命令行调用CMake时Qt5_DIR变量未留存至缓存的问题求助

我有一个基于Qt的大型CMake项目，在命令行或VSCode中执行构建时，发现配置完成后Qt5_DIR变量没有被保存到CMake缓存里。比如执行以下命令后，缓存中找不到Qt5_DIR：

cmake -DQt5_DIR=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5 -S .. -B .

这导致后续用CMake GUI修改其他变量时，会出现Qt5_DIR未设置的错误。但如果在CMake GUI里手动设置Qt5_DIR，它会一直保留，直到再次通过命令行或VSCode执行构建。

已在CMake 3.24.3和3.31.4版本测试过，并且在极简项目中复现了该问题：

复现步骤 #

1. 创建CMakeLists.txt，内容如下：

cmake_minimum_required(VERSION 3.24)
project(testprog)

variable_watch("Qt5_DIR")

if(POLICY CMP0020)
  cmake_policy(SET CMP0020 NEW)
endif()
if(POLICY CMP0043)
  cmake_policy(SET CMP0043 NEW)
endif()

# Qt相关必要设置
set(CMAKE_INCLUDE_CURRENT_DIR ON)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)


find_package(Qt5 COMPONENTS Widgets Core SerialPort QML Quick LinguistTools Sql REQUIRED)

# 将Qt头目录标记为系统目录
include_directories(SYSTEM ${Qt5Widgets_INCLUDE_DIRS})
include_directories(.)

qt5_wrap_ui(UI_HEADERS
        mainwindow.ui)


SET(PROG_SOURCES
    mainwindow.cpp
    main.cpp
    ${UI_HEADERS}
)


add_executable(testprog WIN32 ${PROG_SOURCES})
qt5_use_modules(testprog Widgets Core SerialPort QuickWidgets Sql)

2. 在同目录下创建空文件：

   • main.cpp
   • mainwindow.cpp
   • mainwindow.ui

3. 创建build子目录并进入：

mkdir build && cd build

4. 执行CMake配置命令：

cmake "-DCMAKE_CONFIGURATION_TYPES=Debug;Release;MinSizeRel;RelWithDebInfo" -DQt5_DIR=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5  -G "Visual Studio 17 2022" -A x64 ..

输出显示Qt5_DIR被正常读取：

-- Selecting Windows SDK version 10.0.22621.0 to target Windows 10.0.19045.
-- The C compiler identification is MSVC 19.41.34123.0
-- The CXX compiler identification is MSVC 19.41.34123.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: C:/Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/MSVC/14.41.34120/bin/Hostx64/x64/cl.exe - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: C:/Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/MSVC/14.41.34120/bin/Hostx64/x64/cl.exe - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
CMake Debug Log at CMakeLists.txt:21 (find_package):
  Variable "Qt5_DIR" was accessed using READ_ACCESS with value
  "C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5".


CMake Debug Log at CMakeLists.txt:21 (find_package):
  Variable "Qt5_DIR" was accessed using READ_ACCESS with value
  "C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5".


-- Configuring done (4.0s)
-- Generating done (0.2s)
-- Build files have been written to: C:/Users/Documents/git/testcmakeqt/build

5. 查看缓存内容：

cmake -L

结果中缺失Qt5_DIR：

-- Cache values
CMAKE_INSTALL_PREFIX:PATH=C:/Program Files/testprog
Qt5Core_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Core
Qt5Gui_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Gui
Qt5LinguistTools_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5LinguistTools
Qt5Network_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Network
Qt5QML_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Qml
Qt5Qml_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Qml
Qt5QuickWidgets_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5QuickWidgets
Qt5Quick_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Quick
Qt5SerialPort_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5SerialPort
Qt5Sql_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Sql
Qt5Widgets_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Widgets

6. 改用CMake GUI配置，手动指定Qt5_DIR后，缓存中保留了Qt5_DIR：

-- Cache values
CMAKE_BACKWARDS_COMPATIBILITY:STRING=2.4
CMAKE_CONFIGURATION_TYPES:STRING=Debug;Release;MinSizeRel;RelWithDebInfo
CMAKE_INSTALL_PREFIX:PATH=C:/Program Files/Project
EXECUTABLE_OUTPUT_PATH:PATH=
LIBRARY_OUTPUT_PATH:PATH=
Qt5Core_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Core
Qt5Gui_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Gui
Qt5LinguistTools_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5LinguistTools
Qt5Network_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Network
Qt5QML_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Qml
Qt5Qml_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Qml
Qt5QuickWidgets_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5QuickWidgets
Qt5Quick_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Quick
Qt5SerialPort_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5SerialPort
Qt5Sql_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Sql
Qt5Widgets_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5Widgets
Qt5_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5

原因分析 #

CMake默认不会将所有命令行传入的变量都存入缓存，只有当变量被标记为CACHE类型，或者在配置过程中被显式写入缓存时，才会留存。find_package会使用Qt5_DIR来定位Qt，但Qt的配置脚本并没有将Qt5_DIR本身写入缓存——而CMake GUI在设置变量时，会自动将其添加到缓存中。

解决方案 #

方案1：在CMakeLists.txt中显式将Qt5_DIR写入缓存 #

在调用find_package(Qt5 ...)之前，添加以下代码，强制将Qt5_DIR存入缓存：

# 仅当Qt5_DIR已设置但未在缓存中时，将其添加到缓存
if(DEFINED Qt5_DIR AND NOT Qt5_DIR IN_LIST CACHE_VARIABLES)
  set(Qt5_DIR "${Qt5_DIR}" CACHE PATH "Qt5 CMake配置目录路径" FORCE)
endif()

方案2：使用-DCMAKE_CACHE_ARGS传递变量 #

在命令行中，改用CMAKE_CACHE_ARGS参数来传递Qt5_DIR，这样CMake会直接将变量写入缓存：

cmake "-DCMAKE_CONFIGURATION_TYPES=Debug;Release;MinSizeRel;RelWithDebInfo" -DCMAKE_CACHE_ARGS="-DQt5_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5" -G "Visual Studio 17 2022" -A x64 ..

方案3：手动修改缓存文件 #

在配置完成后，直接编辑build/CMakeCache.txt，添加一行：

Qt5_DIR:PATH=C:/Qt/Qt5.12.12/5.12.12/msvc2017_64/lib/cmake/Qt5

这种方式适合临时解决，但不如前两种方案自动化。

内容的提问来源于stack exchange，提问作者KyleL