/******************************************************************************

 *

 * 

 *

 * Copyright (C) 1997-2008 by Dimitri van Heesch.

 *

 * Permission to use, copy, modify, and distribute this software and its

 * documentation under the terms of the GNU General Public License is hereby 

 * granted. No representations are made about the suitability of this software 

 * for any purpose. It is provided "as is" without express or implied warranty.

 * See the GNU General Public License for more details.

 *

 * Documents produced by Doxygen are derivative works derived from the

 * input used in their production; they are not affected by this license.

 *

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/*! \page preprocessing Preprocessing

Source files that are used as input to doxygen can be parsed by doxygen's

built-in C-preprocessor.

By default doxygen does only partial preprocessing. That is, it 

evaluates conditional compilation statements (like \#if) and 

evaluates macro definitions, but it does not perform macro expansion.

So if you have the following code fragment

\verbatim

#define VERSION 200

#define CONST_STRING const char *

#if VERSION >= 200

  static CONST_STRING version = "2.xx";

#else

  static CONST_STRING version = "1.xx";

#endif

\endverbatim

Then by default doxygen will feed the following to its parser:

\verbatim

#define VERSION

#define CONST_STRING

  static CONST_STRING version = "2.xx";

\endverbatim

You can disable all preprocessing by setting 

\ref cfg_enable_preprocessing "ENABLE_PREPROCESSING" to \c 

NO in the configuation file. In the case above doxygen will then read

both statements, i.e:

\verbatim

  static CONST_STRING version = "2.xx";

  static CONST_STRING version = "1.xx";

\endverbatim

In case you want to expand the \c CONST_STRING macro, you should set the

\ref cfg_macro_expansion "MACRO_EXPANSION" tag in the config file 

to \c YES. Then the result after preprocessing becomes:

\verbatim

#define VERSION

#define CONST_STRING

  static const char * version = "1.xx";

\endverbatim

Note that doxygen will now expand \e all macro definitions 

(recursively if needed). This is often too much. Therefore, doxygen also 

allows you to expand only those defines that you explicitly 

specify. For this you have to set the 

\ref cfg_expand_only_predef "EXPAND_ONLY_PREDEF" tag to \c YES

and specify the macro definitions after 

the \ref cfg_predefined "PREDEFINED" or 

\ref cfg_expand_as_defined "EXPAND_AS_DEFINED" tag.  

A typically example where some help from the preprocessor is needed is

when dealing with Microsoft's __declspec language extension. Here is an

example function.

\verbatim

extern "C" void __declspec(dllexport) ErrorMsg( String aMessage,...);

\endverbatim

When nothing is done, doxygen will be confused and see __declspec as

some sort of function. To help doxygen one typically uses the following

preprocessor settings:

\verbatim  

ENABLE_PREPROCESSING   = YES

MACRO_EXPANSION        = YES

EXPAND_ONLY_PREDEF     = YES

PREDEFINED             = __declspec(x)=

\endverbatim

This will make sure the __declspec(dllexport) is removed before doxygen

parses the source code.

For a more complex example, suppose you have the following obfuscated 

code fragment of an abstract base class called \c IUnknown:

\verbatim

/*! A reference to an IID */

#ifdef __cplusplus

#define REFIID const IID &

#else

#define REFIID const IID *

#endif

/*! The IUnknown interface */

DECLARE_INTERFACE(IUnknown)

{

  STDMETHOD(HRESULT,QueryInterface) (THIS_ REFIID iid, void **ppv) PURE;

  STDMETHOD(ULONG,AddRef) (THIS) PURE;

  STDMETHOD(ULONG,Release) (THIS) PURE;

};

\endverbatim

without macro expansion doxygen will get confused, but we may not want to 

expand the REFIID macro, because it is documented and the user that reads 

the documentation should use it when implementing the interface.

By setting the following in the config file:

\verbatim

ENABLE_PREPROCESSING = YES

MACRO_EXPANSION      = YES

EXPAND_ONLY_PREDEF   = YES

PREDEFINED           = "DECLARE_INTERFACE(name)=class name" \

                       "STDMETHOD(result,name)=virtual result name" \

                       "PURE= = 0" \

                       THIS_= \

                       THIS= \

		       __cplusplus

\endverbatim

we can make sure that the proper result is fed to doxygen's parser:

\verbatim

/*! A reference to an IID */

#define REFIID

/*! The IUnknown interface */

class  IUnknown

{

  virtual  HRESULT   QueryInterface ( REFIID iid, void **ppv) = 0;

  virtual  ULONG   AddRef () = 0;

  virtual  ULONG   Release () = 0;

};

\endverbatim

Note that the \ref cfg_predefined "PREDEFINED" tag accepts function 

like macro definitions

(like \c DECLARE_INTERFACE ), normal macro 

substitutions (like \c PURE and \c THIS) and plain 

defines (like \c __cplusplus).

Note also that preprocessor definitions that are normally defined 

automatically by the preprocessor (like \c __cplusplus), have to be defined 

by hand with doxygen's parser (this is done because these defines

are often platform/compiler specific).

In some cases you may want to substitute a macro name or function by 

something else without exposing the result to further macro substitution.

You can do this but using the <code>:=</code> operator instead of 

<code>=</code>

As an example suppose we have the following piece of code:

\verbatim

#define QList QListT

class QListT

{

};

\endverbatim

Then the only way to get doxygen interpret this as a class definition

for class QList is to define:

\verbatim

PREDEFINED = QListT:=QList

\endverbatim

Here is an example provided by Valter Minute and Reyes Ponce that helps 

doxygen to wade through the boilerplate code in Microsoft's ATL \& MFC 

libraries:

\verbatim

PREDEFINED           = "DECLARE_INTERFACE(name)=class name" \

                       "STDMETHOD(result,name)=virtual result name" \

                       "PURE= = 0" \

                       THIS_= \

                       THIS= \

                       DECLARE_REGISTRY_RESOURCEID=// \

                       DECLARE_PROTECT_FINAL_CONSTRUCT=// \

                       "DECLARE_AGGREGATABLE(Class)= " \

                       "DECLARE_REGISTRY_RESOURCEID(Id)= " \

                       DECLARE_MESSAGE_MAP= \

                       BEGIN_MESSAGE_MAP=/* \

                       END_MESSAGE_MAP=*/// \

                       BEGIN_COM_MAP=/* \

                       END_COM_MAP=*/// \

                       BEGIN_PROP_MAP=/* \

                       END_PROP_MAP=*/// \

                       BEGIN_MSG_MAP=/* \

                       END_MSG_MAP=*/// \

                       BEGIN_PROPERTY_MAP=/* \

                       END_PROPERTY_MAP=*/// \

                       BEGIN_OBJECT_MAP=/* \

                       END_OBJECT_MAP()=*/// \

                       DECLARE_VIEW_STATUS=// \

                       "STDMETHOD(a)=HRESULT a" \

                       "ATL_NO_VTABLE= " \

                       "__declspec(a)= " \

                       BEGIN_CONNECTION_POINT_MAP=/* \

                       END_CONNECTION_POINT_MAP=*/// \

                       "DECLARE_DYNAMIC(class)= " \

                       "IMPLEMENT_DYNAMIC(class1, class2)= " \

                       "DECLARE_DYNCREATE(class)= " \

                       "IMPLEMENT_DYNCREATE(class1, class2)= " \

                       "IMPLEMENT_SERIAL(class1, class2, class3)= " \

                       "DECLARE_MESSAGE_MAP()= " \

                       TRY=try \

                       "CATCH_ALL(e)= catch(...)" \

                       END_CATCH_ALL= \

                       "THROW_LAST()= throw"\

                       "RUNTIME_CLASS(class)=class" \

                       "MAKEINTRESOURCE(nId)=nId" \

                       "IMPLEMENT_REGISTER(v, w, x, y, z)= " \

                       "ASSERT(x)=assert(x)" \

                       "ASSERT_VALID(x)=assert(x)" \

                       "TRACE0(x)=printf(x)" \

                       "OS_ERR(A,B)={ #A, B }" \

                       __cplusplus \

                       "DECLARE_OLECREATE(class)= " \

                       "BEGIN_DISPATCH_MAP(class1, class2)= " \

                       "BEGIN_INTERFACE_MAP(class1, class2)= " \

                       "INTERFACE_PART(class, id, name)= " \

                       "END_INTERFACE_MAP()=" \

                       "DISP_FUNCTION(class, name, function, result, id)=" \

                       "END_DISPATCH_MAP()=" \

                       "IMPLEMENT_OLECREATE2(class, name, id1, id2, id3, id4,\

                        id5, id6, id7, id8, id9, id10, id11)="

\endverbatim

As you can see doxygen's preprocessor is quite powerful, but if you want

even more flexibility you can always write an input filter and specify it 

after the \ref cfg_input_filter "INPUT_FILTER" tag.

If you are unsure what the effect of doxygen's preprocessing will be

you can run doxygen as follows:

\verbatim

  doxygen -d Preprocessor

\endverbatim

This will instruct doxygen to dump the input sources to standard output after

preprocessing has been done (Hint: set <code>QUIET = YES</code> and 

<code>WARNINGS = NO</code> in the configuration file to disable any other 

output).

\htmlonly

Go to the <a href="external.html">next</a> section or return to the

 <a href="index.html">index</a>.

\endhtmlonly

*/