Development/Tutorials/KDevelop/Creating a class template

 

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Creating a project template

Tutorial Series	KDevelop Templates
Previous	Grantlee for theme artists, Django template language
What's Next
Further Reading	Class template specification

Introduction

In this tutorial, we will create a template for a basic C++ class. It will demonstrate using the Grantlee template language, as well features specific to KDevelop class templates.

We chose C++ because even the simplest class definition often consists of two separate files, header and implementation. It is possible to include any number of files in a class template (there should be at least one, obviously).

Creating the directory structure

Note

There is a project template for a C++ class template available here. The resulting project is similar to what we will create here. You may use it to create the files but still follow this tutorial for explanation.

We will start with an empty directory. Name it something unique, like kdev_class_tutorial.

The description file

Every class template needs a description file. It is a regular desktop file with an extension of .desktop. Its base name must match the name of the generated archive, so let's call it kdev_class_tutorial.desktop. Paste in these contents:

[General]
Name=Tutorial
Comment=An example C++ class template, suitable for a tutorial
Category=C++
Files=Header,Implementation

[Header]
Name=Header
File=class.h
OutputFile={{ name }}.h

[Implementation]
Name=Implementation
File=class.cpp
OutputFile={{ name }}.cpp

You can see there are three sections in the description file: one for general properties, and one for each output file. In order for a file to be generated, it has to be listed in the Files entry, as well as have its own section with the same name.

In the output file sections, we already used template variables: {{ name }} will be replaced with the class name. The actual generated header file will not be {{ name }}.h, but rather Exmample.h, assuming we name our class Example.

The class header file

Now, let's move on to the class declaration.

Template header

In most free software projects, source files start with a license header. This header varies with the chosen license, author, and the programming language's comment characters. When using our template, the user choose one of the suggested licenses or write his own. In either case, the full text of the license header is available to the template as the license variable.

Since C++ has block-style comments, we could write the license simply as

/*
{{ license }}
*/

However, it might be easier to read if the license had some kind of formatting. For this purpose, KDevPlatform includes a template filter library that makes it possible to write a nicely-formatted license header. We can use it like this:

{% load kdev_filters %}
/*
{{ license|lines_prepend:" * " }}
 */

The lines_prened filter prepends its argument (in our case " * ") to every line of its input (in our case the license). This can produce licenses like the following

/*
 * This file is part of KDevelop
 * Copyright 2012 Example Developer <[email protected]>
 */

Include guards

If our file is to be included somewhere, it is good practice to provide an include guard macro. KDevPlatform ships a convenience template for generating such a macro called include_guard_cpp.txt. We can use it like this

#ifndef {% include "include_guard_cpp.txt" %}
#define {% include "include_guard_cpp.txt" %}

// Class declaration goes here

#endif // {% include "include_guard_cpp.txt" %}

Note

The included template will expand into an uppercase class name with an appended _H, which could also be achieved by typing {{ name|upper }}_H. However, the include guard template also supports namespaces.

Note

See the Django documentation for details about the include command

Class declaration

Information about inheritance is stored in the baseClasses variable. It is a list that can be iterated using a for loop. Each list element has two properties, inheritanceMode and baseType, which can be accessed using the dot notation, like class members in C++ or Python.

class {{ name }}
{% if baseClasses %}
    : {% for b in baseClasses %}b.inheritanceMode b.baseType{% if not forloop.last %}, {% endif %}{% endfor %}
{% endif %}
{

If our new class inherites from KFoo and KBar, and both inheritances are public, the output from such a template looks like

class Example
    : public KFoo, public KBar
{

The template text could also be written in one line, so that the inheritance declarations (as possible the opening brace) would be on the same line as the class name. It is only written out this way in the tutorial for better readability. Both versions are valid C++.

Members

Classes can have two kinds of members: variables and functions. They are available to templates as members and functions variables, respectively. Both are lists and are intended to be iterated.

For simplicity, we will ignore access modes and various function modifiers (const, virtual, static, etc).

{% for f in functions %}
  {% with f.arguments as arguments %}
    {{ f.returnType|default:"void" }} {{ f.name }}({% include "arguments_types_names.txt" %});
  {% endwith %}
{% endfor %}

Every function in C++ has a return type, if it returns nothing that type is void, which is what the default filter assures. Next, we have included another templates, called arguments_types_names.txt, which creates an argument list for language that require both argument types and names to be specified. This template requires a variables called arguments, which we created within the {% with %} statement.

{% for m in members %}
  {{ m.type }} {{ m.name }};
{% endfor %}

Member variables are simple, they only require a type and a name.

Whole header

The entire header file looks like this

{% load kdev_filters %}
/*
{{ license|lines_prepend:" * " }}
 */

#ifndef {% include "include_guard_cpp.txt" %}
#define {% include "include_guard_cpp.txt" %}

class {{ name }}
{% if baseClasses %}
    : {% for b in baseClasses %}b.inheritanceMode b.baseType{% if not forloop.last %}, {% endif %}{% endfor %}
{% endif %}
{
public:
{% for f in functions %}
  {% with f.arguments as arguments %}
    {{ f.returnType|default:"void" }} {{ f.name }}({% include "arguments_types_names.txt" %});
  {% endwith %}
{% endfor %}

{% for m in members %}
  {{ m.type }} {{ m.name }};
{% endfor %}
};

#endif // {% include "include_guard_cpp.txt" %}

The implementation file

Class implementation has the same license as the header, and needs no include guard. Instead, it has to include the header file and contain bodies for member functions.

Including the header can be done using variables. For every output file in the template, KDevelop adds two variables: output_file_foo with the relative path to file Foo, and output_file_foo_absolute with the absolute path. For C++ includes, we will use the relative path.

#include {{ output_file_header }}

Member functions difinitions are very similar to their declaration, except that static and virtual modifiers are dropped and the class name is prepended to function names. Therefore we can copy the function declaration code and make a slight modification.

{% for f in functions %}
  {% with f.arguments as arguments %}
    {{ f.returnType|default:"void" }} {{ name }}::{{ f.name }}({% include "arguments_types_names.txt" %});
  {% endwith %}
  {

  }
{% endfor %}

The function body is left empty and has to be implemented by the user.

Installing the template

Now we have all the template contents prepared. We only need to compress the directory and make it available to KDevelop.

This can be done in a similar way as installing project templates. The only differences are:

• templates should be saved into ${PREFIX}/share/apps/kdevfiletemplates/templates
• they can be loaded from the Code -> Create class from template dialog
• the CMake macro is named kdevplatform_add_file_templates instead of kdevplatform_add_app_templates.