| <?xml version="1.0" encoding="UTF-8" standalone="no"?> |
| <!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>Chapter 16. Compile Time Checks</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="extensions.html" title="Part III. Extensions" /><link rel="prev" href="ext_preface.html" title="" /><link rel="next" href="debug_mode.html" title="Chapter 17. Debug Mode" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 16. Compile Time Checks</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ext_preface.html">Prev</a> </td><th width="60%" align="center">Part III. |
| Extensions |
| |
| </th><td width="20%" align="right"> <a accesskey="n" href="debug_mode.html">Next</a></td></tr></table><hr /></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a id="manual.ext.compile_checks"></a>Chapter 16. Compile Time Checks</h2></div></div></div><p> |
| Also known as concept checking. |
| </p><p>In 1999, SGI added <span class="emphasis"><em>concept checkers</em></span> to their implementation |
| of the STL: code which checked the template parameters of |
| instantiated pieces of the STL, in order to insure that the parameters |
| being used met the requirements of the standard. For example, |
| the Standard requires that types passed as template parameters to |
| <code class="code">vector</code> be <span class="quote">“<span class="quote">Assignable</span>”</span> (which means what you think |
| it means). The checking was done during compilation, and none of |
| the code was executed at runtime. |
| </p><p>Unfortunately, the size of the compiler files grew significantly |
| as a result. The checking code itself was cumbersome. And bugs |
| were found in it on more than one occasion. |
| </p><p>The primary author of the checking code, Jeremy Siek, had already |
| started work on a replacement implementation. The new code has been |
| formally reviewed and accepted into |
| <a class="link" href="http://www.boost.org/libs/concept_check/concept_check.htm" target="_top">the |
| Boost libraries</a>, and we are pleased to incorporate it into the |
| GNU C++ library. |
| </p><p>The new version imposes a much smaller space overhead on the generated |
| object file. The checks are also cleaner and easier to read and |
| understand. |
| </p><p>They are off by default for all GCC 3.0 and all later versions. |
| They can be enabled at configure time with |
| <a class="link" href="configure.html" title="Configure"><code class="literal">--enable-concept-checks</code></a>. |
| You can enable them on a per-translation-unit basis with |
| <a class="link" href="using_macros.html" title="Macros"><code class="code">#define |
| _GLIBCXX_CONCEPT_CHECKS</code></a> for GCC 3.4 and higher |
| (or with <code class="code">#define _GLIBCPP_CONCEPT_CHECKS</code> for versions |
| 3.1, 3.2 and 3.3). |
| </p><p>Please note that the concept checks only validate the requirements |
| of the old C++03 standard and reject some valid code that meets the relaxed |
| requirements of C++11 and later standards. |
| C++11 was expected to have first-class support for template parameter |
| constraints based on concepts in the core language. |
| This would have obviated the need for the library-simulated concept checking |
| described above, but was not part of C++11. |
| C++20 adds a different model of concepts, which is now used to constrain |
| some new parts of the C++20 library, e.g. the |
| <code class="filename"><ranges></code> header and the new overloads in the |
| <code class="filename"><algorithm></code> header for working with ranges. |
| The old library-simulated concept checks might be removed at a future date. |
| </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ext_preface.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="extensions.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="debug_mode.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top"> </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 17. Debug Mode</td></tr></table></div></body></html> |
