A sample c++ header file that demonstrates the basic syntax of the Doxygen documentation.

SampleDoxygenAnnotations.h

/**
 * @file SampleDoxygenAnnotations.h
 * @author My Self
 * @date 9 Sep 2016
 * @brief File containing examples of doxygen usage for quick reference.
 *
 * Here typically goes a more extensive explanation of what the header
 * defines. Doxygens tags are words preceeded by either a backslash @\
 * or by an at symbol @@.
 * @see http://www.stack.nl/~dimitri/doxygen/docblocks.html
 * @see http://www.stack.nl/~dimitri/doxygen/commands.html
 */

#  ifndef _SAMPLE_DOXYGEN_ANNOTATIONS_H
#  define _SAMPLE_DOXYGEN_ANNOTATIONS_H

#  include <stdio.h>
#  include <math.h>

#  include "local_header.h"

/**
* @brief a macro that marks the functions that will be exported. 
*/ 
#define EXPORT

/**
 * @brief Use brief, otherwise the index won't have a brief explanation.
 *
 * Detailed explanation for an enum type.
 */
typedef enum Enum_enum {
  ENUM_FIRST,  /**< Some documentation for first. */
  ENUM_SECOND, /**< Some documentation for second. */
  
  /**
  * Some documentations for Etc.
  */
  ENUM_ETC     
} SampleEnum;

/**
 * @brief Use brief, otherwise the index won't have a brief explanation.
 *
 * Detailed explanation.
 *
 * @deprecated Starts a paragraph indicating that this documentation block
 * belongs to a deprecated entity. Can be used to describe alternatives, 
 * expected life span, etc.
 */
typedef struct SampleStruct_struct {
  int a;    /**< Some documentation for the member SampleStruct#a. */
  int b;    /**< Some documentation for the member SampleStruct#b. */
  
  /**
  * Some documentation for SampleStruct#c.
  */ 
  double c; 
} SampleStruct;

/**
 * @brief Example showing how to document a function with Doxygen.
 *
 * Description of what the function does. This part may refer to the parameters
 * of the function, like @p param1 or @p param2. A word of code can also be
 * inserted like @c this which is equivalent to <tt>this</tt> and can be useful
 * to say that the function returns a @c void or an @c int. If you want to have
 * more than one word in typewriter font, then just use @<tt@>.
 * We can also include text verbatim,
 * @verbatim like this@endverbatim
 * Sometimes it is also convenient to include an example of usage:
 * @code
 * SampleStruct *out = Sample_The_Function_Name(param1, param2);
 * printf("something...\n");
 * @endcode
 * Or,
 * @code{.py}
 * pyval = python_func(arg1, arg2)
 * print pyval
 * @endcode
 * when the language is not the one used in the current source file (but
 * <b>be careful</b> as this may be supported only by recent versions
 * of Doxygen). By the way, <b>this is how you write bold text</b> or,
 * if it is just one word, then you can just do @b this.
 * @param param1 Description of the first parameter of the function.
 * @param param2 The second one, which follows @p param1.
 * @return Describe what the function returns.
 * @see The_Second_Function
 * @see The_Last_One
 * @see http://samplesite.sm/
 * @note Something to note.
 * @warning Warning.
 */
EXPORT SampleStruct *
The_Function_Name(ParamType1 param1, ParamType2 param2 /*, ...*/);

/**
 * @brief A simple stub function to show how links do work.
 *
 * Links are generated automatically for webpages (like http://www.google.com)
 * and for structures, like SampleStruct_struct. For typedef-ed types use
 * #SampleStruct.
 * For functions, automatic links are generated when the parenthesis () follow
 * the name of the function, like The_Function_Name().
 * Alternatively, you can use #The_Function_Name.
 * @return @c NULL is always returned.
 */
EXPORT void *
The_Second_Function(void);

/**
 * @brief A simple stub function to show how to create bullets and numbered lists.
 * By putting a number of column-aligned minus (-) signs at the start of a line, 
 * a bullet list will automatically be generated. Instead of the minus sign also 
 * plus (+) or asterisk (*) can be used. Numbered lists can also be generated by 
 * using a minus followed by a hash or by using a number followed by a dot. 
 * Nesting of lists is allowed and is based on indentation of the items.
 *  A list of events:
 *    - mouse events
 *         -# mouse move event
 *         -# mouse click event\n
 *            More info about the click event.
 *         -# mouse double click event
 *    - keyboard events
 *         1. key down event
 *         2. key up event
 *
 *  More text here.
 */
EXPORT void * The_Event_Function(void);

/**
 * Brief can be omitted. If you configure Doxygen with
 * <tt>JAVADOC_AUTOBRIEF=YES</tt>, then the first Line of the comment is used
 * instead. In this function this would be as if
 * @verbatim @brief Brief can be omitted. @endverbatim
 * was used instead.
 */
EXPORT void
The_Last_One(void);

/**
 * @brief A sample class 
 *  
 * A sample class. A more elaborate class description. Classes are documented mostly the same way that 
 * structures are documented. 
 * A member can be linked like #publicVar. 
 */
class SampleClass
{
  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.
       */
      SampleClass();
      /**
       * A destructor.
       * A more elaborate description of the destructor. 
       */
     ~SampleClass();
    
      /**
       * a normal member taking two arguments and returning an integer value.
       * @param[in] a an integer argument.
       * @param[in] s a constant character pointer.
       * @param[out] ret some return value
       * @see SampleClass()
       * @see ~SampleClass()
       * @see testMeToo()
       * @see publicVar()
       * @return The test results
       */
       int testMe(int a,const char *s, int* ret);
       
      /**
       * A pure virtual member.
       * @see testMe()
       * @param[in] c1 the first argument.
       * @param[in] c2 the second argument.
       * @param[in] buffer Some buffer that returns some data from the function
       * @param[in, out] bufferSize Gives the length of @paramref buffer to the function and 
       * returns the number of characters written to it upon function termination. 
       */
       virtual void testMeToo(char c1,char c2, char* buffer, int* bufferSize) = 0;
   
      /** 
       * a public variable.
       * Details.
       */
       int publicVar;
       
      /**
       * a function variable.
       * Details.
       */
       int (*handler)(int a,int b);
};

#endif /* _SAMPLE_DOXYGEN_ANNOTATIONS_H */

A sample c++ header file that demonstrates the basic syntax of the Doxygen documentation. The file is largely based on the Doxygen usage example (for C) by Matteo Franchin.