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<title>Apache Qpid : BewareStdStringLiterals</title>
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Apache Qpid : BewareStdStringLiterals
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This page last changed on Oct 19, 2006 by <font color="#0050B2">mmccorma</font>.
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<p>The short story: in C++ code using <tt>std::string</tt> never use string literals except to initialize static-scoped <tt>std::string</tt> constants.<br/>
(And by the way: <a href="http://cwiki.apache.org/confluence/display/qpid/NeverUseStaticLocalVariables" title="Visit page outside Confluence">NeverUseStaticLocalVariables</a></p>
<p>The long story: <tt>std::string</tt> is all about avoiding copies. Reference counting and copy-on-write serve to maximise the sharing of a single heap-allocated char array while maintaining memory safety. When used consistently in a program it works rather nicely.</p>
<p>However, when mixed with classic C-style string literals <tt>std::string</tt> can actually <em>cause</em> needless heap-allocated copies. Consider these innocent looking constructs:</p>
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<pre class="code-java">void f(<span class="code-keyword">const</span> std::string& s);
void g(<span class="code-keyword">const</span> std::string& s = <span class="code-quote">"hello"</span>);
std::string h() { <span class="code-keyword">return</span> <span class="code-quote">"foo"</span>; }
void copy_surprise {
std::string x = <span class="code-quote">"x"</span>; <span class="code-comment">// 1
</span> f(<span class="code-quote">"y"</span>); <span class="code-comment">// 2
</span> g(); <span class="code-comment">// 3
</span> x = h(); <span class="code-comment">//4
</span> <span class="code-keyword">while</span> (x != <span class="code-quote">"end"</span>) { ... } <span class="code-comment">// 4
</span>}</pre>
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<p>Lines 1-4 all cause creation and destruction of an implicit temporary <tt>std::string</tt> to hold the literal value. Line 5 does this for every execution of the while loop. That's a new/memcpy/delete each time. The heap is a heavily used resource, in tight inner loops in multi-threaded code this can be a <em>severe</em> contention bottleneck that cripples scalability.</p>
<p>Use static class <tt>std::string</tt> constants or file-private constants instead. You can make global declarations file-private by using a nameless namespace (this is preferred over the use of the <tt>static</tt> keyword.)</p>
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<pre class="code-java">namespace {
<span class="code-keyword">const</span> std::string end(<span class="code-quote">"end"</span>);
}
void f() { std::string x; <span class="code-keyword">while</span> (x != end) {...} }</pre>
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<p>And once again <a href="http://cwiki.apache.org/confluence/display/qpid/NeverUseStaticLocalVariables" title="Visit page outside Confluence">NeverUseStaticLocalVariables</a></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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