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POThread-safe copy constructor/assignment operator
 

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  1. POThread-safe copy constructor/assignment operator
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    <p>Let's say that we want to make class <code>A</code> thread-safe using an <code>std::mutex</code>. I am having my copy constructor and assignment operator similarly to the code below:</p> <pre><code>#include &lt;mutex&gt; class A { private: int i; mutable std::mutex mtx; public: A() : i(), mtx() { } A(const A&amp; other) : i(), mtx() { std::lock_guard&lt;std::mutex&gt; _lock(other.mtx); i = other.i; } A&amp; operator=(const A&amp; other) { if (this!=&amp;other) { std::lock_guard&lt;std::mutex&gt; _mylock(mtx), _otherlock(other.mtx); i = other.i; } return *this; } int get() const { std::lock_guard&lt;std::mutex&gt; _mylock(mtx); return i; } }; </code></pre> <p>I do not think that it has any problems, other than the possibility of <code>other</code> to be destroyed by another thread before being copied, which I can deal with.</p> <p>My issue is that I haven't seen this pattern anywhere, so I do not know if people just haven't had a need for that or that it is plainly wrong for reasons I currently don't see.</p> <p>Thanks</p> <p><strong>NOTES</strong>: </p> <p>This is just an example. I can have an arbitrary number of member variables of any type, it does not have to be just an <code>int</code>.</p> <p>After Martin York's comment for possible deadlocking, this is an updated version that uses copy-and-swap (copy elision is also possible, but it wouldn't handle efficiently the self-assignment case). </p> <p>I also changed int to T, so people cannot assume that it is a POD.</p> <pre><code>template&lt;typename T&gt; class A { private: T t; mutable std::mutex mtx; public: A() : t(), mtx() { } A(const A&amp; other) : t(), mtx() { std::lock_guard&lt;std::mutex&gt; _lock(other.mtx); t = other.t; } A&amp; operator=(const A&amp; other) { if (this!=&amp;other) { A tmp(other); std::lock_guard&lt;std::mutex&gt; _lock(mtx); std::swap(t, tmp.t); } return *this; } T get() const { std::lock_guard&lt;std::mutex&gt; _lock(mtx); return t; } }; </code></pre>
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      1. POThread-safe copy constructor/assignment operator
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    1. COThread-safety problems in an assignment operator might be a symptom of other things going wrong in your application. For instance, if multiple threads are assigning to the same object simultaneously, making `operator=` thread safe with a mutex will prevent data inconsistencies but will lead to your content being silently discarded/overwritten...
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      1. USAndré Caron
    2. COWhat do you mean discarded/overwritten? My use case is that I want to copy construct or assign to my object and be in a consistent state. I do not expect all copies to be identical, but I do expect to have an internal consistent state.
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    3. CO@André Caron: But that would be expected behavior. If I have 5 threads assigning to the object the value assigned by the last person to do the assign is what I expect in the object. What the others have done is irrelevant. Data consistency is the **ONLY** thing I am worried about.
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      1. USMartin York
 

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