QNANO_PROPERTY(double...) -> "comparing floating point with == or != is unsafe."

[NielsMayer]

On upgrading to QtCreator 4.7.1 and Qt5.11.2, I noticed massive numbers of clang-code-model warnings appearing in qnanopainter related source-code.

In particular, when using QNANO_PROPERTY(double,....) one is greeted by warnings that "comparing floating point with == or != is unsafe." In researching this issue, I was greeted by the sound of a can of worms opening: https://randomascii.wordpress.com/2012/02/25/comparing-floating-point-numbers-2012-edition/

In the above macro, the problem code in question is:
void setter(type const &v) { if(v == variable) return; variable = v; emit getter##Changed(); update(); }

What is the "correct" way of eliminating the above warning and any "danger" concerns it may cause? How unsafe is this in practice? I notice the only instances of such code in qnanopainter source are commented out https://github.com/QUItCoding/qnanopainter/blob/master/examples/gallery/qnanopainter_features/src/galleryitem.h and that custom code is used for examples where Animation float values are passed in from QML.

[QUItCoding]

As said that helper macro isn't used in QNanoPainter sources currently. QNANO_PROPERTY does reduce boilerplate code for simple setter/getter properties, while cons is that Qt Creator code completion doesn't work for methods & variables defined using it.

That comparison isn't really unsafe, worst case scenario is that *Changed() signal will get called even when value didn't actually change as comparison thought they are different. So you can just ignore the warnings or not use QNANO_PROPERTY for float properties. Floating point specific version of that macro could be implemented with qFuzzyCompare & qFuzzyIsNull but not sure if that is needed...

[NielsMayer]

The helper macro is used here in the qnanopainter source:
https://github.com/QUItCoding/qnanopainter/blob/master/examples/gallery/qnanopainter_features/src/galleryitem.h#L14

The equivalent code from the macro is present with the alternative macro version commented out:

//Alternatively could just use:
//QNANO_PROPERTY(float, m_animationTime, animationTime, setAnimationTime)
//QNANO_PROPERTY(float, m_animationSine, animationSine, setAnimationSine)

In the equivalent alternative code as would be generated by the macro, the "unsafe" warning is also emitted for the following two cases -- again within current qnanopainter source code:
https://github.com/QUItCoding/qnanopainter/blob/master/examples/gallery/qnanopainter_features/src/galleryitem.h#L30
https://github.com/QUItCoding/qnanopainter/blob/master/examples/gallery/qnanopainter_features/src/galleryitem.h#L38

[NielsMayer]

In reply to @QUItCoding comment from commit commentary in #32:

"By the way, tried also using qFuzzyCompare() for QNANO_PROPERTY but macros are a bit too dummy for that... Possibly would need separate QNANO_FLOAT_PROPERTY macro for float properties."

A separate QNANO_FLOAT_PROPERTY (also probably QNANO_DOUBLE_PROPERTY) would at least silence the warnings and be safer and less bug-inducing.

Couldn't QNANO_PROPERTY(_,_) special-case match for 'double' or 'float' and in turn invoke QNANO_FLOAT_PROPERTY(_) or QNANO_DOUBLE_PROPERTY(_) ?

[QUItCoding]

In theory yes, but I'm not fluent with macros to implement that =)