Recurring C++ and Qt anti-patterns

JonB

@kshegunov
I am just reporting that the "Pythonic" way to do division, where the divisor might be zero, is to do the divide unconditionally and catch the exception. As soon as I see "Pythonic" I tend to ignore it, and do what I'd do in C++ instead, but that's just me :)

What I can show you is the following output:

>>> z = 1 / 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
>>> z = 1.5 / 0.0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: float division by zero
>>>

So you get a ZeroDivisionError either way (which you could catch in a try ... except ZeroDivisionError), though by the look of it the error message distinguishes between plain division by zero versus float division by zero :)

kshegunov

@JonB said in Recurring C++ and Qt anti-patterns:

As soon as I see "Pythonic" I tend to ignore it, and do what I'd do in C++ instead, but that's just me

You're a wise man ... ;)

What I can show you is the following output

Thanks, curiosity satisfied. So python just raises an exception even if dividing by a double(0) is a valid operation. Fair enough.

JonB

@kshegunov
I don't want to get into a debate (I know what you're like :) ), and I do know about floating point numbers being approximate representations (though zero/0.0 does have an exact representation), but (IMHO!) it is only in your physics/quantum mechanics area that "dividing by a double(0) is a valid operation" (the area where you can magic-away infinities and so on!). In a program it is not. (What have I let myself in for...!)

kshegunov

@JonB said in Recurring C++ and Qt anti-patterns:

I know what you're like

Hey! Words can hurt, you know! ;)

though zero/0.0 does have an exact representation

Yes, actually two representations, as with the actual zero. You have +0.0 and -0.0.

it is only in your physics/quantum mechanics area that "dividing by a double(0) is a valid operation"

Eh, I didn't write the IEEE standard. Take your beef with prof. Kahan.

In a program it is not

Actually if you look through the math.h implementations you're going to see a lot of handling for such cases. For example the people who wrote them had the decency to actually handle these special cases like log(0) returning -inf. While I agree it's not often useful to divide by zero it sometimes can be, so that's the reason to handle it like that, I assume.

JonB

@kshegunov said in Recurring C++ and Qt anti-patterns:

You have +0.0 and -0.0

The next time someone asks how much money I have in my pocket I will remember to give this answer.

I have $123.45 to give away. I want to hand each person $0.00. How many people do I need to meet to get rid of all my cash? :)

kshegunov

Here: https://en.wikipedia.org/wiki/Riemann_series_theorem
Knock yourself out ... ;P

fcarney

Be careful with not doing things the pythonic way in python. A lot of the time doing it the pythonic way leverages the internals of the language. In other words it pushes the execution from the interpreter to the built in methods that are written in C. So it can have an effect on performance. I don't think the exception example does this though. There may be other reasons I am not aware of.

Kent-Dorfman

wait a cotton pickin minute! there is no explicit cast to double in python so the x/double(0) argument is invalid on that basis alone...and x/float(0) behaves as expected.

fcarney

Uncomfortable admission:
I wrote windows specfic code today...

sierdzio

@fcarney said in Recurring C++ and Qt anti-patterns:

Uncomfortable admission:
I wrote windows specfic code today...

We feel for you :D

fcarney

Here is a nice QML anti-pattern:

Column {
    Rectangle {
        height: parent.height
    }
}

This one was "fun". Yeah, it doesn't necessarily detect the loop and it locks up the desktop (Gnome). So you have to kill the process manually from a terminal outside of the desktop (ctrl-alt-f4).

ODБOï

@fcarney said in Recurring C++ and Qt anti-patterns:

Gnome

this code works properly on windows with Qt_5_14_0_MinGW_64_bit

JonB

From https://forum.qt.io/topic/113223/check-whether-a-script-exists-by-script-name/14

QProcess process;
process.setStandardOutputFile(QProcess::nullDevice());
if (!process.startDetached(progName, args))
...

Would anyone care to comment on why C++ allows calling a static method off an instance without (seemingly) offering the option of a warning message for it? :) (C# doesn't let me write this.)

SGaist

AFAIK, there's nothing wrong with that. It's just that in the case you are showing, the static method has a specific behaviour that makes it unsuitable to be called like that.

kshegunov

@JonB said in Recurring C++ and Qt anti-patterns:

Would anyone care to comment on why C++ allows calling a static method off an instance without (seemingly) offering the option of a warning message for it? :) (C# doesn't let me write this.)

Because the class is known and that's all that matters. Whether you call it through an object or with its qualified name makes no difference. Actually, there's one widespread use of that in the Qt documentation:

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    return app.exec(); // QCoreApplication::exec is static
}

JonB

@kshegunov
But that is not my point/question. Which is: this piece of code is not the first (or the last) where someone has mistakenly written this. If C++ wants it this way, would it not be a good idea by now for compilers to offer a warning option? There is reason that e.g. C# does not allow it.

fcarney

"Within C++, there is a much smaller and cleaner language struggling to get out."
...
"And no, that smaller and cleaner language is not Java or C#."
Bjarne Stroustrup

kshegunov

@JonB said in Recurring C++ and Qt anti-patterns:

If C++ wants it this way, would it not be a good idea by now for compilers to offer a warning option?

If this were a potential error, probably. Since this is almost always safe there's no reason to offer a warning.

There is reason that e.g. C# does not allow it.

Which is what exactly?

jsulm

@kshegunov I think the point from @JonB is that people do call static methods on an object by mistake and then wander why the object is not changed (I sometimes see this here in the forums). The compiler could generate a warning, but I doubt people would care enough about those if they do not even notice what they do wrongly :-)

J.Hilk

@jsulm said in Recurring C++ and Qt anti-patterns:

but I doubt people would care enough about those

They don't I have taken over projects that hat on first compile 20k + warnings...

"Every time you compile with warnings, a fairy dies! So don't forget to clap your hands during compile time. Once for each fairy!"