Is it possible to build QT 6.2.2 with C++20?

Dmitriano

What happens if my app that is built with C++20 (/std:c++20 option) uses QT built with C++17 (/std:c++17 option)?

Is there a way (a configuration option) to build QT with /std:c++20 with MSVC2019 and CLang 12?

By default QT is built with C++17:

Using C++ standard ..................... C++17

jsulm

@Dmitriano said in Is it possible to build QT 6.2.2 with C++20?:

What happens if my app that is built with C++20 (/std:c++20 option) uses QT built with C++17 (/std:c++17 option)?

Nothing.
You can use newer C++ standards in your Qt app with Qt which was used with an older C++ standard.

J.Hilk

@Dmitriano said in Is it possible to build QT 6.2.2 with C++20?:

What happens if my app is built with C++20 (/std:c++20 option) uses QT built with C++17 (/std:c++17 option)?

nothing out of the ordinary, should work just fine.

Is there a way (a configuration option) to build QT with /std:c++20 with MSVC2019 and CLang 12?

IIRC you simply pass -std=c++20 to the make call ?
maybe there's an option in the cmake file to force this too, but I do not know enough about cmake to help here :D

Dmitriano

@jsulm what about calling std::string QString::toStdString() const in my app? Is it header only?

jsulm

@Dmitriano said in Is it possible to build QT 6.2.2 with C++20?:

Is it header only?

No, it's not.
Is there a reason why you are asking this?

Dmitriano

@jsulm I afraid that std::string in my app can differ from std::string in QT. At least I am not absolutely sure that it is not.

jsulm

@Dmitriano said in Is it possible to build QT 6.2.2 with C++20?:

std::string in QT

There is no std::string in Qt.
std::string (as anything else from std namespace) comes from C++ standard library.
Which C++ standard library you're using depends on the compiler, not Qt.

Dmitriano

@jsulm are you sure? see https://stackoverflow.com/questions/70241529/c17-and-c20-library-compatibility/70241655, for example, most people say that it is different std::strings

jsulm

@Dmitriano said in Is it possible to build QT 6.2.2 with C++20?:

are you sure?

Yes, I am. Qt does not implement std::string. Or can you point me to its implementation sources in Qt?
"most people say that it is different" - I fail to see that "most" people say that. Most actually tell you same as what I do.
From my own experience I can tell that there is no problem using a newer C++ standatd in a Qt application than the one used to build Qt.

JohanSolo

@J-Hilk said in Is it possible to build QT 6.2.2 with C++20?:

IIRC you simply pass -std=c++20 to the make call ?
maybe there's an option in the cmake file to force this too, but I do not know enough about cmake to help here :D

This can indeed be done: set_property(TARGET tgt PROPERTY CXX_STANDARD 20), according to the cmake documentation. This works for individual targets. You can set it "project-wide" by using set (CMAKE_CXX_STANDARD 20).

Dmitriano

@jsulm It is irrelevant where are the sources, it is about ABI compatibility between C++17 and C++20 runtime libs, but you are right about what people say, I misunderstood some answers on stackoverflow.com.

Dmitriano

@J-Hilk Need to pass -std=c++20 to configure.bat call, not to 'make' call.

J.Hilk

@Dmitriano
alt text

right, I need more coffee

SimonSchroeder

With MSVC not all versions work together. You cannot (always) link VS2013 code with VS2019 code, though you can link VS2017 and VS2019 code. The selected version of the standard normally does not impact compatibility.

Unfortunately, this is actually a reasonable question to ask: GCC once broke ABI compatibility for std::string when introducing C++11. There is a define in that case to compile C++11 against the old ABI. I don't know of any case with MSVC where this was a problem.

Also, this question is quite relevant IMHO in the context of Qt. When we ported our software from wxWidgets to Qt we had a lot of trouble with QString::toStdString() with MSVC. Something wasn't linked properly such that our software always crashed when std::string's destructor was called. We haven't figured it out, but switched over to using QString::toUtf8().data() instead, which honestly is a better solution as this way the encoding is known.

@Dmitriano that is indeed a good and valid question. But, you don't have to fear anything mixing C++17 and C++20 code with MSVC.

Dmitriano

@Dmitriano There is -c++std c++20 configuration parameter, but it is not clear does it work correctly with android builds, because there is still

    -DCMAKE_CXX_STANDARD=17

in configuration log.

Dmitriano

@Dmitriano I am not sure what does it exactly mean but Regarding CMAKE_CXX_STANDARD=17, that's only used for the architecture extraction compile project, see https://bugreports.qt.io/browse/QTBUG-99108