[/ 
  (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_assert Asserting and data types]

The VMD macros for identifying data types work best when the macro logic can take different 
paths depending on the type of data being passed for a macro parameter. But occasionally 
the preprocessor metaprogrammer wants to simply verify that the macro parameter data is of 
the correct data type, else a preprocessing error should be generated to notify the programmer 
invoking the macro that the data passed is the incorrect type.

[heading Using BOOST_VMD_ASSERT]

The Boost PP library has a macro which produces a preprocessing error when the condition
passed to it is 0. This macro is called BOOST_PP_ASSERT. The macro produces a preprocessor
error by forcing a call to an internal macro with the wrong number of arguments. According
to the C++ standard this should always cause an immediate preprocessing error for conforming
compilers.

Unfortunately VC++ will only produce a warning when the wrong number of arguments are passed
to a macro. Therefore the BOOST_PP_ASSERT macro does not produce a preprocessing error using
VC++. Amazingly enough there appears to be no other way in which VC++ can be forced to
issue a preprocessing error by invoking a macro ( if you find one please tell me about it ).
However one can create invalid C++ as the output from a macro invocation which causes VC++
to produce a compiler error when the VC++ compiler later encounters the construct.

This is what the macro BOOST_VMD_ASSERT does. It takes the same conditional argument as 
BOOST_PP_ASSERT and it calls BOOST_PP_ASSERT when not used with VC++, otherwise if the 
condition is 0 it generates a compiler error by generating invalid C++ when used with VC++. 
The compiler error is generated by producing invalid C++ whose form is:

 typedef char BOOST_VMD_ASSERT_ERROR[-1];
 
By passing a second optional argument, whose form is a preprocessing identifier, 
to BOOST_VMD_ASSERT you can generate the invalid C++ for VC++, if the first 
argument is 0, of the form:

 typedef char optional_argument[-1];
 
instead. This may give a little more clarity, if desired, to the C++ error generated.

If the first conditional argument is not 0, BOOST_VMD_ASSERT produces no output.

[heading BOOST_VMD_ASSERT Usage]

To use the BOOST_VMD_ASSERT macro either include the general header:

 #include <boost/vmd/vmd.hpp>
 
or include the specific header:

 #include <boost/vmd/assert.hpp>
 
[heading Assertions for data types ]

The data types have their own assertion macros. These are largely just shortcuts for
passing the result of the identifying macros to BOOST_VMD_ASSERT. These assertion
macros are:

* emptiness, BOOST_VMD_ASSERT_IS_EMPTY
* identifier, BOOST_VMD_ASSERT_IS_IDENTIFIER
* number, BOOST_VMD_ASSERT_IS_NUMBER
* array, BOOST_VMD_ASSERT_IS_ARRAY
* list, BOOST_VMD_ASSERT_IS_LIST
* seq, BOOST_VMD_ASSERT_IS_SEQ
* tuple, BOOST_VMD_ASSERT_IS_TUPLE
* type, BOOST_VMD_ASSERT_IS_TYPE

Each of these macros take as parameters the exact same argument as their
corresponding identifying macros. But instead of returning non-zero or 0, each of these
macros produce a compiler error if the type of the input is not correct. 

Each of these macros only check for its assertion when the macro BOOST_VMD_ASSERT_DATA 
is set to 1. By default BOOST_VMD_ASSERT_DATA is only set to 1 in compiler debug mode.
The programmer can manually set BOOST_VMD_ASSERT_DATA to 1 prior to using one
the data types assert macros if he wishes.

[heading BOOST_VMD_ASSERT_... Usage]

To use the individual BOOST_VMD_ASSERT_... macros either include the general header:

 #include <boost/vmd/vmd.hpp>
 
or include the specific header:

 #include <boost/vmd/assert_is_empty.hpp> // BOOST_VMD_ASSERT_IS_EMPTY
 #include <boost/vmd/assert_is_identifier.hpp> // BOOST_VMD_ASSERT_IS_IDENTIFIER
 #include <boost/vmd/assert_is_number.hpp> // BOOST_VMD_ASSERT_IS_NUMBER
 #include <boost/vmd/assert_is_array.hpp> // BOOST_VMD_ASSERT_IS_ARRAY
 #include <boost/vmd/assert_is_list.hpp> // BOOST_VMD_ASSERT_IS_LIST
 #include <boost/vmd/assert_is_seq.hpp> // BOOST_VMD_ASSERT_IS_SEQ
 #include <boost/vmd/assert_is_tuple.hpp> // BOOST_VMD_ASSERT_IS_TUPLE
 #include <boost/vmd/assert_is_type.hpp> // BOOST_VMD_ASSERT_IS_TYPE
 
[heading Assertions and VC++ ]

The VC++ compiler has a quirk when dealing with BOOST_VMD_ASSERT and the 
data type assert macros. If you invoke one of the assert macros within another
macro which would normally generate output preprocessor tokens, it is necessary when using
VC++ to concatenate the result of the assert macro to whatever other preprocessor data
is being generated, even if the assert macro does not generate an error.

As a simple example let us suppose we have a macro expecting a tuple and generating 1
if the tuple has more than 2 elements, otherwise it generates 0. Ordinarily we could
write:

 #include <boost/preprocessor/comparison/greater.hpp>
 #include <boost/preprocessor/control/iif.hpp>
 #include <boost/preprocessor/tuple/size.hpp>
 #include <boost/vmd/assert_is_tuple.hpp>
 
 #define AMACRO(atuple) \
     BOOST_VMD_ASSERT_IS_TUPLE(atuple) \
     BOOST_PP_IIF(BOOST_PP_GREATER(BOOST_PP_TUPLE_SIZE(atuple), 2),1,0)
     
but for VC++ we must write

 #include <boost/preprocessor/cat.hpp>
 #include <boost/preprocessor/comparison/greater.hpp>
 #include <boost/preprocessor/control/iif.hpp>
 #include <boost/preprocessor/tuple/size.hpp>
 #include <boost/vmd/assert_is_tuple.hpp>
 
 #define AMACRO(atuple) \
     BOOST_PP_CAT \
        ( \
        BOOST_VMD_ASSERT_IS_TUPLE(atuple), \
        BOOST_PP_IIF(BOOST_PP_GREATER(BOOST_PP_TUPLE_SIZE(atuple), 2),1,0) \
        )
        
VC++ does not work correctly in the first instance, erroneously getting confused as far as
compiler output is concerned. But by using BOOST_PP_CAT in the second condition VC++ will
work correctly with VMD assertions.
     
[endsect]
