This chapter covers two topics:
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 next section presents the various styles supported by doxygen.
For Python, VHDL, Fortran, and Tcl code there are different commenting conventions, which can be found in sections Comment blocks in Python, Comment blocks in VHDL, Comment blocks in Fortran, and Comment blocks in Tcl respectively.
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:
You can use the JavaDoc style, which consist of a C-style comment block starting with two *'s, like this:
/** * ... text ... */
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:
/*! * ... text ... */
In both cases the intermediate *'s are optional, so
/*! ... text ... */
is also valid.
A third alternative is to use a block of at least two C++ comment lines, where each line starts with an additional slash or an exclamation mark. Here are examples of the two cases:
/// /// ... text ... ///
//! //!... text ... //!
Note that a blank line ends a documentation block in this case.
Some people like to make their comment blocks more visible in the documentation. For this purpose you can use the following:
/********************************************//** * ... text ***********************************************/
(note the 2 slashes to end the normal comment block and start a special comment block).
///////////////////////////////////////////////// /// ... text ... /////////////////////////////////////////////////
For the brief description there are also several possibilities:
One could use the \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:
/*! \brief Brief description. * Brief description continued. * * Detailed description starts here. */
If JAVADOC_AUTOBRIEF is set to
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:
/** Brief description which ends at this dot. Details follow * here. */
The option has the same effect for multi-line special C++ comments:
/// Brief description which ends at this dot. Details follow /// here.
A third option is to use a special C++ style comment which does not span more than one line. Here are two examples:
/// Brief description. /** Detailed description. */
//! Brief description. //! Detailed description //! starts here.
Note the blank line in the last example, which is required to separate the brief description from the block containing the detailed description. The JAVADOC_AUTOBRIEF should also be set to
NO for this case.
As you can see doxygen is quite flexible. If you have multiple detailed descriptions, like in the following example:
//! Brief description, which is //! really a detailed description since it spans multiple lines. /*! Another detailed description! */
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 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.
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:
int var; /*!< Detailed description after the member */
This block can be used to put a Qt style detailed documentation block after a member. Other ways to do the same are:
int var; /**< Detailed description after the member */
int var; //!< Detailed description after the member //!<
int var; ///< Detailed description after the member ///<
Most often one only wants to put a brief description after a member. This is done as follows:
int var; //!< Brief description after the member
int var; ///< Brief description after the member
For functions one can use the @param command to document the parameters and then use
[in,out] to document the direction. For inline documentation this is also possible by starting with the direction attribute, e.g.
void foo(int v /**< [in] docs for input parameter v. */);
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:
\class) are not allowed inside these comment blocks.
Here is an example of a documented piece of C++ code using the Qt style:
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 BRIEF_MEMBER_DESC to
NO in the config file). By default the brief descriptions become the first sentence of the detailed descriptions (but this can be changed by setting the REPEAT_BRIEF tag to
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 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:
/** Brief description (e.g.\ using only a few words). Details follow. */
Here is the same piece of code as shown above, this time documented using the JavaDoc style and JAVADOC_AUTOBRIEF set to YES:
Similarly, if one wishes the first sentence of a Qt style documentation block to automatically be treated as a brief description, one may set QT_AUTOBRIEF to YES in the configuration file.
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 avoid the use of structural commands unless other requirements force you to do so.
Structural commands (like all other commands) start with a backslash (
\), or an at-sign (
@) if you prefer JavaDoc style, followed by a command name and one or more parameters. For instance, if you want to document the class
Test in the example above, you could have also put the following documentation block somewhere in the input that is read by doxygen:
/*! \class Test \brief A test class. A more detailed class description. */
Here the special command
\class is used to indicate that the comment block contains documentation for the class
Test. Other structural commands are:
\structto document a C-struct.
\unionto document a union.
\enumto document an enumeration type.
\fnto document a function.
\varto document a variable or typedef or enum value.
\defto document a #define.
\typedefto document a type definition.
\fileto document a file.
\namespaceto document a namespace.
\packageto document a Java package.
\interfaceto document an IDL interface.
See section Special 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).
Let's repeat that, because it is often overlooked: to document global objects (functions, typedefs, enum, macros, etc), you must document the file in which they are defined. In other words, there must at least be a
/*! \file */
/** @file */
line in this file.
Here is an example of a C header named
structcmd.h that is documented using structural commands:
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.
For Python there is a standard way of documenting the code using so called documentation strings. Such strings are stored in
doc and can be retrieved at runtime. Doxygen will extract such comments and assume they have to be represented in a preformatted way.
Note that in this case none of doxygen's 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:
Since python looks more like Java than like C or C++, you should set OPTIMIZE_OUTPUT_JAVA to
YES in the config file.
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:
To get proper looking output you need to set OPTIMIZE_OUTPUT_VHDL to
YES in the config 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.
When using doxygen for Fortran code you should set OPTIMIZE_FOR_FORTRAN to
For Fortran "!>" or "!<" starts a comment and "!!" or "!>" can be used to continuate a one line comment into a multi-line comment.
Here is an example of a documented Fortran subroutine:
!> 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
As a alternative you can also use comments in fixed format code:
C> Function comment C> another line of comment function A(i) C> input parameter integer i end function A
Doxygen documentation can be included in normal Tcl comments.
To start a new documentation block start a line with
## (two hashes). All following comment lines and continuation lines will be added to this block. The block ends with a line not starting with a
# (hash sign).
A brief documentation can be added with
;#< (semicolon, hash and lower then sign). The brief documentation also ends at a line not starting with a
# (hash sign).
Inside doxygen comment blocks all normal doxygen markings are supported. The only exceptions are described in the following two paragraphs.
If a doxygen comment block ends with a line containing only
#@code all code until a line only containing
#@endcode is added to the generated documentation as code block.
If a doxygen comment block ends with a line containing only
#@verbatim all code until a line only containing
#@endverbatim is added verbatim to the generated documentation.
To detect namespaces, classes, functions and variables the following Tcl commands are recognized. Documentation blocks can be put on the lines before the command.
namespace eval ..Namespace
common ..Common variable
itcl::body ..Class method body definition
oo::class create ..Class
oo::define ..OO Class definition
method ..Class method definitions
constructor ..Class constructor
destructor ..Class destructor
public ..Set protection level
protected ..Set protection level
private ..Set protection level
Following is a example using doxygen style comments:
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 Markdown syntax, including parts of the Markdown Extra extension.
Markdown is designed to be very easy to read and write. It's 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. See here for more details regards Markdown support.
For programming language specific formatting doxygen has two forms of additional markup on top of Markdown formatting.