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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="metaparse.preface"></a><a class="link" href="preface.html" title="Preface">Preface</a>
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</h2></div></div></div>
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="preface.html#metaparse.preface.description">Description</a></span></dt>
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<dt><span class="section"><a href="preface.html#metaparse.preface.scope">Scope</a></span></dt>
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<dt><span class="section"><a href="preface.html#metaparse.preface.advantages_of_using_this_library">Advantages
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of using this library</a></span></dt>
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<dt><span class="section"><a href="preface.html#metaparse.preface.cost_of_using_metaparse">Cost of using
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Metaparse</a></span></dt>
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<dt><span class="section"><a href="preface.html#metaparse.preface.supported_platforms">Supported platforms</a></span></dt>
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</dl></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.preface.description"></a><a class="link" href="preface.html#metaparse.preface.description" title="Description">Description</a>
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</h3></div></div></div>
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<p>
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Metaparse is a compile-time parser generator library. Metaparse provides
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tools to write parsers parsing the content of string literals at compile-time,
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which makes it possible to embed domain specific languages (DSLs) into C++
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without altering their original syntax (Note that the DSL code snippets will
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be written in string literals, therefore they may need to be escaped).
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</p>
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<p>
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Assuming that the following template class is available for representing
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rational numbers in template metaprogramming:
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</p>
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<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>
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<span class="keyword">struct</span> <span class="identifier">rational</span><span class="special">;</span>
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</pre>
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<p>
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Metaparse can be used to construct such values (instantiate the <code class="computeroutput"><span class="identifier">rational</span></code> template class) from string literals.
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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
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C++11 compiler (and mean the same). This can be implemented using Metaparse
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the following way:
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</p>
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<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>
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<span class="keyword">typedef</span>
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<span class="identifier">sequence_apply2</span><span class="special"><</span>
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<span class="identifier">rational</span><span class="special">,</span>
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<span class="identifier">token</span><span class="special"><</span><span class="identifier">int_</span><span class="special">>,</span>
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<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>
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<span class="special">></span>
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<span class="identifier">rational_grammar</span><span class="special">;</span>
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<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>
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<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>
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<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>
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</pre>
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<p>
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Note that this is the entire implementation. Also note that this implementation
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can be extended to improve the error reports in certain situations.
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</p>
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</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.preface.scope"></a><a class="link" href="preface.html#metaparse.preface.scope" title="Scope">Scope</a>
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</h3></div></div></div>
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="preface.html#metaparse.preface.scope.comparsion_to_boost_proto">Comparsion
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to Boost.Proto</a></span></dt>
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<dt><span class="section"><a href="preface.html#metaparse.preface.scope.comparison_to_boost_spirit">Comparison
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to Boost.Spirit</a></span></dt>
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</dl></div>
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<p>
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Metaparse is intended to be used by library authors to make their APIs follow
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the usual notation of the library's problem domain.
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</p>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<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
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to Boost.Proto</a>
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</h4></div></div></div>
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<p>
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Boost.Proto is a tool for building expression templates. Expression templates
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can be used for DSL embedding by reinterpreting valid C++ expressions as
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expressions written in the DSL to embed.
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</p>
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<p>
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This technique has the advantages over parsing the content of string literals
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(which is Metaparse's approach) that:
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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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is faster in most cases
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</li>
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<li class="listitem">
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APIs using this technique can "emerge" as a process of advancing
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the API of a library step-by-step. Moving to a completely new DSL (with
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its own syntax) is a relatively big step.
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</li>
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</ul></div>
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<p>
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Using expression templates for DSL embedding has the following disadvantages:
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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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the syntax of the embedded DSL is limited. It has to be a valid C++
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expression. For most libraries, people familiar with the original DSL
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usually need to learn the library's syntax to understand the embedded
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code snippets.
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</li></ul></div>
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<p>
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Proto helps embedding DSLs based on expression templates, while Metaparse
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helps embedding DSLs based on parsing the content of string literals.
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<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
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to Boost.Spirit</a>
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</h4></div></div></div>
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<p>
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Spirit is a tool that can be used to build parsers parsing (among others)
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the content of string literals at runtime, while Metaparse is a tool that
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can be used to parse the content of string literals at compile-time.
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</p>
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</div>
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</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.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
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of using this library</a>
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</h3></div></div></div>
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<p>
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This library is useful to provide an API for C++ libraries dealing with a
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problem domain with its own notation. Interfaces built with Metaparse make
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it possible for the users of the interface to use the domain's own notation,
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which makes it easier to write and maintain the code. Users of the interface
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don't need to learn a new notation (trying to follow the problem domain's
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original one) library authors constrained by the C++ syntax can provide.
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Example problem domains are regular expressions and SQL queries.
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</p>
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<p>
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Metaparse can also be useful to build libraries validating the content of
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string literals at compile time instead of doing it at runtime or not doing
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it at all. This can help finding (and fixing) bugs in the code early (during
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compilation). An example problem domain is <code class="computeroutput"><span class="identifier">printf</span></code>.
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</p>
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</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.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
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Metaparse</a>
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</h3></div></div></div>
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<p>
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The parsers built with Metaparse process the content of the string literals
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using template metaprograms. This impacts the library using Metaparse the
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following way:
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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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The maintainer of the API built with Metaparse will need to understand
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template metaprogramming.
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</li>
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<li class="listitem">
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The content of the string literals will be (re)parsed during every compilation.
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This will impact the compiler's memory consumption and the compilation
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speed.
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</li>
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<li class="listitem">
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The users of the library will receive the error reports coming from the
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parsers as template error messages of their compiler. (Note that Metaparse
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actively tries improving their quality and provides <a class="link" href="getting_started_with_boost_metap.html#dealing_with_invalid_input">tools</a>
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for parser authors).
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</li>
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</ul></div>
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</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.preface.supported_platforms"></a><a class="link" href="preface.html#metaparse.preface.supported_platforms" title="Supported platforms">Supported platforms</a>
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</h3></div></div></div>
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<p>
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Metaparse is based on C++98. The only exception is the <a class="link" href="reference.html#BOOST_METAPARSE_STRING">BOOST_METAPARSE_STRING</a>
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macro, which needs C++11 <code class="computeroutput"><span class="keyword">constexpr</span></code>.
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</p>
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<p>
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Compilers Metaparse is actively (in a CI environment) tested on:
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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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GCC 4.6, 4.7, 4.8, 4.9
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</li>
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<li class="listitem">
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Clang 3.4, 3.5, 3.6
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</li>
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<li class="listitem">
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Visual C++ 2015
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</li>
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</ul></div>
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<p>
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Metaparse is expected to work on Visual C++ 2012 and 2010.
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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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