QGIS Coding Standards

These standards should be followed by all QGIS developers.

Classes

Names

Class in QGIS begin with Qgs and are formed using camel case.

Examples:

  • QgsPoint
  • QgsMapCanvas
  • QgsRasterLayer

Members

Class member names begin with a lower case m and are formed using mixed case.

  • mMapCanvas
  • mCurrentExtent

All class members should be private. Public class members are STRONGLY discouraged. Protected members should be avoided when the member may need to be accessed from Python subclasses, since protected members cannot be used from the Python bindings.

Mutable static class member names should begin with a lower case s, but constant static class member names should be all caps:

  • sRefCounter
  • DEFAULT_QUEUE_SIZE

Accessor Functions

Class member values should be obtained through accesssor functions. The function should be named without a get prefix. Accessor functions for the two private members above would be:

  • mapCanvas()
  • currentExtent()

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

Functions

Function names begin with a lowercase letter and are formed using mixed case. The function name should convey something about the purpose of the function.

  • updateMapExtent()
  • setUserOptions()

For consistency with the existing QGIS API and with the Qt API, abbreviations should be avoided. E.g. setDestinationSize instead of setDestSize, setMaximumValue instead of setMaxVal.

Acronyms should also be camel cased for consistency. E.g. setXml instead of setXML.

Function Arguments

Function arguments should use descriptive names. Do not use single letter arguments (e.g. setColor( const QColor& color ) instead of setColor( const QColor& c )).

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 ).

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)

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 best practice and code samples.

Methods

Method descriptions should be written in a descriptive form, using the 3rd person. Methods require a \since tag that defines when they have been introduced. You should add additional \since tags for important changes that were introduced later on.

/**
 * Cleans the laundry by using water and fast rotation.
 * It will use the provided \a detergent during the washing programme.
 *
 * \returns True if everything was successful. If false is returned, use
 * \link error() \endlink to get more information.
 *
 * \note Make sure to manually call dry() after this method.
 *
 * \since QGIS 3.0
 * \see dry()
 */

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.

/**
 * \ingroup core
 * Represents points on the way along the journey to a destination.
 *
 * \since QGIS 2.20
 */
class QgsWaypoint
{
  /**
   * Holds information about results of an operation on a QgsWaypoint.
   *
   * \since QGIS 3.0
   */
  struct OperationResult
  {
    QgsWaypoint::ResultCode resultCode; //!< Indicates if the operation completed successfully.
    QString message; //!< A human readable localized error message. Only set if the resultCode is not QgsWaypoint::Success.
    QVariant result; //!< The result of the operation. The content depends on the method that returned it. \since QGIS 3.2
  };
};

Qt Designer

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

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.

C++ Files

Names

C++ implementation and header files should have a .cpp and .h extension respectively. Filename should be all lowercase and, in the case of classes, match the class name.

Example: Class QgsFeatureAttribute source files are qgsfeatureattribute.cpp and qgsfeatureattribute.h

Note

In case it is not clear from the statement above, for a filename to match a class name it implicitly means that each class should be declared and implemented in its own file. This makes it much easier for newcomers to identify where the code is relating to specific class.

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. To use it, copy it from doc/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.

Variable Names

Local variable names begin with a lower case letter and are formed using mixed case. Do not use prefixes like my or the.

Examples:

  • mapCanvas
  • currentExtent

Enumerated Types

Enumerated types should be named in CamelCase with a leading capital e.g.:

enum UnitType
{
  Meters,
  Feet,
  Degrees,
  UnknownUnit
};

Do not use generic type names that will conflict with other types. e.g. use UnkownUnit rather than Unknown

Global Constants & Macros

Global constants and macros should be written in upper case underscore separated e.g.:

const long GEOCRS_ID = 3344;

Qt Signals and Slots

All signal/slot connects should be made using the “new style” connects available in Qt5. Futher information on this requirement is available in QEP #77.

Avoid use of Qt auto connect slots (i.e. those named void on_mSpinBox_valueChanged). Auto connect slots are fragile and prone to breakage without warning if dialogs are refactored.

Editing

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

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

Indentation

Source code should be indented to improve readability. There is a scripts/prepare-commit.sh that looks up the changed files and reindents them using astyle. This should be run before committing. You can also use scripts/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).

Braces

Braces should start on the line following the expression:

if(foo == 1)
{
  // do stuff
  ...
}
else
{
  // do something else
  ...
}

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 sourcecode e.g.

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

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

  signals:
    /**
     * This signal will is 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();
}

SIP Bindings

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

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 can be found in tests/scripts/sipifyheader.h.

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

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

Older files for which the automatic creation is not enabled yet are listed in python/auto_sip.blacklist.

Improving sipify script

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

Coding Style

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

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

Prefer Having Constants First in Predicates

Prefer to put constants first in predicates.

0 == value instead of value == 0

This will help prevent programmers from accidentally using = when they meant to use ==, which can introduce very subtle logic bugs. The compiler will generate an error if you accidentally use = instead of == for comparisons since constants inherently cannot be assigned values.

Whitespace Can Be Your Friend

Adding spaces between operators, statements, and functions makes it easier for humans to parse code.

Which is easier to read, this:

if (!a&&b)

or this:

if ( ! a && b )

Note

scripts/prepare-commit.sh will take care of this.

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();

Indent access modifiers

Access modifiers structure a class into sections of public API, protected API and private API. Access modifiers themselves group the code into this structure. Indent the access modifier and declarations.

class QgsStructure
{
  public:
    /**
     * Constructor
     */
     explicit QgsStructure();
}

Book recommendations

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

Credits for contributions

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