# QString::toShort problem

• FFFE is simply to big for a signed short ...

Why? Considering Visual Studio 2015 (and I assume also a lot of other compilers), the range for (signed) short is –32,768 to 32,767 (see https://msdn.microsoft.com/nl-be/library/s3f49ktz.aspx). The value -2 (represented by FFFE = two's complement) fals nicely into that range. So that's why I was expecting to be able to go from "FFFE" to -2 using QString::toShort()...

• @Bart_Vandewoestyne
Because 0xFFFE is 65,534. It fits in an unsigned short range, but overflows the signed short's maximum positive value of 32,767. That's what @Christian-Ehrlicher is saying.

You are assuming that `QString::toShort()` will treat the string `0xFFFE` as meaning exactly the same thing as `-2`, but it doesn't. It regards it as a positive number which is beyond the range of signed shorts, not as an alternative way of writing -2.

• @JonB I disagree.
Complement on two for 2:

``````  0000 0010
1111 1101
+ 0000 0001
1111 1110
``````

So, -2 is 1111 1110 or 0xFE - why should this not feet into a signed short?
"You are assuming that QString::toShort() will treat the string 0xFFFE as meaning exactly the same thing as -2, but it doesn't" - why should toShort() not treat 0xFE as -2?

• @jsulm said in QString::toShort problem:

@JonB I disagree.

You shouldn't. ;)

why should toShort() not treat 0xFE as -2?

Quite simply because you don't have a fixed-size data field to work with as input. Why should `toShort` assume that you meant exactly the binary representation. You could've just as well had a data that's too big to fit the type. Say I'm reading some input and I'm trying to get it into a short. Suddenly due to an error or by whatever chance I get a number that's too big for my short, but instead of overflowing the `toShort` would give me an invalid value. It doesn't make sense that the person who implemented `toShort` would just jump the gun on such an assumption.
And lastly, what should we do with overflows of this kind - `0x100FF`, shall `toShort` return 255 in this case?

• Why should toShort assume that you meant exactly the binary representation

Maybe I'm still sleeping and oversee something. What else should it assume? If I say its hex and pass FFFE - how does toShort() interpret it?
0x100FF is too big for a short and toShort() should return 0/false (and it does). But FFFE is a valid signed short number.

• @jsulm said in QString::toShort problem:

Why should toShort assume that you meant exactly the binary representation

Maybe I'm still sleeping and oversee something. What else should it assume? If I say its hex and pass FFFE - how does toShort() interpret it?
0x100FF is too big for a short and toShort() should return 0/false (and it does). But FFFE is a valid signed short number.

@jsulm I completely agree! (although I have the same feeling about sleeping and maybe overseeing something ;-) Maybe it's time to dive into the Qt 4.8.7 source and investigate why QString::toShort() is failing on "FFFE"? (does Qt 5.X also fail on that btw?)

• @jsulm said in QString::toShort problem:

But FFFE is a valid signed short number.

No it isn't, and that's the point. Start doing the math in your head and see for yourself:

``````E * 1 + F * 16 + F * 16^2 + F * 16^3
``````

And the last term overflows, which overflow is caught and voila!
If you have

``````char z = 127;
``````

then:

``````z += 1;
``````

Is overflowing, no matter whether the value you get is "correct".

• @kshegunov I still don't get it.
What is the representation of -2 as signed short? Isn't it 0xFFFE?

``````  0000 0000 0000 0010 - 2
1111 1111 1111 1101 - invert
+ 0000 0000 0000 0001 - add 1
1111 1111 1111 1110
-> 0xFFFE
``````

• OK, here's an exercise to settle the debate. First, assume that QString::toShort() behaves exactly as you expect.

What should each QString (p_*) be initialized to, in order to get `32` for every output line?

``````QString p_oct, p_dec, p_hex, p_r32;

// ... Initialize QStrings here ...

qDebug() << p_dec.toShort(nullptr, 10); // Returns 32
qDebug() << p_hex.toShort(nullptr, 16); // Returns 32

qDebug() << p_oct.toShort(nullptr, 8);  // Returns 32
qDebug() << p_r32.toShort(nullptr, 32); // Returns 32
``````

Next, what should each QString (n_*) be initialized to, in order to get `-32` for every output line?

``````QString n_oct, n_dec, n_hex, n_r32;

// ... Initialize QStrings here ...

qDebug() << n_oct.toShort(nullptr, 8);  // Returns -32
qDebug() << n_dec.toShort(nullptr, 10); // Returns -32
qDebug() << n_hex.toShort(nullptr, 16); // Returns -32
qDebug() << n_r32.toShort(nullptr, 32); // Returns -32
``````

• I don't get what you don't get about: `0xFFFE` is a positive overflow for parsing & storing into a `ushort`. Hence the behaviour.

One thing that is clear: the implementation of `QString::toShort()` is not `static_cast<short>(QString::toUShort())`, even if that might have been the way you were tempted to do it.

Nobody has looked at it "the other way round". I cannot test because I am Python/PyQt not C++, but what does

``````QString("-2").toUShort(&ok, 16)
``````

return? In your theory it should be `0xFFFE`, but I am "hoping"(!) it returns an error, just like `QString("FFFE").toShort(&ok, 16)` does?

Assuming that is the case, this means we do not have an ambiguity/duplication, whereby both `FFFE` and `-2` strings can be parsed as the same number by `toShort()`/`toUShort()` (but `2` is the only way to write +2).

• toShort makes a toLongLong interpretation first and than casts it to short theres where the "error" comes from:

``````short QString::toShort(bool *ok, int base) const
{
long v = toLongLong(ok, base);
if (v < SHRT_MIN || v > SHRT_MAX) {
if (ok)
*ok = false;
v = 0;
}
return (short)v;
}
``````

toLongLong will return ‭65534‬, (0xFFFE in int64 is positve after all), and that is bigger than SHRT_MAX -> 0 and failed conversion

• @JonB said in QString::toShort problem:

In your theory it should be 0xFFFE

No, it would not, because -2 is not a hex number...
"I don't get what you don't get about: 0xFFFE is a positive overflow for parsing & storing into a ushort" - we are not talking about unsigned short, but signed short and 0xFFFE is the representation of -2.

• @J.Hilk
In that case, try passing something like `0xFFFFFFFE` or `0xFFFFFFFFFFFFFFFE` for the string to `toShort()` and those who want `-2` instead of error should get it?!

• @jsulm

No, it would not, because -2 is not a hex number...

Yes it is! It's as much a hex number as some other base.

• @JonB said in QString::toShort problem:

In that case, try passing something like 0xFFFFFFFE or 0xFFFFFFFFFFFFFFFE for the string to toShort()

Come on - these numbers are NOT short. We should stay on topic.

• @jsulm said in QString::toShort problem:

Come on - these numbers are NOT short. We should stay on topic.

I beg your pardon!? I am totally on topic. I was replying to @J-Hilk 's display of the code of `QString::toShort()`. Did you try what I suggested rather than dismissing it as OT? In view of the code shown, I am trying to suggest what `0xFFF....` string `toShort()` will accept as representing a negative number....

• Nobody wants to try my exercises... (sad face)

• @JonB Passing 0xFFFFFFFE returns 0

• @JonB
actually, no take a look at toLongLong

``````qint64 QString::toLongLong(bool *ok, int base) const
{
#if defined(QT_CHECK_RANGE)
if (base != 0 && (base < 2 || base > 36)) {
qWarning("QString::toLongLong: Invalid base (%d)", base);
base = 10;
}
#endif

bool my_ok;
QLocale def_locale;
qint64 result = def_locale.d()->stringToLongLong(*this, base, &my_ok, QLocalePrivate::FailOnGroupSeparators);
if (my_ok) {
if (ok != 0)
*ok = true;
return result;
}

QLocale c_locale(QLocale::C);
return c_locale.d()->stringToLongLong(*this, base, ok, QLocalePrivate::FailOnGroupSeparators);
}
``````

I think, haven't looked stringToLongLong up, that here happens stirng lentgh magic, because every combinaion of FFF..E up to to 0xFFFFFFFE is interpretated as the uint value and everything above as -2 (as returning int64 value)

• @jsulm

@JonB Passing 0xFFFFFFFE returns 0

Since `QString::toLongLong()` returns a `qint64` (8 bytes, not 4), did you try `0xFFFFFFFFFFFFFFFE` ?

• @JonB said in QString::toShort problem:

Since QString::toLongLong() returns a qint64 (8 bytes), did you try 0xFFFFFFFFFFFFFFFE ?

Returns 0 as well.
And I don't see why it should depend on the length.

• @JonB surprisingly enough

``````qDebug() << std::numeric_limits<int64_t>::min() << std::numeric_limits<int64_t>::max()
<< endl << (int64_t)0xFFFFFFFFFFFFFFFE;
QString s("0xFFFFFFFFFFFFFFFE"); bool ok;
short sh =  s.toShort(&ok, 16);
qDebug() <<sh << ok;
long lg = s.toLongLong(&ok,16);
qDebug() << lg << ok;
``````

returns:

``````-9223372036854775808 9223372036854775807
-2
0 false
0 false
``````

• @jsulm
It would "depend on the length", as you put it, because as a 64-bit number `0xFFFFFFFE != 0xFFFFFFFFFFFFFFFE`.

• @JonB I want to convert a signed short number not long or long long or ...
0xFFFE as signed short is -2 - do you agree (I mean independently from what Qt toShort() thinks it is)?

• @JonB

``````qDebug() << (short)0xFFFE;
``````

prints -2 as expected

• @jsulm
I believe the problem here is a confusion between "bit representation" and "string representation".

• It is undoubtedly, unambiguously true that, for signed short, `0xFFFE` as a bit pattern is -2.
• However, for signed short, `0xFFFE` as a string "could" be either -2 (which fits in a short) or 65,534 (which does not fit in a short). And `QString::toShort()` is taking the latter interpretation, and hence erroring.

• @jsulm said in QString::toShort problem:

@JonB

``````qDebug() << (short)0xFFFE;
``````

prints -2 as expected

qDebug() << (short)0xFFFFFFFFFFFFFFFE;

prints also -2, would one expect that

actually yes, the first bytes are simply dropped x)

• @jsulm said in QString::toShort problem:

@JonB

``````qDebug() << (short)0xFFFE;
``````

prints -2 as expected

Yes, that's why I wrote earlier:

One thing that is clear: the implementation of `QString::toShort()` is not `static_cast<short>(QString::toUShort())`, even if that might have been the way you were tempted to do it.

• @JonB said in QString::toShort problem:

However, for signed short, 0xFFFE as a string "could" be either -2 (which fits in a short) or 65,534

No, signed short 0xFFFE is -2 even as string, because I'm calling toShort() not toUShort().
And why does

``````qDebug() << (short)0xFFFE;
``````

print -2?

• @JonB said in QString::toShort problem:

the implementation of QString::toShort() is not static_cast<short>(QString::toUShort())

I never said that

• @jsulm
But you're asking why `qDebug() << (short)0xFFFE;` prints `-2`. And I'm saying that's because of the way "cast-to-short" works in C++, which is simply not what the implementation of Qt's `QString::toShort()` does or purports to do.

Basically, "cast-to-short" (`(short)`) has no concept ever of "overflow/error", but `QString::toShort()` does have a concept of "overflow/error", and that's why they work differently. They are not intended to be equivalent.

[I am beginning to feel the need for @kshegunov 's moral support here, because I feel I am being attacked ( :( ) and it is indeed all to do with the overflowing he mentioned in his earlier reply.]

• @JonB What overflow error do you mean? 0xFFFE is a valid short number in both cases: signed and unsigned.

• Hey, let's do it the russian way and settle this outside, huh? Take a breath people.

@jsulm
Johann, you're wrong simply because `"0xFFFE"` is not a negative number, but a string, that simple. I know that in 2's complement for short this is -2, but that's if you go to the actual implementation of the negative numbers. The fact of the matter is there have been implementations that do not use integer complements. This string is not a binary representation, that is all, so don't expect the function to assume it should convert in binary-like way! Otherwise, as Jonas pointed out earlier `"0xFFFFFFFFFFE"` should just expand to -2 as well due to truncations.

• @jsulm

@JonB What overflow error do you mean? 0xFFFE is a valid short number in both cases: signed and unsigned.

`0xFFFE` as a bit-pattern is indeed a valid signed or unsigned bit-pattern for a short. But as a string to parse, for `QString::toUShort()` it's valid (65,534, which is OK for `ushort`), but for `QString::toShort()` it's a positive number greater than the positive limit of 32,767 for a `short` ("overflow").

• Hey, let's do it the russian way and settle this outside, huh? Take a breath people.

LOL! Phew, that's what I needed from you! I thought you might be Russian: are you "Mafiosa", could you send some "heavies" round to @jsulm for me...? ;-)

• @kshegunov "Hey, let's do it the russian way and settle this outside, huh?" - wait a bit I need to collect some more guys to have better arguments :-)
OK, I see. But actually Qt "knows" for which platform it was built (2's complement or something else) and could interpret such strings accordingly. I guess Qt devs wanted to go safe route :-)
@Bart_Vandewoestyne I would say @kshegunov suggested the correct solution:

``````short hex2 = static_cast<short>(str2.toUShort(&ok2, 16));
``````

• @J.Hilk said in QString::toShort problem:

`QString s("0xFFFFFFFFFFFFFFFE"); bool ok;`
`long lg = s.toLongLong(&ok,16);`

What is it in the implementation of `qint64 QString::toLongLong()` which makes this set `ok=false` instead of returning `-2` ?

• @JonB
thatsa a rapidhole down QString and QLocal ... 😨😨

Still to early in the morning to explore that ;-)

• @JonB said in QString::toShort problem:

I thought you might be Russian: are you "Mafiosa", could you send some "heavies" round to @jsulm for me...?

No, I'm not russian, but let's assume I know a guy who knows a guy, who knows a guy ... ;)

What is it in the implementation of qint64 QString::toLongLong() which makes this set ok=false instead of returning -2 ?

https://code.woboq.org/qt5/qtbase/src/corelib/tools/qlocale.cpp.html#_ZNK11QLocaleData15numberToCLocaleE11QStringView6QFlagsIN7QLocale12NumberOptionEEP15QVarLengthArrayIcLi256EE

• @kshegunov
The link you provide, for `QLocaleData::numberToCLocale()`, is for outputting internal numbers -> external strings. For `QString::toLongLong()` I am looking for function code which is for inputting external string -> internal number, and see why `0xFFFFFFFFFFFFFFFE` is causing a conversion error?