The semantics of the CMake API in Qt 5 and Qt 6 are largely compatible. However, up to Qt 5.14, all imported Qt library targets and commands contained the version number as part of the name. This makes writing CMake code that should work with both Qt 5 and Qt 6 somewhat cumbersome. Qt 5.15 therefore introduced versionless targets and commands to enable writing CMake code that is largely agnostic to the different Qt versions.
除现有导入目标外，Qt 5.15 引入
versionless
目标。也就是说，要链接到
Qt Core
可以引用
Qt6::Core
，或
Qt::Core
:
find_package(Qt6 COMPONENTS Core)
if (NOT Qt6_FOUND)
find_package(Qt5 5.15 REQUIRED COMPONENTS Core)
endif()
add_executable(helloworld
...
)
target_link_libraries(helloworld PRIVATE Qt::Core)
以上片段首先试着查找 Qt 6 安装。若失败，试着查找 Qt 5.15 包。独立于是使用 Qt 6 或 Qt 5，可以使用导入的
Qt::Core
目标。
The versionless targets are defined by default. Set
QT_NO_CREATE_VERSIONLESS_TARGETS
先于第一
find_package()
call to disable them.
注意： The imported Qt::Core target will not feature the target properties that are available in the Qt6::Core target.
从 Qt 5.15 起，Qt 模块还提供了无版本变体的 命令 。例如，现在可以使用 qt_add_translation 编译翻译文件，独立于是使用 Qt 5 还是 Qt 6。
Set
QT_NO_CREATE_VERSIONLESS_FUNCTIONS
先于第一
find_package()
调用以阻止创建无版本命令。
There might be projects that need to load both Qt 5 and Qt 6 in one CMake context (though mixing Qt versions in one library or executable is not supported, so be careful there).
在这样的设置中，无版本目标和命令将隐式引用找到的首个 Qt 版本凭借
find_package
。设置
QT_DEFAULT_MAJOR_VERSION
CMake 变量先于首个
find_package
call to make the version explicit.
若还需要支持早于 5.15 的 Qt 5 版本，可以通过将当前版本存储在 CMake 变量中做到这点：
find_package(QT NAMES Qt6 Qt5 REQUIRED COMPONENTS Core)
find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS Core)
add_executable(helloworld
...
)
target_link_libraries(helloworld PRIVATE Qt${QT_VERSION_MAJOR}::Core)
在此，让
find_package(<PackageName>...)
try to find first Qt 6, and if that fails Qt 5, under the name
QT
。若找到它们中的任一，
find_package
will succeed, and the CMake variable
QT_VERSION_MAJOR
will be defined to either
5
or
6
.
We then do load the package for the determined Qt version again by creating the name
Qt${QT_VERSION_MAJOR}
on the fly. This is needed because
CMAKE_AUTOMOC
expects the package name to be either
Qt5
or
Qt6
，否则将打印错误。
We can use the same pattern to also specify the name of the imported library. Before calling
target_link_libraries
，CMake 将解析
Qt${QT_VERSION_MAJOR}::Widgets
到
Qt5::Widgets
or
Qt6::Widgets
.
Use the versionless variants of the CMake commands where possible.
Use the versioned targets unless you have to support Qt 5 and Qt 6 in the same project.
If you have to use versionless targets, be aware of the 使用无版本目标时的陷阱 .
Use the versioned versions of the CMake commands and targets if you need to support Qt 5 versions older than Qt 5.15, or if you cannot control whether your CMake code is loaded in a context where QT_NO_CREATE_VERSIONLESS_FUNCTIONS or QT_NO_CREATE_VERSIONLESS_TARGETS might be defined. In this case you can still simplify your code by determining the actual command or target name through a variable.
Using the versionless targets has several downsides.
The versionless targets are
ALIAS
targets and lack the target properties of the versioned targets.
Projects must not export targets that expose the versionless targets. For example, a library that is consumed by another project must not export targets that link publicly against versionless targets. Otherwise, transitive dependencies might be broken, or the user of that library mixes Qt5 and Qt6 targets involuntarily.
在 Qt 6，
UNICODE
and
_UNICODE
compiler definitions are set by default for targets that link against Qt modules. This is in line with the qmake behavior, but it is a change compared to the CMake API behavior in Qt 5.
调用 qt_disable_unicode_defines() on the target to not set the definitions.
find_package(Qt6 COMPONENTS Core)
add_executable(helloworld
...
)
qt_disable_unicode_defines(helloworld)
Qt CMake 策略
CMake 命令参考