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    <p>No, you can still use a std::atomic even conditionally.</p> <p>Firstly, if you use <code>std::atomic&lt;unsigned int&gt;</code> then overflow behavoir is well defined (although possibly not what you want). If you use a signed integer overflow isn't well defined but as long as you don't hit it then this doesn't matter.</p> <p>If you absolutely must check for overflow, or otherwise act conditionally, you can use compare-exchange. This lets you read the value, decide whether you want to do work on it and then atomically update the value back if it hasn't changed. And the key part here is the system will tell you if the atomic update <em>failed</em>, in which case you can go back to the start and read the new value and make the decision again.</p> <p>As an example, if we only wanted to set the max value of an atomic integer to 4 (in some kind of refcounting, for instance), we could do:</p> <pre><code>#include &lt;atomic&gt; static std::atomic&lt;int&gt; refcount = 0; int val = refcount; // this doesn't need to be in the loop as on failure compare-exchange-strong updates it while(true) { if(val == 4) { // there's already 4 refs here, maybe come back later? break; } int toChangeTo = val + 1; if(refcount.compare_exchange_strong(val, toChangeTo)) { // we successfully took a ref! break; } // if we fail here, another thread updated the value whilst we were running, just loop back and try again } </code></pre> <p>In the above code you can use <code>compare_exchange_weak</code> instead. This can sometimes spuriously fail and so you need to do it in a loop. However, we have a loop anyway (and in general you always will as you need to handle real failures) and so <code>compare_exchange_weak</code> makes a lot of sense here.</p>
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    1. POIs std::atomic redundant if you have to check for overflow or act conditionally?
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    1. POIs std::atomic redundant if you have to check for overflow or act conditionally?
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    1. COthanks, btw: for incrementing or checking for overflow this might be better, but in case I expensively calculate the value to update with, then the computation is wasted if someone modified the atomic since then, right?, with a lock I only compute when I surely own it
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    2. COThe difference between lock free and non lock free programming can make a massive difference. Yes, you can do redundant calculations (although a compare exchange *failure* means another thread *succeded* which means work always gets done). There's a whole lot of information on the topic of lock free programming available - I'd recommended Herb Sutters 'Atomic Weapons' talk [http://channel9.msdn.com/Shows/Going+Deep/Cpp-and-Beyond-2012-Herb-Sutter-atomic-Weapons-1-of-2] if you want to understand the real nitty gritty. Infact, any of Herb's recent talks are very good.
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    3. COI am going to watch that, thanks again.
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