qt-minimal.md (5379B)
1 # Getting started with QT, without the nonsense 2 3 At work I have recently (yesterday) ended working on a big project and 4 I am now waiting for a new assignment. While I wait, I decided that I 5 should take some time to learn something that could be useful both at 6 work and for my personal projects. After some thought, I settled on QT 7 development with C++. 8 9 Any tutorial on this topic tells you to start by downloading QT Creator, 10 an full-blown IDE, and set up a new project from there. In a few clicks 11 you'll have a couple of source code files and a CMakeLists.txt full of 12 [CMake](https://nl.wikipedia.org/wiki/CMake) magic. Ready to go! 13 14 But there is a problem. I don't like full-blown IDEs (although admittedly 15 QT Creator seems quite snappy), and I don't like being locked into specific 16 tools such as CMake. I tried to look online how to set up a QT project 17 without these, but I could not find anything, so I had to figure it out on 18 my own. And then I wrote this post so you don't have have to waste your 19 time like I did :) 20 21 You can find the source code for this tiny project 22 [here](https://git.tronto.net/minimal-qt) (or 23 [here](https://github.com/sebastianotronto/minimal-qt)) if you have bad 24 tase and prefer github over my [self-hosted](../2022-11-23-git-host/) 25 git instance. If you just want to try it out you can run, from the 26 project's main folder: 27 28 ``` 29 make && run 30 ``` 31 32 But keep reading if you want to understand how this works! 33 34 ## Requirements 35 36 * A C++ compiler (tested with GCC 14.2.1 and Clang 19.1.7, both on Linux-x86_64) 37 * QT libraries and headers 38 * The QT development tools `moc` and `uic` 39 40 On Fedora Linux they are contained in the `qt6-qtbase-devel` package. 41 I have not tested this on other operating systems. 42 43 ## Explanation 44 45 Following the most basic QT Creator template, there are 4 source code files: 46 47 * `main.cpp`: the main source file 48 * `mainwindow.h`: the header file for the main window of the application 49 * `mainwindow.cpp`: the C++ source for the main window 50 * `mainwindow.ui`: the UI file with an XML description of the main window 51 52 The compilation is done in 3 steps: 53 54 1. Pre-process `mainwindow.h` with `moc` to obtain `moc_mainwindow.cpp` 55 2. Compile `mainwindow.ui` with `uic` to obtain `ui_mainwindow.h` 56 3. Compile the C++ files, including `moc_mainwindow.cpp` 57 58 The `Makefile` implements these three steps. 59 60 The first two steps are straightforward: first you need to locate the 61 `moc` and `uic` commands. They are usually in the QT installation 62 folder - for example I have them in `/usr/lib64/qt/libexec`. Then run: 63 64 ``` 65 /usr/lib64/qt/libexec/moc mainwindow.h > moc_mainwindow.cpp 66 /usr/lib64/qt/libexex/uic mainwindow.ui > ui_mainwindow.h 67 ``` 68 69 These steps can be skipped if one prefers to write the `moc_` and `ui_` 70 files directly, removing the dependency on the two tools. 71 72 For the last step, one needs to include the header files and link with 73 the QT libraries. 74 75 On my system the header files are in `/usr/include/qt6`. This means I 76 need to add the option `-I /usr/include/qt6` to my `g++` command. This 77 project also requires headers contained in a subfolder of this, 78 so I will add a second `-I` option (see the full command below). 79 80 As for the shared libraries, in my case they are installed in a system 81 folder that is scanned by default by the linker, so I don't have to 82 specify the path. We need the files `libQt6Widgets.so`, `libQt6Core.so` 83 and `libQt6Gui.so`, which we can include with 84 `-lQt6Widgets -lQt6Core -lQt6Gui`. If your linker does not find them, 85 locate them and include their folder with `-L /path/to/the/folder`. 86 87 So the command for step 3 becomes: 88 89 ``` 90 g++ main.cpp mainwindow.cpp moc_mainwindow.cpp \ 91 -I /usr/include/qt6 -I /usr/include/qt6/QtWidgets \ 92 -lQt6Widgets -lQt6Core -lQt6Gui 93 -o run 94 ``` 95 96 And finally you can enjoy your new app: 97 98 ``` 99 ./run 100 ``` 101 102 ## Update 2025-04-07 - now with AI! 103 104 When I shared this post on LinkedIn, I asked if anyone could prompt an 105 LLM to build a QT app without QT Creator or CMake. A colleague did, 106 and the catbot's (pun intended) response can be found 107 [here](https://chat.mistral.ai/chat/fa3e3a76-0e8c-4135-94a1-ae7735174e93). 108 109 This solution is different from the one I found. The bot opted for using 110 [qmake](https://doc.qt.io/qt-6/qmake-manual.html), another tool bundled 111 with QT, like moc and uic. This tool reads a `.pro` file that describes 112 the project's structure and build configuration, and it generates a 113 `Makefile`. This method is a bit more black-boxy than the one I found, 114 but it does not require more external dependencies; I think I might use 115 this other approach in the future. 116 117 The chat bot's response is great, but it is "only" 99% 118 accurate. Unfortunately this means that if one blindly follows these 119 instructions, they won't get a working QT application. The problem is 120 with the commands: 121 122 ``` 123 qmake -project 124 qmake 125 ``` 126 127 The first one is completely useless, and actually prevents the project 128 from being built by generating a second project file. If one simply 129 ignores the `qmake -project` line, the instructions become correct. 130 131 This story is quite insightful: the chat bot gave me instructions that are 132 almost correct (so technically they are *incorrect*). With my experience 133 (a whole 5 hours of CV-worthy QT development) I was able to fix these 134 instructions easily in a couple of minutes, but someone without the same 135 experience would have probably struggled for much longer.