## Documenting the code

### Special documentation blocks

The following types of special documentation blocks are supported by doxygen:

- The Qt style, where special documentation blocks look like:

      /*!
        ... text ...
      */

  and the one line version:

      //! ... one line of text ...

- The JavaDoc style, where special documentation blocks look like:

      /**
       * ... text ...
       */

  and the one line version:

      /// ... one line of text ...

Doxygen only allows one brief and one detailed description. If there is one brief description before a declaration and one before a definition, only the one before the declaration will be used. If the same situation occurs for a detailed description, the one before the definition is preferred and the one before the declaration will be ignored.

Here is an example of a documented piece of C++ code using the Qt style:

    //!  A test class.
    /*!
      A more elaborate class description.
    */

    class Test
    {
      public:

        //! An enum.
        /*! More detailed enum description. */
        enum TEnum {
                     TVal1, /*!< Enum value TVal1. */
                     TVal2, /*!< Enum value TVal2. */
                     TVal3  /*!< Enum value TVal3. */
                   }
             //! Enum pointer.
             /*! Details. */
             *enumPtr,
             //! Enum variable.
             /*! Details. */
             enumVar;

        //! A constructor.
        /*!
          A more elaborate description of the constructor.
        */
        Test();

        //! A destructor.
        /*!
          A more elaborate description of the destructor.
        */
       ~Test();

        //! A normal member taking two arguments and returning an integer value.
        /*!
          \param a an integer argument.
          \param s a constant character pointer.
          \return The test results
          \sa Test(), ~Test(), testMeToo() and publicVar()
        */
        int testMe(int a,const char *s);

        //! A pure virtual member.
        /*!
          \sa testMe()
          \param c1 the first argument.
          \param c2 the second argument.
        */
        virtual void testMeToo(char c1,char c2) = 0;

        //! A public variable.
        /*!
          Details.
        */
        int publicVar;

        //! A function variable.
        /*!
          Details.
        */
        int (*handler)(int a,int b);
    };

Click here for the corresponding HTML documentation that is generated by doxygen.

The one-line comments should contain a brief description, whereas the multi-line comment blocks contain a more detailed description. Note that consecutive one-line comments are merged together in one brief description. 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 behaviour 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:

    /**
     *  A test class. A more elaborate class description.
     */

    class Test
    {
      public:

        /**
         * An enum.
         * More detailed enum description.
         */

        enum TEnum {
              TVal1, /**< enum value TVal1. */
              TVal2, /**< enum value TVal2. */
              TVal3  /**< enum value TVal3. */
             }
           *enumPtr, /**< enum pointer. Details. */
           enumVar;  /**< enum variable. Details. */

          /**
           * A constructor.
           * A more elaborate description of the constructor.
           */
          Test();

          /**
           * A destructor.
           * A more elaborate description of the destructor.
           */
         ~Test();

          /**
           * a normal member taking two arguments and returning an integer value.
           * @param a an integer argument.
           * @param s a constant character pointer.
           * @see Test()
           * @see ~Test()
           * @see testMeToo()
           * @see publicVar()
           * @return The test results
           */
           int testMe(int a,const char *s);

          /**
           * A pure virtual member.
           * @see testMe()
           * @param c1 the first argument.
           * @param c2 the second argument.
           */
           virtual void testMeToo(char c1,char c2) = 0;

          /**
           * a public variable.
           * Details.
           */
           int publicVar;

          /**
           * a function variable.
           * Details.
           */
           int (*handler)(int a,int b);
    };

Click here for the corresponding HTML documentation that is generated by doxygen.

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.

Note:
  Each entity can only have one brief and one detailed description. If you specify more than one comment block of the same type, only one will be used, and all others are ignored!

### Structural commands

So far we have assumed that the documentation blocks are always located in front of the declaration or definition of a file, class or namespace or in front of one of its members. Although this is often comfortable, it may sometimes be better to put the documentation somewhere else. For some types of documentation blocks (like file documentation) this is even required. 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), as long as you put a structural command inside the documentation block.

Structural commands (like all other commands) start with a backslash (`\`), or an at-sign (`@`) in 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:

- `\struct` to document a C-struct.
- `\union` to document a union.
- `\enum` to document an enumeration type.
- `\fn` to document a function.
- `\var` to document a variable or typedef or enum value.
- `\def` to document a #define.
- `\file` to document a file.
- `\namespace` to document a namespace.

See section Special Commands for detailed information about these and other commands. Note that the documentation block belonging to a file should always contain a structural command.

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 */

or a

     /** @file */

line in this file.

Here is an example of a C header named `structcmd.h` that is documented using structural commands:

    /*! \file structcmd.h
        \brief A Documented file.

        Details.
    */

    /*! \def MAX(a,b)
        \brief A macro that returns the maximum of \a a and \a b.

        Details.
    */

    /*! \var typedef unsigned int UINT32
        \brief A type definition for a .

        Details.
    */

    /*! \var int errno
        \brief Contains the last error code.

        \warning Not thread safe!
    */

    /*! \fn int open(const char *pathname,int flags)
        \brief Opens a file descriptor.

        \param pathname The name of the descriptor.
        \param flags Opening flags.
    */

    /*! \fn int close(int fd)
        \brief Closes the file descriptor \a fd.
        \param fd The descriptor to close.
    */

    /*! \fn size_t write(int fd,const char *buf, size_t count)
        \brief Writes \a count bytes from \a buf to the filedescriptor \a fd.
        \param fd The descriptor to write to.
        \param buf The data buffer to write.
        \param count The number of bytes to write.
    */

    /*! \fn int read(int fd,char *buf,size_t count)
        \brief Read bytes from a file descriptor.
        \param fd The descriptor to read from.
        \param buf The buffer to read into.
        \param count The number of bytes to read.
    */

    #define MAX(a,b) (((a)>(b))?(a):(b))
    typedef unsigned int UINT32;
    int errno;
    int open(const char *,int);
    int close(int);
    size_t write(int,const char *, size_t);
    int read(int,char *,size_t);

Click here for the corresponding HTML documentation that is generated by doxygen.

Note:
  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!

### Documenting compound members.

If you want to document the members of a file, struct, union, class, or enum, and you want to put the documentation for these members inside the compound, it is sometimes desired to place the documentation block after the member instead of before. For this purpose doxygen has the following additional comment blocks:

    /*!< ... */

This block can be used to put a Qt style detailed documentation block after a member. The one line brief description looks as follows:

    //!< ...

There are also JavaDoc versions for detailed documentation:

    /**< ... */

(where the first sentence is the brief description if `JAVADOC_AUTOBRIEF` is set to `YES)` and there is a separate brief description as well:

    ///< ...

Note that these blocks have the same structure and meaning as the special comment blocks above 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:

    /*! A test class */

    class Test
    {
      public:
        /** An enum type.
         *  The documentation block cannot be put after the enum!
         */
        enum EnumType
        {
          int EVal1,     /**< enum value 1 */
          int EVal2      /**< enum value 2 */
        };
        void member();   //!< a member function.

      protected:
        int value;       /*!< an integer value */
    };

Click here for the corresponding HTML documentation that is generated by doxygen.

Warning:
  These blocks can only be used to document members and parameters. They cannot be used to document files, classes, unions, structs, groups, namespaces and enums themselves. Furthermore, the structural commands mentioned in the previous section (like `\class`) are ignored inside these comment blocks.

Go to the next section or return to the index.