[/ 
  (C) Copyright Edward Diener 2011-2015
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]

[section:vmd_identifying Identifying data types]

[heading Identifying macros and BOOST_VMD_IS_EMPTY ]

The various macros for identifying VMD data types complement
the ability to identify emptiness using BOOST_VMD_IS_EMPTY.
The general name I will use in this documentation for these 
specific macros is "identifying macros." The identifying macros
also share with BOOST_VMD_IS_EMPTY the inherent flaw
mentioned when discussing BOOST_VMD_IS_EMPTY prior to the C++20
standard, since they themselves use BOOST_VMD_IS_EMPTY to determine
that the input has ended.

To recapitulate the flaw with BOOST_VMD_IS_EMPTY prior to C++20:

* using a standard C++ compiler if the input ends with the
name of a function-like macro, and that macro takes two or
more parameters, a preprocessing error will occur.
* using the VC++ compiler if the input consists of the name
of a function-like macro, and that macro when invoked with no 
parameters returns a tuple, the macro erroneously returns 1, 
meaning that the input is empty.
* even if the function-like macro takes one parameter, passing
emptiness to that macro could cause a preprocessing error.

The obvious way to avoid the BOOST_VMD_IS_EMPTY problem with the 
identifying macros is to design input so that the name of a function-like
macro is never passed as a parameter. This can be done, if one uses
VMD and has situations where the input could contain
a function-like macro name, by having that function-like macro name placed
within a Boost PP data type, such as a tuple, without attempting to identify
the type of the tuple element using VMD. In other word if the input is:

 ( SOME_FUNCTION_MACRO_NAME )
 
and we have the macro definition:

 #define SOME_FUNCTION_MACRO_NAME(x,y) some_output
 
VMD can still parse the input as a tuple, if desired, using BOOST_VMD_IS_TUPLE
without encountering the BOOST_VMD_IS_EMPTY problem. However if the input is:

 SOME_FUNCTION_MACRO_NAME
 
either directly or through accessing the above tuple's first element, and the 
programmer attempts to use BOOST_VMD_IS_IDENTIFIER with this input, the 
BOOST_VMD_IS_EMPTY problem will occur.

[heading Identifying macros and programming flexibility ]

The VMD identifying macros give the preprocessor metaprogrammer a great amount
of flexibility when designing macros. It is not merely the flexibility of allowing
direct parameters to a macro to be different data types, and having the macro work
differently depending on the type of data passed to it, but it is also the flexibility
of allowing individual elements of the higher level Boost PP data types to be 
different data types and have the macro work correctly depending on the type of data 
type passed as part of those elements.

With this flexibility also comes a greater amount of responsibility. For the macro 
designer this responsibility is twofold: 

* To carefully document the possible combinations of acceptable data and what they mean.
* To balance flexibility with ease of use so that the macro does not become so hard to
understand that the programmer invoking the macro gives up using it entirely.

For the programmer invoking a macro the responsibility is to understand the documentation
and not attempt to pass to the macro data which may cause incorrect results or preprocessing
errors.

[endsect]