1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Preface</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../metaparse.html" title="Chapter 24. Boost.Metaparse"> 10<link rel="prev" href="related_publications_and_blogs.html" title="Related publications and blogs"> 11<link rel="next" href="getting_started_with_boost_metap.html" title="Getting started with Boost.Metaparse"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="related_publications_and_blogs.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../metaparse.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="getting_started_with_boost_metap.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="metaparse.preface"></a><a class="link" href="preface.html" title="Preface">Preface</a> 29</h2></div></div></div> 30<div class="toc"><dl class="toc"> 31<dt><span class="section"><a href="preface.html#metaparse.preface.description">Description</a></span></dt> 32<dt><span class="section"><a href="preface.html#metaparse.preface.scope">Scope</a></span></dt> 33<dt><span class="section"><a href="preface.html#metaparse.preface.advantages_of_using_this_library">Advantages 34 of using this library</a></span></dt> 35<dt><span class="section"><a href="preface.html#metaparse.preface.cost_of_using_metaparse">Cost of using 36 Metaparse</a></span></dt> 37<dt><span class="section"><a href="preface.html#metaparse.preface.supported_platforms">Supported platforms</a></span></dt> 38</dl></div> 39<div class="section"> 40<div class="titlepage"><div><div><h3 class="title"> 41<a name="metaparse.preface.description"></a><a class="link" href="preface.html#metaparse.preface.description" title="Description">Description</a> 42</h3></div></div></div> 43<p> 44 Metaparse is a compile-time parser generator library. Metaparse provides 45 tools to write parsers parsing the content of string literals at compile-time, 46 which makes it possible to embed domain specific languages (DSLs) into C++ 47 without altering their original syntax (Note that the DSL code snippets will 48 be written in string literals, therefore they may need to be escaped). 49 </p> 50<p> 51 Assuming that the following template class is available for representing 52 rational numbers in template metaprogramming: 53 </p> 54<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Num</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Denom</span><span class="special">></span> 55<span class="keyword">struct</span> <span class="identifier">rational</span><span class="special">;</span> 56</pre> 57<p> 58 Metaparse can be used to construct such values (instantiate the <code class="computeroutput"><span class="identifier">rational</span></code> template class) from string literals. 59 Instead of <code class="computeroutput"><span class="identifier">rational</span><span class="special"><</span><span class="number">1</span><span class="special">,</span> <span class="number">3</span><span class="special">></span></code> one can write <code class="computeroutput"><span class="identifier">RATIONAL</span><span class="special">(</span><span class="string">"1/3"</span><span class="special">)</span></code> which can be processed by any standard-compliant 60 C++11 compiler (and mean the same). This can be implemented using Metaparse 61 the following way: 62 </p> 63<pre class="programlisting"><span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">metaparse</span><span class="special">;</span> 64 65<span class="keyword">typedef</span> 66 <span class="identifier">sequence_apply2</span><span class="special"><</span> 67 <span class="identifier">rational</span><span class="special">,</span> 68 69 <span class="identifier">token</span><span class="special"><</span><span class="identifier">int_</span><span class="special">>,</span> 70 <span class="identifier">last_of</span><span class="special"><</span><span class="identifier">lit_c</span><span class="special"><</span><span class="char">'/'</span><span class="special">>,</span> <span class="identifier">token</span><span class="special"><</span><span class="identifier">int_</span><span class="special">>></span> 71 <span class="special">></span> 72 <span class="identifier">rational_grammar</span><span class="special">;</span> 73 74<span class="keyword">typedef</span> <span class="identifier">build_parser</span><span class="special"><</span><span class="identifier">entire_input</span><span class="special"><</span><span class="identifier">rational_grammar</span><span class="special">>></span> <span class="identifier">rational_parser</span><span class="special">;</span> 75 76<span class="preprocessor">#define</span> <span class="identifier">RATIONAL</span><span class="special">(</span><span class="identifier">s</span><span class="special">)</span> <span class="special">\</span> 77 <span class="special">(::</span><span class="identifier">rational_parser</span><span class="special">::</span><span class="identifier">apply</span><span class="special"><</span><span class="identifier">BOOST_METAPARSE_STRING</span><span class="special">(</span><span class="identifier">s</span><span class="special">)>::</span><span class="identifier">type</span><span class="special">::</span><span class="identifier">run</span><span class="special">())</span> 78</pre> 79<p> 80 Note that this is the entire implementation. Also note that this implementation 81 can be extended to improve the error reports in certain situations. 82 </p> 83</div> 84<div class="section"> 85<div class="titlepage"><div><div><h3 class="title"> 86<a name="metaparse.preface.scope"></a><a class="link" href="preface.html#metaparse.preface.scope" title="Scope">Scope</a> 87</h3></div></div></div> 88<div class="toc"><dl class="toc"> 89<dt><span class="section"><a href="preface.html#metaparse.preface.scope.comparsion_to_boost_proto">Comparsion 90 to Boost.Proto</a></span></dt> 91<dt><span class="section"><a href="preface.html#metaparse.preface.scope.comparison_to_boost_spirit">Comparison 92 to Boost.Spirit</a></span></dt> 93</dl></div> 94<p> 95 Metaparse is intended to be used by library authors to make their APIs follow 96 the usual notation of the library's problem domain. 97 </p> 98<div class="section"> 99<div class="titlepage"><div><div><h4 class="title"> 100<a name="metaparse.preface.scope.comparsion_to_boost_proto"></a><a class="link" href="preface.html#metaparse.preface.scope.comparsion_to_boost_proto" title="Comparsion to Boost.Proto">Comparsion 101 to Boost.Proto</a> 102</h4></div></div></div> 103<p> 104 Boost.Proto is a tool for building expression templates. Expression templates 105 can be used for DSL embedding by reinterpreting valid C++ expressions as 106 expressions written in the DSL to embed. 107 </p> 108<p> 109 This technique has the advantages over parsing the content of string literals 110 (which is Metaparse's approach) that: 111 </p> 112<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 113<li class="listitem"> 114 is faster in most cases 115 </li> 116<li class="listitem"> 117 APIs using this technique can "emerge" as a process of advancing 118 the API of a library step-by-step. Moving to a completely new DSL (with 119 its own syntax) is a relatively big step. 120 </li> 121</ul></div> 122<p> 123 Using expression templates for DSL embedding has the following disadvantages: 124 </p> 125<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> 126 the syntax of the embedded DSL is limited. It has to be a valid C++ 127 expression. For most libraries, people familiar with the original DSL 128 usually need to learn the library's syntax to understand the embedded 129 code snippets. 130 </li></ul></div> 131<p> 132 Proto helps embedding DSLs based on expression templates, while Metaparse 133 helps embedding DSLs based on parsing the content of string literals. 134 </p> 135</div> 136<div class="section"> 137<div class="titlepage"><div><div><h4 class="title"> 138<a name="metaparse.preface.scope.comparison_to_boost_spirit"></a><a class="link" href="preface.html#metaparse.preface.scope.comparison_to_boost_spirit" title="Comparison to Boost.Spirit">Comparison 139 to Boost.Spirit</a> 140</h4></div></div></div> 141<p> 142 Spirit is a tool that can be used to build parsers parsing (among others) 143 the content of string literals at runtime, while Metaparse is a tool that 144 can be used to parse the content of string literals at compile-time. 145 </p> 146</div> 147</div> 148<div class="section"> 149<div class="titlepage"><div><div><h3 class="title"> 150<a name="metaparse.preface.advantages_of_using_this_library"></a><a class="link" href="preface.html#metaparse.preface.advantages_of_using_this_library" title="Advantages of using this library">Advantages 151 of using this library</a> 152</h3></div></div></div> 153<p> 154 This library is useful to provide an API for C++ libraries dealing with a 155 problem domain with its own notation. Interfaces built with Metaparse make 156 it possible for the users of the interface to use the domain's own notation, 157 which makes it easier to write and maintain the code. Users of the interface 158 don't need to learn a new notation (trying to follow the problem domain's 159 original one) library authors constrained by the C++ syntax can provide. 160 Example problem domains are regular expressions and SQL queries. 161 </p> 162<p> 163 Metaparse can also be useful to build libraries validating the content of 164 string literals at compile time instead of doing it at runtime or not doing 165 it at all. This can help finding (and fixing) bugs in the code early (during 166 compilation). An example problem domain is <code class="computeroutput"><span class="identifier">printf</span></code>. 167 </p> 168</div> 169<div class="section"> 170<div class="titlepage"><div><div><h3 class="title"> 171<a name="metaparse.preface.cost_of_using_metaparse"></a><a class="link" href="preface.html#metaparse.preface.cost_of_using_metaparse" title="Cost of using Metaparse">Cost of using 172 Metaparse</a> 173</h3></div></div></div> 174<p> 175 The parsers built with Metaparse process the content of the string literals 176 using template metaprograms. This impacts the library using Metaparse the 177 following way: 178 </p> 179<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 180<li class="listitem"> 181 The maintainer of the API built with Metaparse will need to understand 182 template metaprogramming. 183 </li> 184<li class="listitem"> 185 The content of the string literals will be (re)parsed during every compilation. 186 This will impact the compiler's memory consumption and the compilation 187 speed. 188 </li> 189<li class="listitem"> 190 The users of the library will receive the error reports coming from the 191 parsers as template error messages of their compiler. (Note that Metaparse 192 actively tries improving their quality and provides <a class="link" href="getting_started_with_boost_metap.html#dealing_with_invalid_input">tools</a> 193 for parser authors). 194 </li> 195</ul></div> 196</div> 197<div class="section"> 198<div class="titlepage"><div><div><h3 class="title"> 199<a name="metaparse.preface.supported_platforms"></a><a class="link" href="preface.html#metaparse.preface.supported_platforms" title="Supported platforms">Supported platforms</a> 200</h3></div></div></div> 201<p> 202 Metaparse is based on C++98. The only exception is the <a class="link" href="reference.html#BOOST_METAPARSE_STRING">BOOST_METAPARSE_STRING</a> 203 macro, which needs C++11 <code class="computeroutput"><span class="keyword">constexpr</span></code>. 204 </p> 205<p> 206 Compilers Metaparse is actively (in a CI environment) tested on: 207 </p> 208<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 209<li class="listitem"> 210 GCC 4.6, 4.7, 4.8, 4.9 211 </li> 212<li class="listitem"> 213 Clang 3.4, 3.5, 3.6 214 </li> 215<li class="listitem"> 216 Visual C++ 2015 217 </li> 218</ul></div> 219<p> 220 Metaparse is expected to work on Visual C++ 2012 and 2010. 221 </p> 222</div> 223</div> 224<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 225<td align="left"></td> 226<td align="right"><div class="copyright-footer">Copyright © 2015 Abel Sinkovics<p> 227 Distributed under the Boost Software License, Version 1.0. (See accompanying 228 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 229 </p> 230</div></td> 231</tr></table> 232<hr> 233<div class="spirit-nav"> 234<a accesskey="p" href="related_publications_and_blogs.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../metaparse.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="getting_started_with_boost_metap.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 235</div> 236</body> 237</html> 238
