<![CDATA[Fastest way to compare QVariant with UserType]]>I'm looking for a very fast way to compare two QVariants that wrap the same user type.

What do you think about the following approaches:
@bool operator==() const(const QVariant &rhs)
{
if(myData.userType() == qMetaTypeId<MyUserType>())
{
bool method1 = static_cast<MyUserType>(myData.constData()) == static_cast<const MyUserType>(rhs.myData.constData());

bool method2 = myData.value<MyUserType>() == rhs.myData.value<MyUserType>();
}
return something
}@

I would think that the first approach is faster, because no deep copy or ctor call is necessary. It is more risky because of the static_cast and kind of a hack though.

I took a look into the Qt implementation of the second method, but as far as I could see, at least once the copy ctor of MyUserType is called.

]]>https://forum.qt.io/topic/28261/fastest-way-to-compare-qvariant-with-usertypeRSS for NodeSun, 19 Apr 2026 05:09:50 GMTThu, 13 Jun 2013 14:23:41 GMT60<![CDATA[Reply to Fastest way to compare QVariant with UserType on Thu, 13 Jun 2013 15:25:58 GMT]]>Thanks for the answers guys.

I took the advice to compare the two approaches by stepping through them. The result is pretty clear.
The first approach first calls QVariant::constData() twice (for each side) and then MyClass::operator==() is called.

The second approach takes several steps, including multiple checks and a ctor call for each side before !!! method 1 is used (with a reinterpret_cast instead of a static_cast though).

I think I'll stick to the first method, because it is faster and the comparing of the MetaTypeIds beforehand should in my case be enough to avoid illegal static_casts.

]]>https://forum.qt.io/post/182705https://forum.qt.io/post/182705Thu, 13 Jun 2013 15:25:58 GMT<![CDATA[Reply to Fastest way to compare QVariant with UserType on Thu, 13 Jun 2013 14:30:37 GMT]]>i would also tend to the second comparison.
But you should add "QVariant::canConvert()":http://qt-project.org/doc/qt-4.8/qvariant.html#canConvert checks before comparing the values.

]]>https://forum.qt.io/post/182696https://forum.qt.io/post/182696Thu, 13 Jun 2013 14:30:37 GMT<![CDATA[Reply to Fastest way to compare QVariant with UserType on Thu, 13 Jun 2013 14:27:47 GMT]]>Well... why don't you try and see? Seems like something to just try out and doing some measurements on.

]]>https://forum.qt.io/post/182693https://forum.qt.io/post/182693Thu, 13 Jun 2013 14:27:47 GMT