path: root/doc/preprocessing.doc

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 * Copyright (C) 1997-2015 by Dimitri van Heesch.
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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 \c \#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 configuration 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 configuration file 
to \c YES. Then the result after preprocessing becomes:

\verbatim
#define VERSION
#define CONST_STRING

  static const char * version = "2.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 the language extension from Microsoft: \c __declspec. The same goes
for GNU's \c \__attribute__ 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 \c __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 \c __declspec(dllexport) is removed before doxygen
parses the source code.

Similar settings can be used for removing \c \__attribute__ expressions from the input:

\verbatim  
ENABLE_PREPROCESSING   = YES
MACRO_EXPANSION        = YES
EXPAND_ONLY_PREDEF     = YES
PREDEFINED             = __attribute__(x)=
\endverbatim

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 \c 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 configuration 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 \c 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
or when the line numbers are not wanted:
\verbatim
  doxygen -d Preprocessor -d NoLineno
\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).

Note preprocessing is not done for all languages. Preprocessing is enabled for files
that use the "C" scanner (with the exception of 'java', 'd' and 'php'), Fortran files
(only in case the extension contains at least one upper case character) and vhdl files.

\htmlonly
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 <a href="index.html">index</a>.
\endhtmlonly

*/