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POstd::string in a multi-threaded program
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  1. POstd::string in a multi-threaded program
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    copied!<p>Given that:</p> <p>1) The C++03 standard does not address the existence of threads in any way</p> <p>2) The C++03 standard leaves it up to implementations to decide whether <code>std::string</code> should use Copy-on-Write semantics in its copy-constructor</p> <p>3) Copy-on-Write semantics often lead to unpredictable behavior in a multi-threaded program</p> <p>I come to the following, seemingly controversial, conclusion:</p> <p><strong>You simply cannot safely and portably use std::string in a multi-threaded program</strong></p> <p>Obviously, no STL data structure is thread-safe. But at least, with std::vector for example, you can simply use mutexes to protect access to the vector. With an std::string implementation that uses COW, you can't even reliably do that without editing the reference counting semantics deep within the vendor implementation.</p> <p>Real-world example:</p> <p>In my company, we have a multi-threaded application which has been thoroughly unit-tested and run through Valgrind countless times. The application ran for months with no problems whatsoever. One day, I recompile the application on another version of gcc, and all of a sudden I get random segfaults all the time. Valgrind is now reporting invalid memory accesses deep within libstdc++, in the std::string copy constructor.</p> <p>So what is the solution? Well, of course, I could typedef <code>std::vector&lt;char&gt;</code> as a string class - but really, that sucks. I could also wait for C++0x, which I pray will require implementors to forgo COW. Or, (shudder), I could use a custom string class. I personally always rail against developers who implement their own classes when a preexisting library will do fine, but honestly, I need a string class which I can be sure is not using COW semantics; and std::string simply doesn't guarantee that. </p> <p>Am I right that <code>std::string</code> simply cannot be used reliably <em>at all</em> in portable, multi-threaded programs? And what is a good workaround?</p>
 

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