[QDocExamples | Integrating Examples]] page explains how the command works with the QDoc variables to form URLs.
 
Requires the following:
 
'''1.''' A thumbnail image to represent its content. It could be the main screen or a specific area of the screen (for example, just the toolbar).
 
'''2.''' A 3-sentence description (minimum) consisting of ''how to find the example'' and ''what the example is''.
The description should answer the following:
* if the example is runnable from Qt Creator, or from the install directory, or somewhere else.
* what the user should see when running the example.
* should the user: see an application screen? or output from the command-line? a downloaded image or just a rectangle popping up?
* the primary topic the example addresses. ''Is it a Qt Network example or a Network and Connectivity example?''
 
Examples:
* "{Image Download} example is run by opening the "Image Download Example" from within Qt Creator."
* "The {NAME_OF_EXAMPLE} example pops up a screen which shows an image downloaded from the internet."
* "{NAME_OF_EXAMPLE} shows how to download an image using QNetworkAccessManager and how to use the {Container Classes}{container classes}."
'''Note:''' It is acceptable to refer to the name of the example within the example, but use the command to mark the title.
 
'''3.''' Link to relevant material.
* Link to the main Qt topic. The topics are listed in http://doc.qt.io/qt-5/overviews-main.html
* Relate the example to Use ''' to relate to different example groups.
''' Make sure that the example is reachable from other articles as well.
 
'''Note:''' For simple examples, adding the following ''' creates a ''Running the Example'' section. This is for convenience and should not be used if the text is not appropriate.
<code>
examples-run.qdocinc
</code>
This will include the text (taken from qtbase/doc/global):
<code>
\section1 Running the Example
 
To run the example from {Qt Creator Manual}{Qt Creator}, open the Welcome
mode and select the example from Examples. For more information, visit
{Qt Creator: Building and Running an Example}{Building and Running an Example}.
</code>
 
== "Tutorials", "Walkthroughs", or Detailed Example Documentation ==
''' ''Tutorials'', ''walkthroughs'' can be created with the ''' command or by the''' command (if there is no source-code included).
* These guidelines also apply to example documentation that have detailed documentation.
 
These are considered tutorials:
http://doc.qt.io/qtcreator-3.0/creator-running-targets.html
https://doc.qt.io/qt-5/qtwidgets-widgets-digitalclock-example.html
 
Requires the following:
'''1.''' The same content as an example documentation. See ''Example Documentation'' section.
 
'''2.''' Enhanced content. Choose at least one of a or b, or both:
* a. Show code snippets and a paragraph describing the snippet in addition to 1.
* b. a list of steps
 
'''3.''' For tutorials, or walkthroughs, use an explicit title or address a specific use. Choose either a or b, but not both:
* a. Add "Tutorial" at the end of the title
* b. Begin the title with a gerund (-ing words).
 
Examples:
* mage Download Tutorial
* ownloading an Image
 
 
== Writing Style ==
What is considered good writing style is inherently subjective. There is still some good advice that it normally pays to follow. Let's look at a few issues when Qt example documentation is concerned.
 
Try not to simply state what the code does line for line (the reader can easily figure that out by looking at the code and refer to the API documentation if he/she needs to). Focus on how things work behind the scenes and on issues that it is not easy to read from the code. On the other hand, we do not leave snippets uncommented; if you have nothing to say about a particular snippet, it is better to state what the code does line for line than leaving the space between two snippets empty. When implementing an advanced example, you can generally assume that the reader is familiar with basic Qt programming and concepts. So there is no need to explain everything in detail; again, focus on what the example tries to teach. For instance, an example that shows how to paint items in an item view does not need to explain what an item view is. There are no absolute rules to follow when deciding what needs explaining and what does not.
 
=== Example Code Guidelines ===
The example code should:
* follow the [[Coding Conventions]]
* should be compileable by the end-user
* be easier to understand
* runnable by the end-user
 
== File Names ==
The examples are as modularized as Qt itself. Each submodule can have a <code>examples</code> directory containing examples. Examples within a submodule are organized in a directory hierarchy, ''e.g.'' <code>qtbase/examples/itemviews/dirview</code> or <code>qtbase/examples/widgets/tetrix</code>. The hierarchy is there only to group related examples together. The examples should have unique names even if the hierarchy didn't exist. All examples should be at the same depth in the hierarchy.
 
The example's <code>.pro</code> file should have the same name as the directory it's in (''e.g.'' <code>tetrix.pro</code> for <code>widgets/tetrix</code>). The example name should only contain lowercase letters and digits, no hyphens, no underscores.
 
In general, each class should be defined in its own header file and implemented in a corresponding .cpp file, ''e.g.'' MainWindow in mainwindow.h and mainwindow.cpp. Try to avoid defining more than one class per header file, since it confuses Java people and it's usually unnecessary, unless you have lots of small classes (''e.g.'' an undo/redo framework, with one class per type of action).
 
Local classes may be defined in a .cpp file. Do not use the <code>#include "foo.moc"</code> trick in an example. (Don't worry if you don't understand what I'm talking about.)
 
Always put your main() function in a separate file called main.cpp, and try to keep it as simple as possible. Most users have understood that main() is usually boring and seldom take the time to read it. It's therefore not the place to do unusual stuff.
 
* '''Exception:''' If your example is very simple and requires only one file containing a main() function and (hopefully) a few other functions, call this file the same as your example (''e.g.'' tetrix.cpp), not main.cpp.
 
If you need images or other resources, use the resource mechanism. Call the resource file the same thing as the example, but with the .qrc extension (''e.g.'' tetrix.qrc). See the mainwindows/application example for an example of how to do this right.
 
=== Include Files and Forward Declarations ===
Use the Qt 4/Qt 5 syntax for #include files, and order them alphabetically:
 
<code>
#include <QApplication>
#include <QComboBox>
#include <QStringList>
</code>
 
In your .h files, try to include as few headers as possible, using forward declarations when feasible in alphabetical order. Unless the example is meant to be a very simple one, appealing to a novice programmer, make your the example "Qt namespace aware" by wrapping the declarations of Qt classes in QT_BEGIN_NAMESPACE/QT_END_NAMESPACE:
 
<code>
#include <QDialog>
 
class MyClass;
class YourClass;
 
QT_BEGIN_NAMESPACE
class QCheckBox;
class QLineEdit;
class QPushButton;
QT_END_NAMESPACE
</code>
 
Best practice is to avoid "module includes", such as <code>#include <QtGui></code>, and use individual <code>#include <QClass></code> preprocessor directives instead. However, as the module includes reduces visual clutter you may use the <code><QtGui></code>, <code><QtXml></code>, etc., headers.
 
If you need standard C++ headers, use the new style of headers (''e.g.'' <code><iostream></code>) and put a <code>using namespace std;</code> after all the includes in .cpp files. Never use <code>using</code> directives in global namespace in header files.
 
* '''Exception:''' For C headers, check whether you have used the .h name (<code><math.h></code>, not <code><cmath></code>) because the new name isn't supported by all compilers.
 
=== Include Order ===
Arrange <code>#includes</code> in blocks, from most-specfic to most-common to make sure your headers are as self-contained as possible. Within a block of similar <code>#includes</code>, try to keep the order alphabetically.
 
<code>
#include "mydialog.h"
#include "yourdialog.h"
 
#include <QtGui>
#include <QtNetwork>
 
#include <iostream>
 
using namespace std;
</code>
 
 
=== Variable Names ===
In non-trivial projects there are often special rules on the naming of member variables, like prefixing them with "_" or "m_", or suffixing with "_". If your example can live without such markers, that's fine. If it can't, use the "m_" convention.
 
<code>
void MyClass::setColor(const QColor &color)
{
m_color = color;
}
</code>
 
or
 
<code>
void MyClass::setColor(const QColor &c)
{
color = c;
}
</code>
 
Use the same variable name for parameters in header files as in the implementation. Don't drop the parameter names even if C++ lets you do so.
 
 
=== C++ Preprocessor ===
Use the preprocessor exclusively to protect against multiple inclusion, ''e.g.''
 
<code>
#ifndef FOO_H
#define FOO_H
 
…
 
#endif
</code>
 
Don't use the QT_NO_xxx macros in examples. Instead, disable the example if it doesn't compile. We don't want the example source code, which is supposed to reflect end-user application code and be a model of clarity, to be littered with that junk.
 
== Breaking Long Lines ==
Example code should wrap at 80 characters, because it's often quoted in the documentation and not all web browsers can display (or print) many more characters than that. This limit is stricter than the standard Trolltech limit of 100.
 
The 80 character limit is liveable with. After all, Qt Quarterly is wrapped at 58 characters, and the entire Qt book is wrapped at 68 characters.
 
When breaking long lines, break right ''before'' an operator and try to align the lines in the standard Trolltech way:
 
<code>
int xxxxxx = alpha + (beta
* gamma);
</code>
 
Use a monospaced font when editing code. Never attempt to align code if you're using a proportional font.
 
This rule applies to basically all operators:
 
<code>
outStream << "Foo"
<< "Bar";
</code>
 
One exception: With <code>if</code> and <code>&&</code> or <code>||</code>, you can rapidly end up having an unfortunate alignment problem:
 
<code>
if (dsfljfsfskjldsjkljklsjdk
&& fdsljsjdsdljklsjsjkdfs
&& dsfljkdfjkldksdfjdjkfdksfdkjld)
sadjdjddadhsad;
</code>
 
The problem is that it's not clear where the if condition ends (the third line) and where the if body starts (the fourth line). My ad hoc solution is to add four spaces to the continuation lines:
 
<code>
if (dsfljfsfskjldsjkljklsjdk
&& fdsljsjdsdljklsjsjkdfs
&& dsfljkdfjkldksdfjdjkfdksfdkjld)
sadjdjddadhsad;
</code>
 
Fortunately, you don't need to do this for <code>else if</code> or <code>while</code>; unlike <code>if</code>, these aren't two-letter words.
 
<code>
while (dsfljfsfskjldsjkljklsjdk
&& fdsljsjdsdljklsjsjkdfs
&& dsfljkdfjkldksdfjdjkfdksfdkjld)
sadjdjddadhsad;
</code>
 
 
=== Spaces, Blank Lines, and Comments ===
Apart when breaking lines, avoid aligning things together. This looks really bad with proportional fonts and actually looks bad with monospaced fonts as well. ''I.e.'' avoid
 
<code>
x = rect.x();
y = rect.y();
width = rect.width();
height = rect.height();
</code>
 
Use blank lines consistently in your source files to separate functions in the .cpp file. Never use more than one blank line in a row, as this normally leads to inconsistent style.
 
Avoid comments. Documentation belongs exclusively in the example's walkthrough. (We don't have the resources to document the examples twice!) One case were comments are useful is when a parameter is unused:
 
<code>
void MandelbrotWidget::paintEvent(QPaintEvent * /* event */)
</code>
 
This is better than
 
<code>
void MandelbrotWidget::paintEvent(QPaintEvent''')
</code>
 
 
== Braces ==
If syntax:
 
<code>
if (x)
y;
</code>
 
Never:
 
<code>
if (x) y;
</code>
 
If y is complicated, you can use braces to make it clearer:
 
<code>
if (x) {
// do something strange
yyyyyyyyy = yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy
''zzzzzzzzzzzzzzzzzzzzzz;
}
</code>
 
Same thing applies to for and while loops:
 
<code>
for (int i = 0; i < n;i)
qDebug() << i;
</code>
 
In the interest of readability, use braces whenever things start getting sightly complicated:
 
<code>
for (int i = 0; i < n;i) {
for (int j = 0; j < n;j) {
for (int k = 0; k < n;k) {
data[i][j][k] = i'' j * k;
}
}
}
</code>
 
If you put braces around one case, do it around all:
 
<code>
if (x) {
foo;
} else if (y) {
bar;
baz;
} else {
blah;
}
</code>
 
The advantage of this style is that you get nicely aligned braces at the bottom of the code, corresponding to the indentation level.
 
=== Signal-Slot Connections ===
Always use the four- or five-parameter overload of the
<code>QObject::connect()</code> call:
 
<code>
connect(obj1, SIGNAL (foo()), this, SLOT (bar()));
</code>
 
This is clearer than
 
<code>
connect(obj1, SIGNAL (foo()), SLOT (bar()));
</code>
 
Beginners might think that we're connecting to obj1's bar() slot, when we actually are connecting to <code>this</code>.
 
Always specify the full types of the arguments, ''i.e.'' <code>const</code> <code>QString &</code> instead of <code>QString</code>, in the interest of keeping things simple for beginners. We'll mention that you can write the latter in the documentation, but stick to a consistent, simple subset of the Qt language in examples.

Latest revision as of 15:25, 20 November 2024

This page is obsolete and deprecated. For Qt 6 equivalent, see Writing Example Documentation and Tutorials

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