Odd format in Qurl c'tor

[mzimmers]

Hi all -

I created a Qt Quick project from Creator, and got this in my main.cpp:

    const QUrl url(u"qrc:/xxx/main.qml"_qs);

I don't understand the argument; can someone fill me in?

Thanks...

[SGaist]

Hi,

AFAIK, it is a string literal notation. See the Qt StringLiterals namespace.

Basically, you create a QString out of a UTF-16 string. See here for the C++ doc about them.

[JoeCFD]

https://doc.qt.io/qt-6.2/newclasses62.html#new-functions-in-namespaces
operator""_qs(const char16_t *str, size_t size)
https://doc.qt.io/qt-6.2/qstring.html#operator-22-22_qs

[mzimmers]

Thanks, guys. So, the "u" preceding the string creates a string literal (16-bit flavor), and the "_qs" after the string converts it to a QString. (According to the docs that JoeCFD referenced, _qs needs a 16-bit string literal.)

So...if _qs is a function, why doesn't it look like one in the line I posted above? Is this more new C++ stuff? I'm starting to feel antediluvian...

[JonB]

@mzimmers said in Odd format in Qurl c'tor:

So...if _qs is a function,

It isn't, it's an operator :) QString operator""_qs(const char16_t *str, size_t size). So, doubtless by some piece of C++2000 magic, ""_qs does its (compile-time) magic on the string in between "..." quotes and followed by _qs. Gotta love C++.

BTW, u"qrc:/xxx/main.qml"_qs equates to

QStringLiteral("qrc:/xxx/main.qml")

which is a bit more understandable.

[Chris Kawa]

u prefix signifies a char16_t string literal.
_qs suffix is a user defined literal that produces a QString using that literal as the data directly, without runtime allocation.

It's nothing new. Both of these are C++11, so way over decade old by now.

[mzimmers]

@JonB said in Odd format in Qurl c'tor:

It isn't, it's an operator :)

OK, so, to further my education:

operator""_qs(const char16_t *str, size_t size)

How does one go about providing str and size to this operator - is it somehow implicit from the line of code I posted originally?

[mzimmers]

@Chris-Kawa said in Odd format in Qurl c'tor:

It's nothing new. Both of these are C++11, so way over decade old by now.

Chris - anything newer than the Reagan administration is new to me. Nice haircut, BTW...

[Chris Kawa]

How does one go about providing str and size to this operator - is it somehow implicit from the line of code I posted originally?

u"qrc:/xxx/main.qml" becomes the str argument and size is calculated by the compiler.

The size argument is optional too, so if you'd like to create your own suffix, for example one that converts a string literal to an int at compile time you would do something like:

int operator"" _mysuffix(const char* str) { return atoi(str); }

and then you can use it like this:

int foo = "42"_mysuffix;

Obviously this is not a very useful example, but the string can come from anywhere, so you could do something like take a preprocessor macro with a version string and convert it to an int at compile time. Or if you have a complex number class you could define MyComplex operator"" _cx(const char*, size_t) and have it parse at compile time to create a variable of class MyComplex like this: "4+3i"_cx;.

It's worth pointing out that apart from string literals this operator can also take numbers. For example the std::chrono namespace defines a bunch of them for time units, so you can write something like sleep(250ms) and no longer wonder what unit the parameter is.