ceph/src/boost/libs/vmd/doc/vmd_dynamic_typing.qbk

   1 [/
   2   (C) Copyright Edward Diener 2011-2015
   3   Distributed under the Boost Software License, Version 1.0.
   4   (See accompanying file LICENSE_1_0.txt or copy at
   5   http://www.boost.org/LICENSE_1_0.txt).
   6 ]
   7
   8 [section:vmd_dynamic_typing Input as dynamic types]
   9
  10 Within the constraints based on the top-level types which VMD can parse,
  11 the libraries gives the end-user the ability to design macros with
  12 dynamic data types. By this I mean that a macro could be designed to handle
  13 different data types based on some documented agreement of different combinations
  14 of macro input meaning slightly different things. Add to this the ability
  15 to design such macros with variadic parameters and we have a preprocessor
  16 system of macro creation which to a lesser extent rivals the DSELS of template
  17 metaprogramming. Of course the preprocessor is not nearly as flexible as
  18 C++ templates, but still the sort of preprocessor metaprogramming one could
  19 do with VMD, and the underlying Boost PP, in creating flexible macros which
  20 can handle different combinations of data types is very interesting.
  21
  22 Of course macros need to be usable by an end-user so the syntactical ability
  23 of sequences to represent different types of input data must
  24 be balanced against ease of use and understanding when using a macro. But because
  25 certain sequences can mimic C++ function calls to some extent it is possible
  26 to represent macros as a language closer to C++ with VMD.
  27
  28 What is important when designing a macro in which you parse input to decide which type
  29 of data the invoker is passing to your macro is that you are aware of the constraints
  30 when parsing a data type. As an example if you design a macro where some input can either
  31 be a number, an identifier, or some other data type top-level input then attempting
  32 to parse the data to see if it is a number or identifier could fail with a preprocessor
  33 error and nullify your design if the data is not a VMD data type. So designing a macro with
  34 data types in mind often means restricting data to parseable top-level types.
  35
  36 [endsect]