Create QString with QStringLiteral and arg

tham

Purpose of QStringLiteral is avoid dynamic buffer allocation, but create string with arg do need to know the buffer size at runtime, could I gain any benefits by creating QString by QStringLiteral under this kind of case?

example :

QString const birth = QStringLiteral("%1/%2/%3").arg(year).arg(date).arg(time);

Rondog

@tham said in Create QString with QStringLiteral and arg:

QString const birth = QStringLiteral("%1/%2/%3").arg(year).arg(date).arg(time);

I pretty sure you can't use arguments with a QStringLiteral so this won't work (not on my system at least).

mrjj

Hi
It does seems to work

alt text

With QStringLiteral it makes a QStaticStringData
else not.

Rondog

I also tried it and found it worked. My bad.

I remember trying this at some point in the past and found it didn't work (compile error). For this reason I have always been in the habit of using QString instead of QStringLiteral for anything with arguments. From what I understand QStringLiteral is a macro wrapper that makes static text more memory and CPU friendly (it create a QString directly at compile time and not through a constructor from a char pointer) so I always assumed it was specifically aimed at static text.

mrjj

@Rondog

Hi
I also though it would not work as it not very static in nature but it seems
to create this static struct regardless.
But since we alter it a runtime, it cant be in read only memory so there is still something
fishy here :)

JonB

I don't see the problem/issue.
https://woboq.com/blog/qstringliteral.html, http://blog.qt.io/blog/2014/06/13/qt-weekly-13-qstringliteral/, https://www.kdab.com/qstringview-diaries-advances-qstringliteral/ may help.

And I assume that while QStringLiteral("%1/%2/%3") is "a QStringLiteral" (sort of), once you go QStringLiteral("%1/%2/%3").arg(...) then what you get back is a QString, as per e.g. http://doc.qt.io/qt-5/qstring.html#arg

?

mrjj

@JonB
Hi
good links. There is nothing wrong but clearly the whole expression cannot be read only so
i guess only "%1/%2/%3" is in readonly section and rest is dynamic.

JonB

@mrjj
I believe:

"%1/%2/%3" => read-only/literal string
QStringLiteral("%1/%2/%3") => read-only/literal string
QStringLiteral("%1/%2/%3").arg(...) => function QString::arg() returns a plain QString => whatever rules for QString only, not per se read-only/literal string

That's the point --- it's the use of the arg() function that changes the "type".

tham

I guess we can say QStringLiteral may offer some small optimization in this case(part of the string could construct at compile time, no dynamic allocation), unless it wouldn't worse than construct by QString directly?

kshegunov

QStringLiteral will construct a QString object once in the program lifetime from the const char * you pass it. This is happening through a bit of static initialization magic. After that you can use that QString object wherever you feel like it. The idea is that shallow copies of QString (due to it having implicit sharing) is very cheap. If you use "write" operations on it the data will be detached, as it's the case here - arg will copy out the literal part to do the replacement. However you'd still get some small optimization for the initial string construction.

JonB

Yes, the initial use of QStringLiteral may save you at least 1 nanosecond or 10 bytes in your overall program... :)

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