QVariant comparison with own types working?

Question

Update

I have created an qt bugticket hoping the documentation will be extended.

Original Question

Believing an Question from 2010 and the Qt Documen

Answer 1

I'm afraid you'll need to rely on the code (and, being behaviour, it can't be changed without breaking), and not documentation. There's a surprise just below, though.

Here's the relevant code.

QVariant::operator== for types with unregistered operators will just use memcmp. The relevant snippet (in 5.1) is this:

bool QVariant::cmp(const QVariant &v) const
{
    QVariant v1 = *this;
    QVariant v2 = v;
    if (d.type != v2.d.type) 
        // handle conversions....

    return handlerManager[v1.d.type]->compare(&v1.d, &v2.d);
}

handlerManager is a global object that gets used to perform type-aware manipulations. It contains an array of QVariant::Handler objects; each of such objects contains pointers to perform certain operations on the types they know how to handle:

struct Handler {
    f_construct construct;
    f_clear clear;
    f_null isNull;
    f_load load;
    f_save save;
    f_compare compare;
    f_convert convert;
    f_canConvert canConvert;
    f_debugStream debugStream;
};

Each and every of those members is actually a pointer to a function.

_{The reason for having this array of global objects is a bit complicated -- it's for allowing other Qt libraries (say, QtGui) to install custom handlers for the types defined in those libs (f.i. QColor).}

The operator[] on the handlerManager will perform some extra magic, namely get the right per-module handler given the type:

return Handlers[QModulesPrivate::moduleForType(typeId)];

Now the type is of course a custom type, so the Handler returned here is the one the Unknown module. That Handler will use the customCompare function in qvariant.cpp, which does this:

static bool customCompare(const QVariant::Private *a, const QVariant::Private *b)
{
    const char *const typeName = QMetaType::typeName(a->type);
    if (Q_UNLIKELY(!typeName) && Q_LIKELY(!QMetaType::isRegistered(a->type)))
        qFatal("QVariant::compare: type %d unknown to QVariant.", a->type);

    const void *a_ptr = a->is_shared ? a->data.shared->ptr : &(a->data.ptr);
    const void *b_ptr = b->is_shared ? b->data.shared->ptr : &(b->data.ptr);

    uint typeNameLen = qstrlen(typeName);
    if (typeNameLen > 0 && typeName[typeNameLen - 1] == '*')
        return *static_cast<void *const *>(a_ptr) == *static_cast<void *const *>(b_ptr);

    if (a->is_null && b->is_null)
        return true;

    return !memcmp(a_ptr, b_ptr, QMetaType::sizeOf(a->type));
}

Which, apart from a bit of error checking and handling shared and null variants in a special way, uses memcmp on the contents.

_{... only if the type is not a pointer type, it seems. Wonder why there's that code there...}

---

Good news!

Starting with Qt 5.2, you can use QMetaType::registerComparator (see here) to make Qt invoke operator< and operator== on your custom type. Just add to your main:

qRegisterMetaType<MyClass>();
QMetaType::registerComparators<MyClass>();

And voilà, you'll hit the assert in your equality operator. QVariant::cmp now is:

QVariant v1 = *this;
QVariant v2 = v;
if (d.type != v2.d.type) 
    // handle conversions, like before

// *NEW IMPORTANT CODE*
if (v1.d.type >= QMetaType::User) {
    // non-builtin types (MyClass, MyEnum...)
    int result;
    // will invoke the comparator for v1's type, if ever registered
    if (QMetaType::compare(QT_PREPEND_NAMESPACE(constData(v1.d)), QT_PREPEND_NAMESPACE(constData(v2.d)), v1.d.type, &result))
        return result == 0;
}
// as before
return handlerManager[v1.d.type]->compare(&v1.d, &v2.d);