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    <p>Given types Key and T, the following code achieves the goal, assuming that type T is a type for which tbb::atomic is supported. </p> <pre><code>class AtomicValue { mutable tbb::atomic&lt;T&gt; content; public: AtomicValue() {} AtomicValue( T value ) {content=value;} bool cas( T value, T comparand ) const { return content.compare_and_swap(value,comparand)==comparand; } }; typedef tbb::concurrent_hash_map&lt;Key,AtomicValue&gt; table; bool update( table&amp; x, Key key, T value, T comparand ) { table::const_accessor a; if( !x.insert(a,table::value_type(key,value) ) ) { // value is already there return a-&gt;second.cas(value,comparand); } return true; } </code></pre> <p>The tricky part is using a const_accessor to do the update. Using a regular accessor would serialize the updates. But a const_accessor allows multiple threads to access the same table entry at the same time. It's called a "const_accessor" because the usual use case involves reading the value. But here the code uses CAS to arbitrate updates. The wrapper class "AtomicValue" enables doing CAS on a const object. </p> <p>A similar solution should work for tbb::concurrent_unordered_map, which might be better if non-blocking is the key criterion, because concurrent_unordered_map has a non-blocking implementation. </p> <p>Better yet, if you have the latest TBB <strong>and</strong> a compiler that supports the C++11 features of constexpr and defaulted/deleted member functions, the following should work:</p> <pre><code> typedef tbb::concurrent_unordered_map&lt;Key,tbb::atomic&lt;T&gt; &gt; table; bool update( table&amp; x, Key key, T value, T comparand ) { auto p = x.insert(table::value_type(key,value) ); if( !p.second ) { // value is already there return p.first-&gt;second.compare_and_swap(value,comparand) == comparand; } return true; } </code></pre> <p>It worked for me with gcc 4.7 when compiled with "g++ -std=c++0x".</p>
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    1. COThis is what I was looking for. Thanks a lot.
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