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- [/
- (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_dynamic_typing Input as dynamic types]
- Within the constraints based on the top-level types which VMD can parse,
- the libraries gives the end-user the ability to design macros with
- dynamic data types. By this I mean that a macro could be designed to handle
- different data types based on some documented agreement of different combinations
- of macro input meaning slightly different things. Add to this the ability
- to design such macros with variadic parameters and we have a preprocessor
- system of macro creation which to a lesser extent rivals the DSELS of template
- metaprogramming. Of course the preprocessor is not nearly as flexible as
- C++ templates, but still the sort of preprocessor metaprogramming one could
- do with VMD, and the underlying Boost PP, in creating flexible macros which
- can handle different combinations of data types is very interesting.
- Of course macros need to be usable by an end-user so the syntactical ability
- of sequences to represent different types of input data must
- be balanced against ease of use and understanding when using a macro. But because
- certain sequences can mimic C++ function calls to some extent it is possible
- to represent macros as a language closer to C++ with VMD.
- What is important when designing a macro in which you parse input to decide which type
- of data the invoker is passing to your macro is that you are aware of the constraints
- when parsing a data type. As an example if you design a macro where some input can either
- be a number, an identifier, or some other data type top-level input then attempting
- to parse the data to see if it is a number or identifier could fail with a preprocessor
- error and nullify your design if the data is not a VMD data type. So designing a macro with
- data types in mind often means restricting data to parseable top-level types.
- [endsect]
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