Compiling in g++

[A Former User]

Currently, one of my personal goals in developing software is to move from compiling my Qt apps in the Qt Creator Software to compiling them in g++; I am relatively new to using g++, and I have had trouble following some online tutorials. What do I need to do to successfully compile my app (especially with the resource and project files) in g++ ?

[ChrisW67]

Qt Creator is not a compiler. It was running qmake/cmake for you to create a Makefile (or equivalent), then running make. Make is responsible for running the compiler and linker. To do the same thing outside Qt Creator is fairly straightforward:

• Write your C++ code
• Write PRO file that lists your project components (SOURCES, HEADERS, RESOURCES: https://doc.qt.io/qt-5/qmake-tutorial.html)
• Run qmake from the Qt library set you wish to build with
• Run make
• Happy days

[A Former User]

@ChrisW67
Thank you; I'm a little confused once you get to step 3, is there a good webpage/video to look at?

[ChrisW67]

I am going to assume you are using the Qt versions installed by the online installer. On Linux this creates a folder Qt containing one folder for each Qt library version you chose to install (and some other stuff to support Qt Creator). For example:

chrisw@newton:~/Qt$ ls -l
total 31492
drwxrwxr-x  3 chrisw chrisw     4096 Jun  8  2020 5.15.0
drwxrwxr-x  3 chrisw chrisw     4096 Jun 20 13:24 6.1.1
-rw-r--r--  1 chrisw chrisw    10174 Jul  4 12:48 components.xml
...

Inside each Qt version folder is a complete set of Qt library components and tools identified by compiler(s):

chrisw@newton:~/Qt$ ls -l 5.15.0/
total 8
drwxrwxr-x 13 chrisw chrisw 4096 Jun  8  2020 gcc_64
-rw-rw-r--  1 chrisw chrisw 2271 May 14  2020 sha1s.txt

chrisw@newton:~/Qt$ ls -l 5.15.0/gcc_64/
total 76
drwxr-xr-x  2 chrisw chrisw  4096 Jun  8  2020 bin
drwxr-xr-x  4 chrisw chrisw  4096 Jun  8  2020 doc
drwxr-xr-x 88 chrisw chrisw  4096 Jun  8  2020 include
drwxr-xr-x  5 chrisw chrisw 28672 Jun  8  2020 lib
drwxr-xr-x  2 chrisw chrisw  4096 Jun  8  2020 libexec
drwxr-xr-x 76 chrisw chrisw  4096 Jun  8  2020 mkspecs
drwxr-xr-x  2 chrisw chrisw  4096 Jun  8  2020 phrasebooks
drwxr-xr-x 33 chrisw chrisw  4096 Jun  8  2020 plugins
drwxr-xr-x 23 chrisw chrisw  4096 Jun  8  2020 qml
drwxr-xr-x  2 chrisw chrisw  4096 Jun  8  2020 resources
drwxr-xr-x  3 chrisw chrisw 12288 Jun  8  2020 translations

If you run the qmake executable from the ~Qt/5.15.0/gcc_64/bin folder against your PRO file it will generate a Makefile that uses the tools and library under ~Qt/5.15.0/gcc_64 and the (in this example) the 64-bit GCC compiler. When you run make your project will be built with those tools and libraries. A Windows install will be similar but you may need to ensure the bundled MingW bin folder is in your PATH.

So, from top to bottom:

chrisw@newton:/tmp/demo$ cat main.cpp
#include <QCoreApplication>
#include <QDebug>

int main(int argc, char **argv) {
        QCoreApplication app(argc, argv);
        qDebug() << "Excellent!";
        return 0;
}

A basic PRO file starter can be created for you (one time only, after that you maintain this file yourself):

chrisw@newton:/tmp/demo$ ~/Qt/5.15.0/gcc_64/bin/qmake -project

chrisw@newton:/tmp/demo$ cat demo.pro
######################################################################
# Automatically generated by qmake (3.1) Sat Aug 28 09:32:14 2021
######################################################################

TEMPLATE = app
TARGET = demo
INCLUDEPATH += .

# You can make your code fail to compile if you use deprecated APIs.
# In order to do so, uncomment the following line.
# Please consult the documentation of the deprecated API in order to know
# how to port your code away from it.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

# Input
SOURCES += main.cpp

Then you create the Makefile:

chrisw@newton:/tmp/demo$ ~/Qt/5.15.0/gcc_64/bin/qmake 

chrisw@newton:/tmp/demo$ ls 
demo.pro  main.cpp  Makefile

and build the project:

chrisw@newton:/tmp/demo$ make
g++ -c -pipe -O2 -Wall -Wextra -D_REENTRANT -fPIC -DQT_NO_DEBUG -DQT_GUI_LIB -DQT_CORE_LIB -I. -I. -I/home/chrisw/Qt/5.15.0/gcc_64/include -I/home/chrisw/Qt/5.15.0/gcc_64/include/QtGui -I/home/chrisw/Qt/5.15.0/gcc_64/include/QtCore -I. -isystem /usr/include/libdrm -I/home/chrisw/Qt/5.15.0/gcc_64/mkspecs/linux-g++ -o main.o main.cpp
g++ -Wl,-O1 -Wl,-rpath,/home/chrisw/Qt/5.15.0/gcc_64/lib -o demo main.o   /home/chrisw/Qt/5.15.0/gcc_64/lib/libQt5Gui.so /home/chrisw/Qt/5.15.0/gcc_64/lib/libQt5Core.so -lGL -lpthread   

chrisw@newton:/tmp/demo$ ./demo 
Excellent!