[Interest] Warning about QHash keys().toVector()

[Ahmad Samir]

On 08/07/2021 08:41, Oliver Knoll wrote:
> 
> 
>>     template<typename K, typename V> static QVector<K>
>> keysToVector(QHash<K,V> const& hash)
>>     {
>>         QVector<K> v;
>>         v.reserve(hash.size());
>>
>>         for (auto it = hash.keyBegin(); it != hash.keyEnd(); ++it)
>>             v.append(*it);
>>
>>         return v;
>>     }
>>
>>     QVector<int> CompatibilityInterfaceImpl::getActionIds() const
>>     {
>>         return keysToVector(_actions);
>>     }
> 
> Dear all,
> 
> I am sorry to kind of hijack this thread with a follow-up question, but I am trying to better understand what‘s going on „under the C++ hood“, especially given the solution above.
> 
> When we look at the original code again:
> 
> QVector<int> CompatibilityInterfaceImpl::getActionIds() const
>     {
>         return _actions.keys().toVector();   // _actions is a QHash
>     }
> 
> And the warning it generates:
> 
>    allocating an unneeded temporary container [clazy-container-anti-pattern]
> 

What clazy is warning about here is the, temporary intermediate, container created by QHash::keys(), 
this creates a QList of keys (which is then converted to a QVector). What's more efficient is to 
iterate right over the QHash itself, and use iterator.key() to fill the vector.

[...]


-- 
Ahmad Samir