Recurring C++ and Qt anti-patterns

kshegunov

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

I like this: your claim/view is that Python is so slow in the first place that it can't get much worse with exceptions :)

No, not exactly. I claim that you should compare things that are comparable to begin with. Python runs in a VM, while C++ runs on metal. It is context that defines the term, and "costly" in C++ terms isn't applicable, or justifiable in python. Think of it like this, fopen calls into the kernel, do you consider this costly? It can be, if you do it all the time, like opening a file reading a couple of bytes and closing it. But the point is, is this costly? "Well, it depends" is the correct answer. Or, as I mentioned already, is longjmp costly?

What I'm trying to convey is that in python you don't even consider this stuff, because the almighty VM shields you from it, and you can say your exceptions are cheap, which I don't know, they might very well be. But you still pay for the VM, exceptions or no exceptions. On the other hand I can choose to use exceptions where they make sense and pay the handling code price, or I may choose not to in some places and declare things with noexcept, or I can do what Qt does and disable them altogether.

But even when you're talking only C++ it's not absolute. As I stated, and I do stand by it, throwing isn't that much different than unwinding stack frame by stack frame, until you meet the handler. And in C/C++ the stack is king, it's god and spirit and the holy mother, and all that; and it's very fast, and it is hardware supported for a reason. I would hardly believe anyone claiming push and pop are coincidences, a technical curiosity if you will. So yeah, unwinding the stack "is as inevitable as the demise of capitalism", but it's done all the time for all reasons, some of which were mentioned, and it is by far a bad reasoning, rotten logic if I may, to say it makes throwing "costly".

PS.
That's one of my better missives, if I may be so bold to say so myself. ;P

jsulm

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

try:
abc = dict["key"]
except:

Isn't pythonic way

if "key" in dict:
   ...

?!

JonB

@jsulm
No, the whole point is that the "Pythonic" way is precisely to go try ... except instead of checking via in, that's my point! Same with division by 0, and other cases. This is Python's "Ask for forgiveness, not for permission" philosophy. Note that personally I wrote

Am I good Python citizen? Am I, heck no! If nothing else, it gets in the way of having the choice to break on any exception in a debugger.

:)

kshegunov

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

Same with division by 0, and other cases.

Out of curiosity: How does python handle that, because as far as I know (I don't really know that much, but bear with me for a second) there's no (strong) typing and while dividing by int(0) is invalid, division by double(0) is valid?

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.