1*36ac495dSmrg<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2*36ac495dSmrg<!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 Stylesheets Vsnapshot" /><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. 3*36ac495dSmrg Diagnostics 4*36ac495dSmrg 5*36ac495dSmrg</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> 6*36ac495dSmrg In 1999, SGI added <span class="quote">“<span class="quote">concept checkers</span>”</span> to their 7*36ac495dSmrg implementation of the STL: code which checked the template 8*36ac495dSmrg parameters of instantiated pieces of the STL, in order to insure 9*36ac495dSmrg that the parameters being used met the requirements of the 10*36ac495dSmrg standard. For example, the Standard requires that types passed as 11*36ac495dSmrg template parameters to <code class="classname">vector</code> be 12*36ac495dSmrg "Assignable" (which means what you think it means). The 13*36ac495dSmrg checking was done during compilation, and none of the code was 14*36ac495dSmrg executed at runtime. 15*36ac495dSmrg </p><p> 16*36ac495dSmrg Unfortunately, the size of the compiler files grew significantly 17*36ac495dSmrg as a result. The checking code itself was cumbersome. And bugs 18*36ac495dSmrg were found in it on more than one occasion. 19*36ac495dSmrg </p><p> 20*36ac495dSmrg The primary author of the checking code, Jeremy Siek, had already 21*36ac495dSmrg started work on a replacement implementation. The new code was 22*36ac495dSmrg formally reviewed and accepted into 23*36ac495dSmrg <a class="link" href="http://www.boost.org/libs/concept_check/concept_check.htm" target="_top">the 24*36ac495dSmrg Boost libraries</a>, and we are pleased to incorporate it into the 25*36ac495dSmrg GNU C++ library. 26*36ac495dSmrg </p><p> 27*36ac495dSmrg The new version imposes a much smaller space overhead on the generated 28*36ac495dSmrg object file. The checks are also cleaner and easier to read and 29*36ac495dSmrg understand. 30*36ac495dSmrg </p><p> 31*36ac495dSmrg They are off by default for all versions of GCC. 32*36ac495dSmrg They can be enabled at configure time with 33*36ac495dSmrg <a class="link" href="configure.html" title="Configure"><code class="literal">--enable-concept-checks</code></a>. 34*36ac495dSmrg You can enable them on a per-translation-unit basis with 35*36ac495dSmrg <code class="literal">-D_GLIBCXX_CONCEPT_CHECKS</code>. 36*36ac495dSmrg </p><p> 37*36ac495dSmrg Please note that the checks are based on the requirements in the original 38*36ac495dSmrg C++ standard, many of which were relaxed in the C++11 standard and so valid 39*36ac495dSmrg C++11 code may be incorrectly rejected by the concept checks. Additionally, 40*36ac495dSmrg some correct C++03 code might be rejected by the concept checks, 41*36ac495dSmrg for example template argument types may need to be complete when used in 42*36ac495dSmrg a template definition, rather than at the point of instantiation. 43*36ac495dSmrg There are no plans to address these shortcomings. 44*36ac495dSmrg </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. 45*36ac495dSmrg Utilities 46*36ac495dSmrg 47*36ac495dSmrg</td></tr></table></div></body></html>