QMultiMap with QVariant as key-CodePudding

I have a multimap with QVariant as key, but it's not working with QByteArray.
The funcion map.values("\xc2\x39\xc7\xe1") is returning all the values of the map.
This is a small example:

#include <QCoreApplication>
#include <QMultiMap>
#include <QVariant>

int main(int argc, char *argv[])
{
  QCoreApplication a(argc, argv);

  QMultiMap<QVariant, QString> map;
  QByteArray b1("\xc1\x39\xc7\xe1");
  QByteArray b2("\xc1\x39\xc7\xe2");

  map.insert(QVariant(b1), "TEST1");
  map.insert(QVariant(b2), "TEST2");

  QStringList values = map.values(QByteArray("\xc1\x39\xc7\xe1"));

  return a.exec();
}

I tried also using a QMap to see what happens and it adds only an element to the map.
Can someone explain me this behavior?
What am I doing wrong?

CodePudding user response：

The problem is the unavailability of a suitable operator<. You can use this hack to show the desired behaviour:

bool operator<(const QVariant& lhs, const QVariant& rhs)
{
    if (lhs.userType() == QMetaType::QByteArray && rhs.userType() == QMetaType::QByteArray)
    {
        return lhs.toByteArray() < rhs.toByteArray();
    }
    // The rest is up to you.
    return true;
}

CodePudding user response：

Use QMultiHash instead of QMultiMap. QMultiHash does not require < operator for the key type. Hashed container should be preferred over map whenever you do not need sorted keys. And I bet you do not not need them sorted because sorting variants (i.e. values of different underlying types) does not make any sense.

CodePudding user response：

It appears to be a bug in Qt, because the operator QVariant::operator<() does not provide a total ordering, even though QByteArray::operator<() does. And QMap relies on that (see QMap documentation).

QByteArray b1("\xc1\x39\xc7\xe1");
QByteArray b2("\xc1\x39\xc7\xe2");
QVariant v1(b1);
QVariant v2(b2);

assert(b1 < b2 != b2 < b1);  // works as expected for QByteArray
assert(v1 != v2);            // make sure the values are actually not equal
assert(v1 < v2 != v2 < v1);  // fails for QVariant(QByteArray)

So a QByteArray works as a key to a QMap, but a QVariant(QByteArray) does not.