QSet with custom compare functor

xtech

Hi,

is there any way to define custom compare functor for QSet? For std::set we can define

std::set<MemoryLayoutInfo,MemoryLayoutInfo::Compare> set;

and the same for std::unordered set but in documentation about QSet class I didnt see similar constructor so... this class does not support this feature?

BjornW

From the docs:

"QSet's value data type must be an assignable data type. You cannot, for example, store a QWidget as a value; instead, store a QWidget *. In addition, the type must provide operator==(), and there must also be a global qHash() function that returns a hash value for an argument of the key's type. See the QHash documentation for a list of types supported by qHash()."

If you look at QHash docs there are examples of how to implement this custom qHash.

http://doc.qt.io/qt-5/qhash.html

xtech

I read about qHash but I do not understand how this can help me to reorder items in QSet in custom way? qHash is not the same as compare function.

For example using std::set I can implement custom comparator like below

class Compare {
    public:
        bool operator()(const MemoryLayoutInfo &x,const MemoryLayoutInfo &y) {
            return x.bits < y.bits ? true  :
                   x.bits > y.bits ? false :
                   x.data <= y.data  ? true  : false;
        }
    };

BjornW

From the docs:

"QSet<T> is one of Qt's generic container classes. It stores values in an unspecified order and provides very fast lookup of the values." So no ordering.

Do you need to use a Qt container? If not, std::set<T> is ordered. (std::unordered_set<T> is unordered)

I'm unsure what's the best Qt approach for a ordered container. QMap is ordered by key, but that seems like overkill for the job.

SGaist

Hi and welcome to devnet,

From what you wrote QSet is not the class you want to use since as @BjornW already took from the documentation, it's an unordered container.

There's no equivalent of std::set in Qt. QSet matches std::unordered_set.

You can find a good analysis of Qt's containers here.