QGIS Coding Standards¶

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 argments (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.

• int maximumValue() const
• const LayerSet& layers() const
• QString title() const (QString is implicitly shared)
• QList< QgsMapLayer* > layers() const (QList is implicitly shared)

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.

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

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.
 *
 ***************************************************************************/

Tabs¶

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

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
  ...
}

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 )

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¶

• Effective Modern C++, Scott Meyers
• More Effective C++, Scott Meyers
• Effective STL, Scott Meyers
• Design Patterns, GoF

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