path: root/doc/docblocks.doc

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/*! \page docblocks Documenting the code
\tableofcontents{html,latex}

This chapter covers two topics:
1. How to put comments in your code such that doxygen incorporates them in
   the documentation it generates. 
   This is further detailed in the \ref specialblock "next section".
2. Ways to structure the contents of a comment block such that the output
   looks good, as explained in section \ref docstructure.

\section specialblock Special comment blocks 

A special comment block is a C or C++ style comment block with some 
additional markings, so doxygen knows it is a piece of structured text that
needs to end up in the generated documentation. The \ref cppblock "next" section
presents the various styles supported by doxygen.

For Python, VHDL, and Fortran code there are different commenting 
conventions, which can be found in sections \ref pythonblocks, \ref vhdlblocks, and
\ref fortranblocks respectively.

\subsection cppblock Comment blocks for C-like languages (C/C++/C#/Objective-C/PHP/Java)

For each entity in the code there are two (or in some cases three) types of descriptions, 
which together form the documentation for that entity; a *brief* description and *detailed*
description, both are optional. For methods and functions there is also a third
type of description, the so called *in body* description, which consists of 
the concatenation of all comment blocks found within the body of the method or function.

Having more than one brief or detailed description is allowed (but not recommended,
as the order in which the descriptions will appear is not specified).

As the name suggest, a brief description is
a short one-liner, whereas the detailed description provides longer, 
more detailed documentation. An "in body" description can also act as a detailed
description or can describe a collection of implementation details.
For the HTML output brief descriptions are also
used to provide tooltips at places where an item is referenced.

There are several ways to mark a comment block as a detailed description:
<ol>
<li> You can use the Javadoc style, which consist of a C-style comment
block starting with two *'s, like this:

\verbatim
/**
 * ... text ...
 */
\endverbatim

<li> or you can use the Qt style and add an exclamation mark (!) 
after the opening of a C-style comment block, as shown in this example:

\verbatim
/*!
 * ... text ...
 */
\endverbatim

In both cases the intermediate *'s are optional, so

\verbatim
/*!
 ... text ...
*/
\endverbatim

is also valid.

<li> A third alternative is to use a block of <i>at least two</i> C++ comment 
lines, where each line starts with an additional slash or an 
exclamation mark. Here are examples of the two cases:

\verbatim
///
/// ... text ...
///
\endverbatim

or

\verbatim
//!
//!... text ...
//!
\endverbatim

Note that a blank line ends a documentation block in this case.

<li>

Some people like to make their comment blocks more visible in the
documentation. For this purpose you can use the following:

\verbatim
/********************************************//**
 *  ... text
 ***********************************************/
\endverbatim
(note the 2 slashes to end the normal comment block and start a special comment block).

or

\verbatim
/////////////////////////////////////////////////
/// ... text ...
/////////////////////////////////////////////////
\endverbatim

or

\verbatim
/************************************************
 *  ... text
 ***********************************************/
\endverbatim

as long as JAVADOC_BANNER = YES is used.
\include javadoc-banner.h
 \htmlonly
 Click <a href="examples/javadoc-banner/html/index.html">here</a>
 for the corresponding HTML documentation that is generated by doxygen.
 \endhtmlonly
 \latexonly
 See \hyperlink{javadoc_banner_example}{Javadoc Banner example}
 for the corresponding \mbox{\LaTeX} documentation that is generated by doxygen.
 \endlatexonly

</ol>

For the brief description there are also several possibilities:
<ol>
<li>One could use the \ref cmdbrief "\\brief" command with one of the 
above comment blocks. This command ends at the end of a paragraph, 
so the detailed description follows after an empty line.

Here is an example:

\verbatim
/*! \brief Brief description.
 *         Brief description continued.
 *
 *  Detailed description starts here.
 */
\endverbatim

<li>If \ref cfg_javadoc_autobrief "JAVADOC_AUTOBRIEF" is set to \c YES 
    in the configuration file, 
    then using Javadoc style comment
    blocks will automatically start a brief description which ends at the
    first dot followed by a space or new line. Here is an example:

\verbatim
/** Brief description which ends at this dot. Details follow
 *  here.
 */
\endverbatim
The option has the same effect for multi-line special C++ comments:
\verbatim
/// Brief description which ends at this dot. Details follow
/// here.
\endverbatim

<li>A third option is to use a special C++ style comment which does not 
    span more than one line. Here are two examples:
\verbatim
/// Brief description.
/** Detailed description. */
\endverbatim

or

\verbatim
//! Brief description.

//! Detailed description 
//! starts here.
\endverbatim

Note the blank line in the last example, which is required to separate the 
brief description from the block containing the detailed description. The
\ref cfg_javadoc_autobrief "JAVADOC_AUTOBRIEF" should also be set to \c NO
for this case.

</ol>

As you can see doxygen is quite flexible. If you have multiple
detailed descriptions, like in the following example:

\verbatim
//! Brief description, which is
//! really a detailed description since it spans multiple lines.
/*! Another detailed description!
 */
\endverbatim

They will be joined. Note that this is also the case if the descriptions
are at different places in the code! In this case the order will depend
on the order in which doxygen parses the code.


Unlike most other documentation systems, doxygen also allows you to put
the documentation of members (including global functions) in front of 
the \e definition. This way the documentation can be placed in the source 
file instead of the header file. This keeps the header file compact, and allows the 
implementer of the members more direct access to the documentation.
As a compromise the brief description could be placed before the
declaration and the detailed description before the member definition.

\subsubsection memberdoc Putting documentation after members 

If you want to document the members of a file, struct, union, class, or enum,
it is sometimes desired to place the documentation block after the member 
instead of before. For this purpose you have to put an additional \< marker
in the comment block. Note that this also works for the parameters 
of a function.

Here are some examples:
\verbatim
int var; /*!< Detailed description after the member */
\endverbatim
This block can be used to put a Qt style detailed 
documentation block \e after a member. Other ways to do the
same are:
\verbatim
int var; /**< Detailed description after the member */
\endverbatim
or 
\verbatim
int var; //!< Detailed description after the member
         //!< 
\endverbatim
or 
\verbatim
int var; ///< Detailed description after the member
         ///< 
\endverbatim

Most often one only wants to put a brief description after a member.
This is done as follows:
\verbatim
int var; //!< Brief description after the member
\endverbatim
or
\verbatim
int var; ///< Brief description after the member
\endverbatim

For functions one can use the \ref cmdparam "\@param" command to document the parameters
and then use <code>[in]</code>, <code>[out]</code>, <code>[in,out]</code> 
to document the direction. For inline documentation this is also possible 
by starting with the direction attribute, e.g.
\verbatim
void foo(int v /**< [in] docs for input parameter v. */);
\endverbatim

Note that these blocks have the same structure and meaning as the 
special comment blocks in the previous section 
only the \< indicates that the member is 
located in front of the block instead of after the block.

Here is an example of the use of these comment blocks:
\include afterdoc.h
 \htmlonly
 Click <a href="examples/afterdoc/html/class_afterdoc___test.html">here</a>
 for the corresponding HTML documentation that is generated by doxygen.
 \endhtmlonly
 \latexonly
 See \hyperlink{afterdoc_example}{After Block example}
 for the corresponding \mbox{\LaTeX} documentation that is generated by doxygen.
 \endlatexonly

\warning These blocks can only be used to document \e members and \e parameters.
         They cannot be used to document files, classes, unions, structs,
         groups, namespaces and enums themselves. Furthermore, the structural 
         commands mentioned in the next section 
         (like <code>\\class</code>) are not allowed 
         inside these comment blocks.

\subsubsection docexamples Examples

Here is an example of a documented piece of C++ code using the Qt style:
\include qtstyle.cpp
 \htmlonly
 Click <a href="examples/qtstyle/html/class_q_tstyle___test.html">here</a>
 for the corresponding HTML documentation that is generated by doxygen.
 \endhtmlonly
 \latexonly
 See \hyperlink{qtstyle_example}{QT Style example}
 for the corresponding \mbox{\LaTeX} documentation that is generated by doxygen.
 \endlatexonly

The brief descriptions are included in the member overview of a 
class, namespace or file and are printed using a small italic font 
(this description can be hidden by setting 
\ref cfg_brief_member_desc "BRIEF_MEMBER_DESC" to \c NO in 
the configuration file). By default the brief descriptions become the first 
sentence of the detailed descriptions 
(but this can be changed by setting the \ref cfg_repeat_brief "REPEAT_BRIEF" 
tag to \c NO). Both the brief and the detailed descriptions are optional 
for the Qt style. 

By default a Javadoc style documentation block behaves the same way as a
Qt style documentation block. This is not according the Javadoc specification
however, where the first sentence of the documentation block is automatically
treated as a brief description. To enable this behavior you should set
\ref cfg_javadoc_autobrief "JAVADOC_AUTOBRIEF" to YES in the configuration
file. If you enable this option and want to put a dot in the middle of a
sentence without ending it, you should put a backslash and a space after it.
Here is an example:
\verbatim
  /** Brief description (e.g.\ using only a few words). Details follow. */
\endverbatim

Here is the same piece of code as shown above, this time documented using the 
Javadoc style and \ref cfg_javadoc_autobrief "JAVADOC_AUTOBRIEF" set to YES:
\include jdstyle.cpp
 \htmlonly
 Click <a href="examples/jdstyle/html/class_javadoc___test.html">here</a>
 for the corresponding HTML documentation that is generated by doxygen.
 \endhtmlonly
 \latexonly
 See \hyperlink{jdstyle_example}{Javadoc Style example}
 for the corresponding \mbox{\LaTeX} documentation that is generated by doxygen.
 \endlatexonly

Similarly, if one wishes the first sentence of a Qt style documentation
block to automatically be treated as a brief description, one may set
\ref cfg_qt_autobrief "QT_AUTOBRIEF" to YES in the configuration file.

\subsubsection structuralcommands Documentation at other places

In the examples in the previous section the comment blocks were always located *in 
front* of the declaration or definition of a file, class or namespace or *in
front* or *after* one of its members. 
Although this is often comfortable, there may sometimes be reasons to put the 
documentation somewhere else. For documenting a file this is even 
required since there is no such thing as "in front of a file". 

Doxygen allows you to put your documentation blocks practically 
anywhere (the exception is inside the body of a function or inside a 
normal C style comment block). 

The price you pay for not putting the
documentation block directly before (or after) an item is the need to put a  
structural command inside the documentation block, which leads to some
duplication of information. So in practice you should \e avoid the use of
structural commands \e unless other requirements force you to do so.

Structural commands (like \ref cmd_intro "all other commands") start with a backslash 
(<tt>\\</tt>), or an at-sign (<tt>\@</tt>) if you prefer Javadoc style, 
followed by a command name and one or more parameters.
For instance, if you want to document the class \c Test in the example
above, you could have also put the following documentation block somewhere
in the input that is read by doxygen:
\verbatim
/*! \class Test
    \brief A test class.

    A more detailed class description.
*/
\endverbatim

Here the special command \c \\class is used to indicate that the
comment block contains documentation for the class \c Test.
Other structural commands are:
<ul>
<li>\c \\struct to document a C-struct.
<li>\c \\union to document a union.
<li>\c \\enum to document an enumeration type.
<li>\c \\fn to document a function.
<li>\c \\var to document a variable or typedef or enum value.
<li>\c \\def to document a \#define.
<li>\c \\typedef to document a type definition.
<li>\c \\file to document a file.
<li>\c \\namespace to document a namespace.
<li>\c \\package to document a Java package.
<li>\c \\interface to document an IDL interface.
</ul>
See section \ref commands for detailed information about these and many other 
commands. 

To document a member of a C++ class, you must also document the class 
itself. The same holds for namespaces. To document a global C function, 
typedef, enum or preprocessor definition you must first document the file 
that contains it (usually this will be a header file, because that file 
contains the information that is exported to other source files).

@attention Let's repeat that, because it is often overlooked:
to document global objects (functions, typedefs, enum, macros, etc), you
<em>must</em> document the file in which they are defined. In other words, 
there <em>must</em> at least be a \verbatim /*! \file */ \endverbatim
or a \verbatim /** @file */ \endverbatim line in this file.

Here is an example of a C header named \c structcmd.h that is documented 
using structural commands:
\include structcmd.h
 \htmlonly
 Click <a href="examples/structcmd/html/structcmd_8h.html">here</a>
 for the corresponding HTML documentation that is generated by doxygen.
 \endhtmlonly
 \latexonly
 See \hyperlink{structcmd_example}{Structural Commands example}
 for the corresponding \mbox{\LaTeX} documentation that is generated by doxygen.
 \endlatexonly

 Because each comment block in the example above contains a structural command, all
 the comment blocks could be moved to another location or input file 
 (the source file for instance), without affecting the generated 
 documentation. The disadvantage of this approach is that prototypes are
 duplicated, so all changes have to be made twice! Because of this you
 should first consider if this is really needed, and avoid structural
 commands if possible. I often receive examples that contain \\fn command
 in comment blocks which are place in front of a function. This is clearly
 a case where the \\fn command is redundant and will only lead to problems.

 When you place a comment block in a file with one of the following extensions
 `.dox`, `.txt`, or `.doc` then doxygen will hide this file from the file list.

 If you have a file that doxygen cannot parse but still would like to document it,
 you can show it as-is using \ref cmdverbinclude "\\verbinclude", e.g.

\verbatim
/*! \file myscript.sh
 *  Look at this nice script:
 *  \verbinclude myscript.sh
 */
\endverbatim

Make sure that the script is explicitly listed in the \ref cfg_input "INPUT" or
that \ref cfg_file_patterns "FILE_PATTERNS" includes the `.sh` extension and the
the script can be found in the path set via \ref cfg_example_path "EXAMPLE_PATH".

\subsection pythonblocks Comment blocks in Python

For Python there is a standard way of documenting the code using 
so called documentation strings. Such strings are stored in \c __doc__
and can be retrieved at runtime. Doxygen will extract such comments
and assume they have to be represented in a preformatted way.

\include docstring.py
 \htmlonly
 Click <a href="examples/docstring/html/index.html">here</a>
 for the corresponding HTML documentation that is generated by doxygen.
 \endhtmlonly
 \latexonly
 See \hyperlink{python_example}{Python Docstring example}
 for the corresponding \mbox{\LaTeX} documentation that is generated by doxygen.
 \endlatexonly

Note that in this case none of doxygen's \ref cmd_intro "special commands" 
are supported.

There is also another way to document Python code using comments that 
start with "##". These type of comment blocks are more in line with the 
way documentation blocks work for the other languages supported by doxygen 
and this also allows the use of special commands. 

Here is the same example again but now using doxygen style comments:

\include pyexample.py
 \htmlonly
 Click <a href="examples/pyexample/html/index.html">here</a>
 for the corresponding HTML documentation that is generated by doxygen.
 \endhtmlonly
 \latexonly
 See \hyperlink{py_example}{Python example}
 for the corresponding \mbox{\LaTeX} documentation that is generated by doxygen.
 \endlatexonly

Since python looks more like Java than like C or C++, you should set 
\ref cfg_optimize_output_java "OPTIMIZE_OUTPUT_JAVA" to \c YES in the
configuration file. 


\subsection vhdlblocks Comment blocks in VHDL

For VHDL a comment normally start with "--". Doxygen will extract comments
starting with "--!". There are only two types of comment blocks in VHDL;
a one line "--!" comment representing a brief description, and a multi-line
"--!" comment (where the "--!" prefix is repeated for each line) representing
a detailed description.

Comments are always located in front of the item that is being documented
with one exception: for ports the comment can also be after the item
and is then treated as a brief description for the port.

Here is an example VHDL file with doxygen comments:

\include  mux.vhdl
 \htmlonly
 Click <a href="examples/mux/html/index.html">here</a>
 for the corresponding HTML documentation that is generated by doxygen.
 \endhtmlonly
 \latexonly
 See \hyperlink{vhdl_example}{VHDL example}
 for the corresponding \mbox{\LaTeX} documentation that is generated by doxygen.
 \endlatexonly

As of VHDL 2008 it is also possible to use `/&zwj;*` style comments.
Doxygen will handle `/&zwj;* ... *&zwj;/`as plain comments and `/&zwj;*! ... *&zwj;/`
style comments as special comments to be parsed by doxygen.

To get proper looking output you need to set
\ref cfg_optimize_output_vhdl "OPTIMIZE_OUTPUT_VHDL" to \c YES in the
configuration file. This will also affect a number of other settings. When they
were not already set correctly doxygen will produce a warning telling which
settings where overruled.

\subsection fortranblocks Comment blocks in Fortran

When using doxygen for Fortran code you should
set \ref cfg_optimize_for_fortran "OPTIMIZE_FOR_FORTRAN" to \c YES.

The parser tries to guess if the source code is fixed format Fortran or
free format Fortran code. This may not always be correct. If not
one should use \ref cfg_extension_mapping "EXTENSION_MAPPING" to correct this.
By setting `EXTENSION_MAPPING =  f=FortranFixed f90=FortranFree` files with
extension \c f are interpreted as fixed format Fortran code and files with
extension \c f90 are interpreted as free format Fortran code.

For Fortran "!>" or "!<" starts a comment and "!!" or "!>" can be used to 
continue an one line comment into a multi-line comment.

Here is an example of a documented Fortran subroutine:
\code{.f}
  !> Build the restriction matrix for the aggregation 
  !! method.
  !! @param aggr information about the aggregates
  !! @todo Handle special case
  subroutine IntRestBuild(A,aggr,Restrict,A_ghost)
    implicit none
    Type(SpMtx), intent(in) :: A !< our fine level matrix
    Type(Aggrs), intent(in) :: aggr
    Type(SpMtx), intent(out) :: Restrict !< Our restriction matrix
    !...
  end subroutine
\endcode

As an alternative you can also use comments in fixed format code:

\code{.f}
C> Function comment
C> another line of comment
      function A(i)
C> input parameter
        integer i
      end function A
\endcode


\section docstructure Anatomy of a comment block

The previous section focused on how to make the comments in your code known
to doxygen, it explained the difference between a brief and a detailed description, and
the use of structural commands.

In this section we look at the contents of the comment block itself.

Doxygen supports various styles of formatting your comments.

The simplest form is to use plain text. This will appear as-is in the output 
and is ideal for a short description.

For longer descriptions you often will find the 
need for some more structure, like a block of verbatim text, a list, or a 
simple table. For this doxygen supports the 
<a href="https://daringfireball.net/projects/markdown/syntax">Markdown</a> 
syntax, including parts of the
<a href="https://michelf.ca/projects/php-markdown/extra/">Markdown Extra</a>
extension. 

Markdown is designed to be very easy to read and write. 
Its formatting is inspired by plain text mail.
Markdown works great for simple, generic formatting, like an introduction
page for your project. Doxygen also supports reading of markdown files 
directly. For more details see chapter \ref markdown.

For programming language specific formatting doxygen has two
forms of additional markup on top of Markdown formatting.

1. <a href="https://en.wikipedia.org/wiki/Javadoc">Javadoc</a> like markup.
   See \ref commands for a complete overview of all commands supported by doxygen.
2. <a href="https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/xmldoc/recommended-tags-for-documentation-comments">XML</a> markup
   as specified in the <a href="http://standards.iso.org/ittf/PubliclyAvailableStandards/c042926_ISO_IEC_23270_2006(E).zip">C# standard</a>.
   See \ref xmlcmds for the XML commands supported by doxygen.

If this is still not enough doxygen also supports a \ref htmlcmds "subset" of 
the <a href="https://en.wikipedia.org/wiki/HTML">HTML</a> markup language.

\htmlonly
Go to the <a href="markdown.html">next</a> section or return to the
 <a href="index.html">index</a>.
\endhtmlonly

*/