| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>Why scoped attributes don't override existing attributes?</title> |
| <link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> |
| <link rel="home" href="../../index.html" title="Chapter 1. Boost.Log v2"> |
| <link rel="up" href="../rationale.html" title="Rationale and FAQ"> |
| <link rel="prev" href="../rationale.html" title="Rationale and FAQ"> |
| <link rel="next" href="why_weak_record_ordering.html" title="Why log records are weakly ordered in a multithreaded application?"> |
| </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></tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="../rationale.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../rationale.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="why_weak_record_ordering.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="log.rationale.why_weak_scoped_attributes"></a><a class="link" href="why_weak_scoped_attributes.html" title="Why scoped attributes don't override existing attributes?">Why scoped |
| attributes don't override existing attributes?</a> |
| </h3></div></div></div> |
| <p> |
| Initially scoped attributes were able to override other attributes with the |
| same name if they were already registered by the time when a scoped attribute |
| encountered. This allowed some interesting use cases like this: |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_LOG_DECLARE_GLOBAL_LOGGER</span><span class="special">(</span><span class="identifier">my_logger</span><span class="special">,</span> <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger_mt</span><span class="special">)</span> |
| |
| <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="comment">// This scoped attribute would temporarily replace the existing tag</span> |
| <span class="identifier">BOOST_LOG_SCOPED_THREAD_TAG</span><span class="special">(</span><span class="string">"Section"</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="string">"In foo"</span><span class="special">);</span> |
| |
| <span class="comment">// This log record will have a "Section" attribute with value "In foo"</span> |
| <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">get_my_logger</span><span class="special">())</span> <span class="special"><<</span> <span class="string">"We're in foo section"</span><span class="special">;</span> |
| <span class="special">}</span> |
| |
| <span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">char</span><span class="special">*[])</span> |
| <span class="special">{</span> |
| <span class="identifier">BOOST_LOG_SCOPED_THREAD_TAG</span><span class="special">(</span><span class="string">"Section"</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="string">"In main"</span><span class="special">);</span> |
| |
| <span class="comment">// This log record will have a "Section" attribute with value "In main"</span> |
| <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">get_my_logger</span><span class="special">())</span> <span class="special"><<</span> <span class="string">"We're in main section"</span><span class="special">;</span> |
| |
| <span class="identifier">foo</span><span class="special">();</span> |
| |
| <span class="comment">// This log record will have a "Section" attribute with value "In main" again</span> |
| <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">get_my_logger</span><span class="special">())</span> <span class="special"><<</span> <span class="string">"We're in main section again"</span><span class="special">;</span> |
| |
| <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| However, this feature introduced a number of safety problems, including thread |
| safety issues, that could be difficult to track down. For example, it was |
| no longer safe to use logger-wide scoped attributes on the same logger from |
| different threads, because the resulting attribute would be undefined: |
| </p> |
| <pre class="programlisting"><span class="identifier">BOOST_LOG_DECLARE_GLOBAL_LOGGER</span><span class="special">(</span><span class="identifier">my_logger</span><span class="special">,</span> <span class="identifier">src</span><span class="special">::</span><span class="identifier">logger_mt</span><span class="special">)</span> |
| |
| <span class="keyword">void</span> <span class="identifier">thread1</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="identifier">BOOST_LOG_SCOPED_LOGGER_TAG</span><span class="special">(</span><span class="identifier">get_my_logger</span><span class="special">(),</span> <span class="string">"Tag"</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="string">"thread1"</span><span class="special">);</span> |
| <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">get_my_logger</span><span class="special">())</span> <span class="special"><<</span> <span class="string">"We're in thread1"</span><span class="special">;</span> |
| <span class="special">}</span> |
| |
| <span class="keyword">void</span> <span class="identifier">thread2</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="identifier">BOOST_LOG_SCOPED_LOGGER_TAG</span><span class="special">(</span><span class="identifier">get_my_logger</span><span class="special">(),</span> <span class="string">"Tag"</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="number">10</span><span class="special">);</span> |
| <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">get_my_logger</span><span class="special">())</span> <span class="special"><<</span> <span class="string">"We're in thread2"</span><span class="special">;</span> |
| <span class="special">}</span> |
| |
| <span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">char</span><span class="special">*[])</span> |
| <span class="special">{</span> |
| <span class="identifier">BOOST_LOG_SCOPED_LOGGER_TAG</span><span class="special">(</span><span class="identifier">get_my_logger</span><span class="special">(),</span> <span class="string">"Tag"</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="special">-</span><span class="number">2.2</span><span class="special">);</span> |
| |
| <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">get_my_logger</span><span class="special">())</span> <span class="special"><<</span> <span class="string">"We're in main"</span><span class="special">;</span> |
| |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread</span> <span class="identifier">t1</span><span class="special">(&</span><span class="identifier">thread1</span><span class="special">);</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">thread</span> <span class="identifier">t2</span><span class="special">(&</span><span class="identifier">thread2</span><span class="special">);</span> |
| |
| <span class="identifier">t1</span><span class="special">.</span><span class="identifier">join</span><span class="special">();</span> |
| <span class="identifier">t2</span><span class="special">.</span><span class="identifier">join</span><span class="special">();</span> |
| |
| <span class="comment">// Which "Tag" is registered here?</span> |
| <span class="identifier">BOOST_LOG</span><span class="special">(</span><span class="identifier">get_my_logger</span><span class="special">())</span> <span class="special"><<</span> <span class="string">"We're in main again"</span><span class="special">;</span> |
| |
| <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| There were other issues, like having an attribute set iterator that points |
| to one attribute object, then suddenly without seemingly modifying it it |
| becomes pointing to a different attribute object (of, possibly, a different |
| type). Such behavior could lead to tricky failures that would be difficult |
| to investigate. Therefore this feature was eventually dropped, which simplified |
| the scoped attributes implementation significantly. |
| </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 © 2007-2015 Andrey |
| Semashev<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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>). |
| </p> |
| </div></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="../rationale.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../rationale.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="why_weak_record_ordering.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| </body> |
| </html> |
