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<title>Input as dynamic types</title>
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<a name="variadic_macro_data.vmd_dynamic_typing"></a><a class="link" href="vmd_dynamic_typing.html" title="Input as dynamic types">Input as dynamic
    types</a>
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<p>
      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.
    </p>
<p>
      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.
    </p>
<p>
      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.
    </p>
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