1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Input as dynamic types</title> 5<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../index.html" title="Chapter 1. The Variadic Macro Data Library 1.9"> 8<link rel="up" href="../index.html" title="Chapter 1. The Variadic Macro Data Library 1.9"> 9<link rel="prev" href="vmd_reentrant.html" title="Boost PP re-entrant versions"> 10<link rel="next" href="vmd_vc_isms.html" title="Visual C++ gotchas in VMD"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="vmd_reentrant.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="vmd_vc_isms.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 27<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 28 types</a> 29</h2></div></div></div> 30<p> 31 Within the constraints based on the top-level types which VMD can parse, the 32 libraries gives the end-user the ability to design macros with dynamic data 33 types. By this I mean that a macro could be designed to handle different data 34 types based on some documented agreement of different combinations of macro 35 input meaning slightly different things. Add to this the ability to design 36 such macros with variadic parameters and we have a preprocessor system of macro 37 creation which to a lesser extent rivals the DSELS of template metaprogramming. 38 Of course the preprocessor is not nearly as flexible as C++ templates, but 39 still the sort of preprocessor metaprogramming one could do with VMD, and the 40 underlying Boost PP, in creating flexible macros which can handle different 41 combinations of data types is very interesting. 42 </p> 43<p> 44 Of course macros need to be usable by an end-user so the syntactical ability 45 of sequences to represent different types of input data must be balanced against 46 ease of use and understanding when using a macro. But because certain sequences 47 can mimic C++ function calls to some extent it is possible to represent macros 48 as a language closer to C++ with VMD. 49 </p> 50<p> 51 What is important when designing a macro in which you parse input to decide 52 which type of data the invoker is passing to your macro is that you are aware 53 of the constraints when parsing a data type. As an example if you design a 54 macro where some input can either be a number, an identifier, or some other 55 data type top-level input then attempting to parse the data to see if it is 56 a number or identifier could fail with a preprocessor error and nullify your 57 design if the data is not a VMD data type. So designing a macro with data types 58 in mind often means restricting data to parseable top-level types. 59 </p> 60</div> 61<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 62<td align="left"></td> 63<td align="right"><div class="copyright-footer">Copyright © 2010-2017 Tropic Software 64 East Inc</div></td> 65</tr></table> 66<hr> 67<div class="spirit-nav"> 68<a accesskey="p" href="vmd_reentrant.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="vmd_vc_isms.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 69</div> 70</body> 71</html> 72
