Convert QVariantList to QList<Type> list

ikim

Hi All

I have a QVariantList which is passed as a parameter to my function.

void myfunc(const QVariantList &list)
{
  // I want to convert this to  QList<Type> newList
  // where Type is the type() of list.at(0)
  // the variant list consists of variants all of the same type 
}

What I want to do is

QList<Type> newList;   //how do I declare this if I dont know the Type
  foreach(QVariant v, list) 
    newList << v.value();

Then create a QSet from the newList

Is there a way of easily achieving this

Regards

Ikim

SGaist

Hi and welcome to devnet,

Do you mean

QSet<Type> newSet = newList.toSet();

?

If so, why not:

QSet<Type> newSet;
foreach(QVariant v, list) {
    newSet << v.value<Type>();
}

?

ikim

Hi and thanks

How do I declare QSet<Type> at compile time when I dont know Type

Thanks

SGaist

What is your use case ?

mcosta

@ikim said:

How do I declare QSet<Type> at compile time when I dont know Type

You can't! In C++ you must define the type of templates.

Question: Why cannot you use QSet<QVariant>??

ikim

Hi

I need to compare two variantlist both containing variants of the same type and determine the variants that are common to both lists

Thanks

mcosta

You can use QVariant directly without conversion because QVariant supports operator==()

SGaist

QVariant has the == operator

ikim

@SGaist yes I could compare each QVariant in the list but that would mean iterating over the entire list to determine if the item exists in the second list. Is that my only option ?

Thanks

SGaist

No, use the QSet features e.g.

QSet<QVariant> commonSet = mySet1 & mySet2;

ikim

@SGaist Thanks, so I can use a QSet<QVariant> out of the box ?

other forums are suggesting that I need to define a qhash() function for the QVariant.

mcosta

QSet is based on a hash table; this means that internally it uses qHash()

MayEnjoy

@ikim why not use QVariant::Type method? even you can use it with c++ template.

ikim

@mcosta Thanks but this gives a compile error

QSet<QVariant> existingValues;
existingValues.insert(1);

ikim

@MayEnjoy Thanks, but how would this enable me to achieve my goal ?

mcosta

Which kind of error?

ikim

@mcosta /usr/include/QtCore/qhash.h:882:24: error: no matching function for call to 'qHash(const QVariant&)'

mcosta

mmmm, You're right.

So I suggest to create a template method

This code works for me

template <typename T>
QSet<T> mergeList(const QVariantList& l1, const QVariantList& l2) {
    QSet<T> s1, s2;

    for (const QVariant& v: l1) {
        s1.insert(v.value<T>());
    }

    for (const QVariant& v: l2) {
        s2.insert(v.value<T>());
    }

    return s1 & s2;
}

Used as

    QVariantList l1, l2;
    l1 << 1 << 2 << 3;
    l2 << 1 << 3 << 5;

    QSet<int> s1 = mergeList<int> (l1, l2);
    qDebug() << s1;

    l1.clear();
    l2.clear();

    l1 << "Foo" << "Bar";
    l2 << "Bar" << "Fred";

    QSet<QString> s2 = mergeList<QString> (l1, l2);
    qDebug() << s2;

ikim

@mcosta Thanks for your effort, but this still doesn't solve the problem as I dont know the Type at compile time

mcosta

Hi,

I'm not a C++ super-guru but I don't think you can do it because the compiler must know the type of each variable at compile time.

A solution could be implement qHash(const QVariant &v) and use QSet<QVariant>