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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Concept Checking</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.78.1" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="diagnostics.html" title="Chapter 5. Diagnostics" /><link rel="prev" href="errno.html" title="Use of errno by the library" /><link rel="next" href="utilities.html" title="Chapter 6. Utilities" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Concept Checking</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="errno.html">Prev</a> </td><th width="60%" align="center">Chapter 5.
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Diagnostics
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</th><td width="20%" align="right"> <a accesskey="n" href="utilities.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.diagnostics.concept_checking"></a>Concept Checking</h2></div></div></div><p>
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In 1999, SGI added <span class="quote">“<span class="quote">concept checkers</span>”</span> to their
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implementation of the STL: code which checked the template
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parameters of instantiated pieces of the STL, in order to insure
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that the parameters being used met the requirements of the
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standard. For example, the Standard requires that types passed as
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template parameters to <code class="classname">vector</code> be
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"Assignable" (which means what you think it means). The
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checking was done during compilation, and none of the code was
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executed at runtime.
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</p><p>
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Unfortunately, the size of the compiler files grew significantly
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as a result. The checking code itself was cumbersome. And bugs
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were found in it on more than one occasion.
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</p><p>
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The primary author of the checking code, Jeremy Siek, had already
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started work on a replacement implementation. The new code was
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formally reviewed and accepted into
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<a class="link" href="http://www.boost.org/libs/concept_check/concept_check.htm" target="_top">the
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Boost libraries</a>, and we are pleased to incorporate it into the
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GNU C++ library.
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</p><p>
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The new version imposes a much smaller space overhead on the generated
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object file. The checks are also cleaner and easier to read and
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understand.
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</p><p>
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They are off by default for all versions of GCC.
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They can be enabled at configure time with
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<a class="link" href="configure.html" title="Configure"><code class="literal">--enable-concept-checks</code></a>.
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You can enable them on a per-translation-unit basis with
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<code class="literal">-D_GLIBCXX_CONCEPT_CHECKS</code>.
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</p><p>
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Please note that the checks are based on the requirements in the original
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C++ standard, many of which were relaxed in the C++11 standard and so valid
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C++11 code may be incorrectly rejected by the concept checks. Additionally,
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some correct C++03 code might be rejected by the concept checks,
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for example template argument types may need to be complete when used in
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a template definition, rather than at the point of instantiation.
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There are no plans to address these shortcomings.
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</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="errno.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="diagnostics.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="utilities.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Use of errno by the library </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 6.
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Utilities
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