Problem with QString and it arg()

aalderweireldt

If I reorder my "%"-placeholders, QString with arg can't process them correctly.
I can't find any documentation on what I am doing wrong. This happens on Qt 4.8.5 and Qt 5.2
Code below demonstrates the issue:
Test 4 fails.. it fails to replace a placeholder..

@ void UnitTests::test_stringArgs()
{
QString expected = QString("Area52");
QString test1 = QString("%1%2%3").arg("Area").arg(5).arg(2);
QString test2 = QString("%1%3_%2_").arg("Area").arg(2).arg(5);
QString test3 = QString("%1%3%2").arg("Area").arg(2);
QString test4 = QString("%1%3%2").arg("Area").arg(2).arg(5);

    QCOMPARE(expected, test1);
    QCOMPARE(QString("_Area_5_2_"), test2);
    // code block remove my %3, hence the " ' " after it 
    QCOMPARE(QString("_Area%3'2_"), test3);  
    QCOMPARE(expected, test4);
}

FAIL!  on line 13: test_stringArgs() Compared values are not the same
  Actual (expected): _Area52_
  Expected (test4): _Area5_@

Is this a QString bug?

roseicollis

Hi, dunno how this works exactly but could the problem be in the order?

u have:
QString test4 = QString("%1%3%2").arg("Area").arg(2).arg(5);
it should be:
QString test4 = QString("%1%3%2").arg("Area").arg(5).arg(2);

aalderweireldt

Swapping the arguments is done on purpose..
Notice that %3 is place in front of %2

Constructing a "%1%2%3" string works fine
Constructing a "%1%3%2" fails.. %2 gets lost somehow?

[quote author="roseicollis" date="1421756070"]Hi, dunno how this works exactly but could the problem be in the order?

u have:
QString test4 = QString("%1%3%2").arg("Area").arg(2).arg(5);
it should be:
QString test4 = QString("%1%3%2").arg("Area").arg(5).arg(2);[/quote]

alex_malyu

I can confirm that on Windows Qt 4.8.6 VS 2010 I see the following behavior:
@
QString test4 = QString("%1%3%2").arg("Area").arg(20).arg(50);
QString test4m = QString("%1%2%3").arg("Area").arg(50).arg(20);
Q_ASSERT (test4 == test4m);

Assert fails due to:

test4 "Area500" QString
test4m "Area5020" QString
@
It looks like first digit of the %2 is skipped
I suggest to report bug.
It is funny bug was not found yet.

Alex

hskoglund

Hi, I see the bug also on my Mac with Qt 5.4: if the first character in %2 is a digit 0-9 it gets eaten. Good find!

SysTech

I to can confirm this on Qt 5.4.x on Win 8.1 using MSVC2013 32 bit:

"Area500"
"Area5020"

aalderweireldt

Seems to be wanted behavior.....

The %1%3%2 with arg("Area") becomes Area%3%2
Area%3%2 with arg(20) becomes Area%3 20
Now note how the documentation says that it will replace % up to 99, so...
Area%3 20 with arg(50) becomes Area500 as it will only replace 2, not %3

Although I think this is unwanted behavior for most of the people using the qt library. We heavily use placeholders for translations and thus don't control the arg input. I don't want arg to dynamicly change my placeholders..

https://bugreports.qt.io/browse/QTBUG-44044

hskoglund

Aha, thanks for posting this, I've seen in the docs for QString that there's actually 2 ways you can use the arg() function:

string.arg(1).arg(2).arg(3); or
string.arg(1,2,3);

and the docs says that the difference is that the 2nd version "replaces the arguments in one pass". Until know I didn't really pay attention to what that meant. So this implies that the 1st version iterates the replacements, so indeed this is wanted (designed) behavior.
But it sure looked like a bug :-)

alex_malyu

I would say it seems reasonable if you count that arg is just a function,
Qt documentation should have warning about usage of the string.arg(1).arg(2).arg(3); pattern

I just hope that in my Qt career I have not introduced such disaster. The only hope is that my strings always had some kind of additional separators.

SGaist

Documentation improvement is on its way