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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="metaparse.the_design_of_the_library"></a><a class="link" href="the_design_of_the_library.html" title="The design of the library">The design of the
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library</a>
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</h2></div></div></div>
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<div class="toc"><dl class="toc"><dt><span class="section"><a href="the_design_of_the_library.html#metaparse.the_design_of_the_library.design_rationale">Design
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rationale</a></span></dt></dl></div>
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<p>
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The purpose of the library is to provide tools to build template metaprograms
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being able to interpret the content of a string literal and generate code,
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display error messages, etc based on the content of the string literal. Such
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metaprograms are called <a class="link" href="reference.html#parser">parser</a>s. Metaparse is
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based on <a href="https://en.wikipedia.org/wiki/Parser_combinator" target="_top">parser
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combinators</a>.
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</p>
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<p>
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The key components of the library:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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<a class="link" href="reference.html#ref-string">Compile-time string representation</a>. These
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are tools for representing the content of a string literal in a way that
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makes it possible for template metaprograms to work on them. For this the
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library provides the <a class="link" href="reference.html#string"><code class="computeroutput"><span class="identifier">string</span></code></a>
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template class, which is a drop-in replacement of Boost.MPL's <code class="computeroutput"><span class="identifier">string</span></code> implementation, and the <a class="link" href="reference.html#BOOST_METAPARSE_STRING"><code class="computeroutput"><span class="identifier">BOOST_METAPARSE_STRING</span></code></a>
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macro.
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</li>
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<li class="listitem">
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<a class="link" href="reference.html#parsers">Parsers</a>. These are template metafunction
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classes parsing a prefix of a string literal. These are simple <a class="link" href="reference.html#parser">parser</a>s
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providing the basic building blocks for more complicated ones doing some
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useful work.
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</li>
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<li class="listitem">
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<a class="link" href="reference.html#combinators">Parser combinators</a>. These are <a class="link" href="reference.html#metafunction">template metafunction</a>s taking <a class="link" href="reference.html#parser">parser</a>s
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as argument and/or returning <a class="link" href="reference.html#parser">parser</a>s as their
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result. They can be used to build more and more complex <a class="link" href="reference.html#parser">parser</a>s
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out of the simple ones.
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</li>
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</ul></div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="metaparse.the_design_of_the_library.design_rationale"></a><a class="link" href="the_design_of_the_library.html#metaparse.the_design_of_the_library.design_rationale" title="Design rationale">Design
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rationale</a>
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</h3></div></div></div>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
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<span class="bold"><strong>Why template metaprogramming?</strong></span>
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</li></ul></div>
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<p>
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An alternative is using <code class="computeroutput"><span class="keyword">constexpr</span></code>
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functions instead of template metaprograms. There are certain things that
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are difficult (if possible) using <code class="computeroutput"><span class="keyword">constexpr</span></code>
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functions: building containers (at compile-time) the length of which depend
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on the parsed text (eg. parsing a JSON list), generating and validating types
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(eg. <code class="computeroutput"><span class="identifier">printf</span></code>).
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
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<span class="bold"><strong>Why are there so many folding parsers?</strong></span>
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</li></ul></div>
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<p>
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Compilation speed and memory consumption is a critical part of template metaprogramming-based
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libraries. Users of the library interfaces built with Metaparse will have
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to pay for that every time they compile their code. Therefore it is important
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to provide the parser authors the ability to use the parser combinators with
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minimal overhead, while it is also important to provide convenient combinators
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for beginners and for the cases where that is the best option anyway.
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</p>
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<p>
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<a class="link" href="reference.html#repeated"><code class="computeroutput"><span class="identifier">repeated</span></code></a>
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combined with <a class="link" href="reference.html#sequence"><code class="computeroutput"><span class="identifier">sequence</span></code></a>,
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<a class="link" href="reference.html#accept_when"><code class="computeroutput"><span class="identifier">accept_when</span></code></a>
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and <a class="link" href="reference.html#transform"><code class="computeroutput"><span class="identifier">transform</span></code></a>
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can replace any of the folding parsers, however, for the cost of constructing
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intermediate containers, that are (usually) processed sequentially after
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that.
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
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<span class="bold"><strong>Why external code generator for <code class="computeroutput"><span class="identifier">BOOST_METAPARSE_STRING</span></code>?</strong></span>
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</li></ul></div>
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<p>
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To be able to support longer strings. It generates code using macros to reduce
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the size of the header files (the reducion is multiples of MBs).
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
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<span class="bold"><strong>Why defining a custom version of Boost.Preprocessor
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macros?</strong></span>
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</li></ul></div>
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<p>
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There are two reasons for the library defining its own set of preprocessor
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metaprogramming macros: to have control over the upper limit of iteration
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steps and to be able to clean the macros up once they have done their job
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(and avoid polluting the macros of the users).
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</p>
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<p>
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Note that these macros live in the <code class="computeroutput"><span class="identifier">impl</span></code>
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directory, which means that they are an implementation detail of the library
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and should be used internally only.
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</p>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2015 Abel Sinkovics<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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</p>
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</div></td>
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</tr></table>
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