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<title>Apache Qpid : BewareOfStringPromotion</title>
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Apache Qpid : BewareOfStringPromotion
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This page last changed on Oct 19, 2006 by <font color="#0050B2">mmccorma</font>.
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<p><tt>std::string</tt> is a useful tool for simplifying memory management of strings and avoiding unnecessary copies by reference counting. However there is one common gotcha where it <em>causes</em> unnecessary copies. Consider: </p>
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<pre class="code-java">void f(<span class="code-keyword">const</span> std::string& s) { cout << s << endl };
void g() {
<span class="code-keyword">for</span> (<span class="code-object">int</span> i = 0; i < 1000; ++i) { f(<span class="code-quote">"hello"</span>); };
}</pre>
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<p>This actually allocates, copies and deletes 1000 heap buffers with the string "hello"! The problem here is that "hello" is <em>not</em> an instance of <tt>std::string</tt>. It is a char[5] that must be converted to a temporary <tt>std::string</tt> using the appropriate constructor. However <tt>std::string</tt> always wants to manage its own memory, so the constructor allocates a new buffer and copies the string. Once f() returns and we go round the loop again the temporary is deleted along with its buffer.</p>
<p>Here's a better solution:</p>
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<pre class="code-java">void f(<span class="code-keyword">const</span> std::string& s) { cout << s << endl };
namespace { <span class="code-keyword">const</span> std::string hello(<span class="code-quote">"hello"</span>); }
void g() {
<span class="code-keyword">for</span> (<span class="code-object">int</span> i = 0; i < 1000; ++i) { f(hello); };
}</pre>
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<p>This time we have a constant <tt>std::string</tt> that is created once at start up and destroyed once at shut-down. The anonymous namespace makes the constant private to this .cpp file so we wont have name clashes. (Its similar to using the static keyword on a global declaration in C, but anonymous namespaces are the preferred way to do it in modern C++) </p>
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<td align="center"><font color="grey">Document generated by Confluence on Apr 22, 2008 02:47</font></td>
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