[/ 
  (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_modifiers Macros with modifiers]

The basic functionality for VMD macros parsing data types
has been given using the required parameters of those
macros. This basic functionality may be perfectly adequate for
macro programmers to use VMD effectively in their programming efforts.

A number of those macros take optional parameters,
called in general "modifiers", which enhance or change the
functionality of those macros in various ways. All modifiers
are VMD identifiers.

In all situations modifiers are optional parameters which are parsed 
by VMD to provide enhanced functionality for some of its macros. 
They are never required as part of the basic functionality of a macro.

When modifiers are used as optional arguments to a macro they
can be input after the required parameters in any order and 
VMD will still handle the optional parameters correctly.

There are two general types of modifiers, 'specific modifiers' and
'user-defined modifiers'. Specific modifers start with BOOST_VMD_
and are both registered and pre-detected identifiers known to VMD.
The specific modifiers change the expansion of particular macros
in various ways which will be subsequently explained. User-defined
modifiers are user-defined identifiers which the end-user of
specific macros must register and pre-detect for himself. They also
change the expansion of particular macros in various ways which 
will be subsequently explained.

For any particular macro if a specific modifier
is not appropriate it is just ignored. This means that VMD never
generates a preprocessing error or gives an incorrect result 
just because a specific modifier does not apply for a particular 
macro. Any modifier which is not recognized as a specific modifier 
is treated as a user-defined modifier. In cases where a user-defined 
modifier is not appropriate it is also just ignored.

The situations where modifiers can be used to enhance the basic
functionality of VMD macros can be divided by particular types
of specific modifiers. Each particular type of a specific modifier
has a name given to it, functionality, and set of identifiers
associated with that particular type.
Each particular type of a specific modifier may be used as optional
parameters in one or more designated macros depending on the specific
modifier type. 

When more than one mutually exclusive specific modifier from a particular type of modifier is 
specified as an optional parameter the last specified takes effect. This
allows the programmer to override a specific modifier by adding the
overridden identifier as an optional argument to the end of the
macro's invocation.

Header files for specific modifiers are automatically included when
the header files for macros taking those specific modifiers are
included.

Header files for user-defined modifiers, which register and pre-detect
those user-defined modifiers, must be included as needed by the programmer 
using those modifiers.

The following topics will explain each particular type of modifier 
and where it may be used.

[include vmd_modifiers_return_type.qbk]
[include vmd_modifiers_filter.qbk]
[include vmd_modifiers_identifier.qbk]
[include vmd_modifiers_splitting.qbk]
[include vmd_modifiers_index.qbk]
[include vmd_modifiers_single_element.qbk]

[endsect]