Cannot link Qt libs to C++ project manually.

JonB

@Sirzhatina
Like I said, you have to tell it which .lib files there you want it to link with, just as the case with includes.

The syntax for LINK allows .lib files to be named, I believe your example would require

/link /LIBPATH:C:\Qt\5.15.2\msvc2019_64\lib Qt5Widgets.lib ...

The ... is because you will need more than just Qt5Widgets library, you will need whatever it depends on.

This is all why most people allow Creator or cmake/qmake to do the work.

Sirzhatina

@JonB I got it, thank you. Honestly, it's surprising me, because I had no such problem when I was building Boost project - I mean, I just write /LIBPATH:... without necessity to list .lib files
UPD: Ive checked solution, it doesn't work - at least the same mistakes remain without new information

JonB

@Sirzhatina
I don't know how that worked. I don't actually use MSVC :)

You should test what I said in case I'm talking rubbish!

However, I seem to recall that there might be cl command line options which cause it to stamp generated .obj files with what .lib files they reference. And then use that information when linking. Could that be different in your compiling of Boost?

This question https://stackoverflow.com/questions/24813060/how-do-i-link-boost-to-my-program/24813134 shows Boost needing linking with gcc (ld) at least. It also states that most(?) of Boost is header-only. Could your usage happen to fall under this?

JoeCFD

https://download.qt.io/official_releases/vsaddin/2.7.1/
download this and add it into vs. Then create your qt project.

Sirzhatina

@JonB thank you for help, but I'm ok with Boost. It was just an example, that I didn't need to list .lib files excplicitly.

most(?) of Boost is header-only.

I meant exactly separately-compiled part:)
So, the problem isn't out yet

hskoglund

Well if you promise only to use the MSVC compiler, then it's easy:

#include "C:/Qt/5.15.2/msvc2019/include/QtWidgets/qapplication.h"
#include "C:/Qt/5.15.2/msvc2019/include/QtWidgets/qmessagebox.h"

#pragma comment( lib, "C:/Qt/5.15.2/msvc2019/lib/qt5core" )
#pragma comment( lib, "C:/Qt/5.15.2/msvc2019/lib/qt5widgets" )

int main(int argc, char* argv[])
{
    QApplication a(argc, argv);
    QMessageBox::information(nullptr,"Look Ma no Qt Creator","Hello from the command line");
    return a.exec();
}

Sirzhatina

@JoeCFD links don't work

JoeCFD

@Sirzhatina Have you installed any qt vs addin? If not, you have problem to compile qt code with VS. The link is for you to download it for installation. The links works fine for me.
If you still can not open the link, look for: Qt vs addin download in google. Find a place to download it.

JonB

@hskoglund said in Cannot link Qt libs to C++ project manually.:

#pragma comment( lib, "C:/Qt/5.15.2/msvc2019/lib/qt5core" )

Thanks, that was the MSVC I was trying to think of when I said

which cause it to stamp generated .obj files with what .lib files they reference

hskoglund

@JonB Haven't used that #pragma directive in a loooong time, it's probably fallen out of fashion nowadays. But it still works if you need it :-)

JonB

@hskoglund
It was just a thought. I was trying to think how come the OP claims that when he used Boost with MSVC he did not have to add any boost....lib onto his link line and yet it "worked". My experience/understanding is that you do have to add the .libs you want when using MSVC (e.g. for the Qt libraries), it is not enough to just specify /LIBPATH:... and miraculously the linker knows to look in which files. Am I right? Please read my posts above and comment if I am, or if I have misled the OP?

SGaist

@JonB some of boost is header-only so there's nothing to link. He may have been lucky and only used these parts of boost.

Christian Ehrlicher

@SGaist Boost is also using this (imo) crappy msvc auto-link feature. It gaves me a lot of headaches due to static linking and the fact the the sources are also build on linux.

JonB

@SGaist said in Cannot link Qt libs to C++ project manually.:

@JonB some of boost is header-only so there's nothing to link. He may have been lucky and only used these parts of boost.

Which is why I wrote earlier

It also states that most(?) of Boost is header-only. Could your usage happen to fall under this?

But the OP replied

I meant exactly separately-compiled part:)

So, the problem isn't out yet

Hence I don't know!

@Christian-Ehrlicher said in Cannot link Qt libs to C++ project manually.:

@SGaist Boost is also using this (imo) crappy msvc auto-link feature. It gaves me a lot of headaches due to static linking and the fact the the sources are also build on linux.

Which I also suggested to the OP might be why Boost but not Qt works without specifying the .libs! :)

Sirzhatina

@SGaist It's really interesting, so let's make it clear.
I build the next code

boost::asio::io_context io;
boost::asio::steady_timer t(io, boost::asio::chrono::seconds(5));

std::cout << "Hey ";
t.wait();
std::cout << "lala ley ";

And without next command
/link /LIBPATH:C:\Program Files (x86)\Boost\boost_1_76_0\stage\lib
I get linker error (no date-time library and so on - it's separately-compiled).
And, of course, adding this command, I have the program built successfully. So, as you can see I have no need to link exact .lib file excplicitly.

Christian Ehrlicher

@Sirzhatina said in Cannot link Qt libs to C++ project manually.:

So, as you can see I have no need to link exact .lib file excplicitly.

We already explained why this works... but Qt does fortunately not support this.

Sirzhatina

@Christian-Ehrlicher okay, thanks. But you said that's what msvc does. And I still use msvc to compile Qt libs, so what's the mystery here? I mean why does msvc auto-link one libraries and not auto-link others

Christian Ehrlicher

@Sirzhatina said in Cannot link Qt libs to C++ project manually.:

I mean why does msvc auto-link one libraries and not auto-link others

see the answer from @hskoglund - this is basically what the boost libs are doing and Qt not (because it's crappy, error-prone and non-portable)