1. QGIS Coding Standards

QGIS coding standards are described in the policy document available at [QEP #314](https://github.com/qgis/QGIS-Enhancement-Proposals/blob/master/qep-314-coding-style.md). All developers are required to follow those policies. Please note that QEP #314 is a live document, and that these policies may change over time.

1.1. Classes

1.1.1. Accessor Functions

Ensure that accessors are correctly marked with const. Where appropriate, this may require that cached value type member variables are marked with mutable.

1.1.2. Function Arguments

Pay careful attention to when arguments should be passed by reference. Unless argument objects are small and trivially copied (such as QPoint objects), they should be passed by const reference. For consistency with the Qt API, even implicitly shared objects are passed by const reference (e.g. setTitle( const QString& title ) instead of setTitle( QString title ).

1.1.3. Function Return Values

Return small and trivially copied objects as values. Larger objects should be returned by const reference. The one exception to this is implicitly shared objects, which are always returned by value. Return QObject or subclassed objects as pointers.

  • int maximumValue() const

  • const LayerSet& layers() const

  • QString title() const (QString is implicitly shared)

  • QList< QgsMapLayer* > layers() const (QList is implicitly shared)

  • QgsVectorLayer *layer() const; (QgsVectorLayer inherits QObject)

  • QgsAbstractGeometry *geometry() const; (QgsAbstractGeometry is abstract and will probably need to be casted)

1.2. API Documentation

It is required to write API documentation for every class, method, enum and other code that is available in the public API.

QGIS uses Doxygen for documentation. Write descriptive and meaningful comments that give a reader information about what to expect, what happens in edge cases and give hints about other interfaces he could be looking for, best practices and code samples.

1.2.1. Members Variables

Member variables should normally be in the private section and made available via getters and setters. One exception to this is for data containers like for error reporting. In such cases do not prefix the member with an m.

1.3. Qt Designer

1.3.1. Generated Classes

QGIS classes that are generated from Qt Designer (ui) files should have a Base suffix. This identifies the class as a generated base class.

Examples:

  • QgsPluginManagerBase

  • QgsUserOptionsBase

1.3.2. Dialogs

All dialogs should implement tooltip help for all toolbar icons and other relevant widgets. Tooltips add greatly to feature discoverability for both new and experienced users.

Ensure that the tab order for widgets is updated whenever the layout of a dialog changes.

1.4. C++ Files

1.4.1. Standard Header and License

Each source file should contain a header section patterned after the following example:

/***************************************************************************
  qgsfield.cpp - Describes a field in a layer or table
  --------------------------------------
  Date : 01-Jan-2004
  Copyright: (C) 2004 by Gary E.Sherman
  Email: sherman at mrcc.com
/***************************************************************************
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 ***************************************************************************/

Note

There is a template for Qt Creator in git repository. To use it, copy it from qt_creator_license_template to a local location, adjust the mail address and - if required - the name and configure QtCreator to use it: Tools ► Options ► C++ ► File Naming.

1.5. Editing

Any text editor/IDE can be used to edit QGIS code, providing the following requirements are met.

1.5.1. Tabs

Set your editor to emulate tabs with spaces. Tab spacing should be set to 2 spaces.

Note

In vim this is done with set expandtab ts=2

1.5.2. Indentation

Source code should be indented to improve readability. There is a prepare_commit.sh file that looks up the changed files and reindents them using astyle. This should be run before committing. You can also use astyle.sh to indent individual files.

As newer versions of astyle indent differently than the version used to do a complete reindentation of the source, the script uses an old astyle version, that we include in our repository (enable WITH_ASTYLE in cmake to include it in the build).

1.6. API Compatibility

There is API documentation for C++.

We try to keep the API stable and backwards compatible. Cleanups to the API should be done in a manner similar to the Qt source code e.g.

class Foo
{
  public:
    /**
     * This method will be deprecated, you are encouraged to use
     * doSomethingBetter() rather.
     * \deprecated use doSomethingBetter()
     */
    Q_DECL_DEPRECATED bool doSomething();

    /**
     * Does something a better way.
     * \note added in 1.1
     */
    bool doSomethingBetter();

  signals:
    /**
     * This signal will be deprecated, you are encouraged to
     * connect to somethingHappenedBetter() rather.
     * \deprecated use somethingHappenedBetter()
     */
#ifndef Q_MOC_RUN
    Q_DECL_DEPRECATED
#endif
    bool somethingHappened();

    /**
     * Something happened
     * \note added in 1.1
     */
    bool somethingHappenedBetter();
}

1.7. SIP Bindings

Some of the SIP files are automatically generated using a dedicated script.

1.7.1. Header pre-processing

All the information to properly build the SIP file must be found in the C++ header file. Some macros are available for such definition:

  • Use #ifdef SIP_RUN to generate code only in SIP files or #ifndef SIP_RUN for C++ code only. #else statements are handled in both cases.

  • Use SIP_SKIP to discard a line

  • The following annotations are handled:

    • SIP_FACTORY: /Factory/

    • SIP_OUT: /Out/

    • SIP_INOUT: /In,Out/

    • SIP_TRANSFER: /Transfer/

    • SIP_PYNAME(name): /PyName=name/

    • SIP_KEEPREFERENCE: /KeepReference/

    • SIP_TRANSFERTHIS: /TransferThis/

    • SIP_TRANSFERBACK: /TransferBack/

  • private sections are not displayed, except if you use a #ifdef SIP_RUN statement in this block.

  • SIP_PYDEFAULTVALUE(value) can be used to define an alternative default value of the python method. If the default value contains a comma ,, the value should be surrounded by single quotes '

  • SIP_PYTYPE(type) can be used to define an alternative type for an argument of the python method. If the type contains a comma ,, the type should be surrounded by single quotes '

A demo file, sipifyheader.h, is also available.

1.7.2. Generating the SIP file

The SIP file can be generated using a dedicated script. For instance:

scripts/sipify.pl src/core/qgsvectorlayer.h > python/core/qgsvectorlayer.sip

To automatically generate the SIP file of a newly added C++ file sip_include.sh needs to be executed.

As soon as a SIP file is added to one of the source file (core_auto.sip, gui_auto.sip or analysis_auto.sip), it will be considered as generated automatically. A test on will ensure that this file is up to date with its corresponding header.

To force recreation of SIP files, sipify_all.sh shall be executed.

1.7.3. Improving sipify script

If some improvements are required for sipify script, please add the missing bits to the demo file sipifyheader.h and create the expected header sipifyheader.expected.sip. This will also be automatically tested as a unit test of the script itself.

1.8. Settings

QGIS code base offers a mechanism to declare, register and use settings.

  • settings should be defined using one of the available implementations (QgsSettingsEntryString, QgsSettingsEntryInteger, …).

  • settings must be integrated in the settings tree (QgsSettingsTree), this is automatically done when using the constructor with a parent node (QgsSettingsTreeNode).

  • they are declared as const static either in a dedicated class or in the registry directly (core, gui, app, …).

  • the setting key should be using a kebab-case.

1.9. Coding Style

Here are described some programming hints and tips that will hopefully reduce errors, development time and maintenance.

1.9.1. Where-ever Possible Generalize Code

If you are cut-n-pasting code, or otherwise writing the same thing more than once, consider consolidating the code into a single function.

This will:

  • allow changes to be made in one location instead of in multiple places

  • help prevent code bloat

  • make it more difficult for multiple copies to evolve differences over time, thus making it harder to understand and maintain for others

1.9.2. Put commands on separate lines

When reading code it’s easy to miss commands, if they are not at the beginning of the line. When quickly reading through code, it’s common to skip lines if they don’t look like what you are looking for in the first few characters. It’s also common to expect a command after a conditional like if.

Consider:

if (foo) bar();

baz(); bar();

It’s very easy to miss part of what the flow of control. Instead use

if (foo)
  bar();

baz();
bar();

1.9.3. Book recommendations

You should also really read this article from Qt Quarterly on designing Qt style (APIs)

1.10. Credits for contributions

Contributors of new functions are encouraged to let people know about their contribution by: