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[/
(C) Copyright Edward Diener 2011-2015,2020
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 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 with its default preprocessor 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.
[endsect]
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