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Library/boost/doc/html/align/rationale.html
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19 дек 2024, 19:11
19 дек 2024, 19:11
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Rationale</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../align.html" title="Chapter 3. Boost.Align"> <link rel="prev" href="../align.html" title="Chapter 3. Boost.Align"> <link rel="next" href="examples.html" title="Examples"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="../align.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../align.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="examples.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="align.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a> </h2></div></div></div> <h4> <a name="align.rationale.h0"></a> <span class="phrase"><a name="align.rationale.dynamic_allocation"></a></span><a class="link" href="rationale.html#align.rationale.dynamic_allocation">Dynamic allocation</a> </h4> <p> C++11 added the ability to specify increased alignment (over-alignment) for class types. Unfortunately, <code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span> <span class="keyword">new</span></code> allocation functions, <code class="computeroutput"><span class="keyword">new</span></code> expressions and the Default Allocator, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator</span></code>, do not support dynamic memory allocation of over-aligned data. This library provides allocation functions and allocators that respect the alignment requirements of a type and so are suitable for allocating memory for over-aligned types. </p> <div class="variablelist"> <p class="title"><b></b></p> <dl class="variablelist"> <dt><span class="term"><code class="computeroutput"><span class="identifier">aligned_alloc</span><span class="special">(</span><span class="identifier">alignment</span><span class="special">,</span> <span class="identifier">size</span><span class="special">)</span></code></span></dt> <dd><p> Replaces <code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span> <span class="keyword">new</span><span class="special">(</span><span class="identifier">size</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">nothrow</span><span class="special">)</span></code> </p></dd> <dt><span class="term"><code class="computeroutput"><span class="identifier">aligned_free</span><span class="special">(</span><span class="identifier">pointer</span><span class="special">)</span></code></span></dt> <dd><p> Replaces <code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span> <span class="keyword">delete</span><span class="special">(</span><span class="identifier">pointer</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">nothrow</span><span class="special">)</span></code> </p></dd> <dt><span class="term"><code class="computeroutput"><span class="identifier">aligned_allocator</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code></span></dt> <dd><p> Replaces <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> </p></dd> <dt><span class="term"><code class="computeroutput"><span class="identifier">aligned_allocator_adaptor</span><span class="special"><</span><span class="identifier">Allocator</span><span class="special">></span></code></span></dt> <dd><p> Replaces use of Allocator </p></dd> <dt><span class="term"><code class="computeroutput"><span class="identifier">aligned_delete</span></code></span></dt> <dd><p> Replaces <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">default_delete</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> </p></dd> </dl> </div> <h4> <a name="align.rationale.h1"></a> <span class="phrase"><a name="align.rationale.pointer_alignment"></a></span><a class="link" href="rationale.html#align.rationale.pointer_alignment">Pointer alignment</a> </h4> <p> C++11 provided <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">align</span></code> in the standard library to align a pointer value. Unfortunately some C++ standard library implementations do not support it yet (libstdc++ as far as gcc 4.8.0) and other standard library implementations implement it incorrectly (dinkumware in msvc11.0). This library provides it for those implementations and also for C++03 compilers where it is equally useful. </p> <h4> <a name="align.rationale.h2"></a> <span class="phrase"><a name="align.rationale.querying_alignment"></a></span><a class="link" href="rationale.html#align.rationale.querying_alignment">Querying alignment</a> </h4> <p> C++11 provided the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">alignment_of</span></code> trait in the standard library to query the alignment requirement of a type. Unfortunately some C++ standard library vendors do not implement it in an entirely standard conforming manner, such as for array types (libc++ as far as clang 3.4). Other vendor implementations report incorrect values for certain types, such as pointer to members (msvc 14.0). This library provides it for those implementations and also for C++03 compilers where it is equally useful. </p> <h4> <a name="align.rationale.h3"></a> <span class="phrase"><a name="align.rationale.hinting_alignment"></a></span><a class="link" href="rationale.html#align.rationale.hinting_alignment">Hinting alignment</a> </h4> <p> Allocating aligned memory is sometimes not enough to ensure that optimal code is generated. Developers use specific compiler intrinsics to notify the compiler of a given alignment property of a memory block. This library provides a macro, <code class="computeroutput"><span class="identifier">BOOST_ALIGN_ASSUME_ALIGNED</span></code>, to abstract that functionality for compilers with the appropriate intrinsics. </p> <h4> <a name="align.rationale.h4"></a> <span class="phrase"><a name="align.rationale.checking_alignment"></a></span><a class="link" href="rationale.html#align.rationale.checking_alignment">Checking alignment</a> </h4> <p> This library provides a function, <code class="computeroutput"><span class="identifier">is_aligned</span></code> to test the alignment of a pointer value. It is generally useful in assertions to validate that memory is correctly aligned. </p> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2014-2017 Glen Joseph Fernandes<p> Distributed under the Boost Software License, Version 1.0. </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="../align.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../align.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="examples.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>