为何CMake中unset指令未按预期生效？技术问询

调用CMake的Shell脚本 #

rm -rf build && mkdir build && cd build && cmake .. -DPULISH_VERSION=ON 

CMakeLists.txt内容 #

cmake_minimum_required(VERSION 3.13)

option(WITH_FOO "whether FOO is enabled or not" on)
option(PULISH_VERSION "whether is the pulished verion or not" on)

if(PULISH_VERSION)
    if(WITH_FOO)
        message("FOO should not be enabled for the published version.")
        unset(WITH_FOO) # It seems `WITH_FOO` does not work, what a surprise!
    endif()
endif()

if(WITH_FOO)
    message("Attention! FOO is enabled!")
    add_compile_definitions(WITH_FOO_ENABLED)
endif()

add_executable(demo main.c)

main.c内容 #

int main(){return 0;}

执行输出 #

cmake .. -DPULISH_VERSION=ON                               
-- The C compiler identification is GNU 4.9.4
-- The CXX compiler identification is GNU 4.9.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting C compile features - done
FOO should not be enabled for the published version.
Attention! FOO is enabled!
-- Configuring done
-- Generating done
-- Build files have been written to: /home/John/demo/build

CMake的option命令会将定义的变量存入**缓存（Cache）**中，普通的unset(WITH_FOO)仅能移除当前作用域内的普通变量，无法清除缓存里的WITH_FOO值。后续的if(WITH_FOO)判断会优先读取缓存中的变量值，因此即使执行了unset，WITH_FOO依然被判定为ON，导致后续逻辑触发。

有两种可行的修复方式：

方式一：清除缓存中的变量 #

将unset(WITH_FOO)替换为：

unset(WITH_FOO CACHE)

这条命令会彻底清除缓存中的WITH_FOO变量，后续的if(WITH_FOO)会因为变量未定义而判定为FALSE。

方式二：直接修改缓存值为OFF #

如果需要明确将WITH_FOO设置为禁用状态，也可以直接覆盖缓存值：

set(WITH_FOO OFF CACHE BOOL "whether FOO is enabled or not" FORCE)

FORCE参数确保强制覆盖缓存中已有的值，这样后续的判断会直接使用OFF状态。

修改后的核心代码片段示例：

if(PULISH_VERSION)
    if(WITH_FOO)
        message("FOO should not be enabled for the published version.")
        # 二选一即可
        unset(WITH_FOO CACHE)
        # 或者 set(WITH_FOO OFF CACHE BOOL "whether FOO is enabled or not" FORCE)
    endif()
endif()

内容的提问来源于stack exchange，提问作者John