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| <a name="log.rationale.why_not_lazy_streaming"></a><a class="link" href="why_not_lazy_streaming.html" title="Why not using lazy streaming?">Why not using lazy |
| streaming?</a> |
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| <p> |
| One of the possible library implementations would be using lazy expressions |
| to delay log record formatting. In essence, the expression: |
| </p> |
| <pre class="programlisting"><span class="identifier">logger</span> <span class="special"><<</span> <span class="string">"Hello, world!"</span><span class="special">;</span> |
| </pre> |
| <p> |
| would become a lambda-expression that is only invoked if the filtering is |
| successful. Although this approach has advantages, it must be noted that |
| lazy expression construction is not zero-cost in terms of performance, code |
| size and compile times. The following expression: |
| </p> |
| <pre class="programlisting"><span class="identifier">logger</span> <span class="special"><<</span> <span class="string">"Received packet from "</span> <span class="special"><<</span> <span class="identifier">ip</span> <span class="special"><<</span> <span class="string">" of "</span> <span class="special"><<</span> <span class="identifier">packet</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span> <span class="special"><<</span> <span class="string">" bytes"</span><span class="special">;</span> |
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| <p> |
| would generate a considerable amount of code (proportional to the number |
| of streaming operators) to be executed before filtering takes place. Another |
| drawback is that the <code class="computeroutput"><span class="identifier">packet</span><span class="special">.</span><span class="identifier">size</span><span class="special">()</span></code> |
| is always called, whether or not the record is actually written to the log. |
| In order to delay this call, yet more scaffolding is needed, possibly involving |
| <a href="http://www.boost.org/doc/libs/release/libs/bind/bind.html" target="_top">Boost.Bind</a>, |
| <a href="http://www.boost.org/doc/libs/release/doc/html/lambda.html" target="_top">Boost.Lambda</a> |
| or <a href="http://www.boost.org/doc/libs/release/libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a>. |
| This complication is not acceptable for such a basic use case, like this. |
| </p> |
| <p> |
| Although lazy streaming is not provided by the library out of the box, nothing |
| prevents developing it in a separate hierarchy of loggers. See the <a class="link" href="../extension/sources.html" title="Writing your own sources">Extending the library</a> section for |
| more information. |
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| <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>). |
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