Alright, let's tackle how to switch the qmake version for a specific sub-project in your Qt subdirs setup. Here's a step-by-step breakdown based on common scenarios:

1. Locate your target qmake executable #

First, you need the full path to the qmake binary from the Qt version you want to use for your specific sub-project. For example:

• On Linux/macOS: /opt/Qt/5.15.2/gcc_64/bin/qmake
• On Windows: C:\Qt\5.15.2\mingw81_64\bin\qmake.exe

This path varies depending on your Qt installation directory and the compiler variant you're targeting.

2. Using Qt Creator (GUI Approach) #

If you're using Qt Creator for development, this is the most straightforward method:

• Open your project and switch to the Projects mode (left sidebar icon that looks like a folder with a wrench).
• Select the specific sub-project (e.g., sub_project_2) from the list on the left.
• In the Build & Run tab, click Add under Kits to create a new kit, or modify an existing kit:
  • In the kit settings, set the Qt version to your target Qt release (which will automatically map to the correct qmake).
• Assign this custom kit to your target sub-project. All other sub-projects can remain on their original kits.
• Don't forget to clean the build directory for the sub-project before rebuilding to avoid cached artifacts.

3. Manual Command-Line Build #

If you prefer building via terminal/command prompt:

• Navigate directly to the directory of your target sub-project:

  cd project/sub_project_2
  

• Run the target qmake executable against the sub-project's .pro file:

  # Linux/macOS example
  /opt/Qt/5.15.2/gcc_64/bin/qmake sub_project_2.pro
  

  :: Windows example (Command Prompt)
  C:\Qt\5.15.2\mingw81_64\bin\qmake.exe sub_project_2.pro
  

• Compile as usual with make, mingw32-make, or nmake depending on your compiler.

4. Integrate into the Main Subdirs Project #

If you want the main project.pro to handle building the specific sub-project with the alternate qmake version automatically:

• Open your main project.pro file and modify the SUBDIRS section to explicitly specify the qmake path for the target sub-project:

  SUBDIRS += sub_project_1 sub_project_2 sub_project_3
  
  # Override qmake for sub_project_2
  sub_project_2.subdir = sub_project_2
  sub_project_2.qmake = /opt/Qt/5.15.2/gcc_64/bin/qmake
  

  This tells the main project's build system to use the specified qmake when processing sub_project_2, while using the default qmake for other sub-projects.

Critical Notes #

• ABI Compatibility: If your sub-projects depend on each other (e.g., sub_project_1 links against sub_project_2), ensure their Qt versions are compatible. Mixing Qt versions with different ABIs will cause linker errors.
• Clean Builds: Always delete the sub-project's build directory when switching qmake versions to avoid leftover files from the previous build configuration.
• Common Pri Files: Verify that your common/common.pri uses syntax and Qt modules supported by the target qmake version. Older Qt versions may not support newer module syntax or features.

内容的提问来源于stack exchange，提问作者C. Haas