检查CMake变量

Question

我的CMakeLists.txt中有以下CMake代码段:

cmake_minimum_required(VERSION 2.8.8 FATAL_ERROR)

message(STATUS "Before setting - ${MY_VARIABLE}")

# first check
if(NOT DEFINED ${MY_VARIABLE})
  set(MY_VARIABLE true)
endif(NOT DEFINED ${MY_VARIABLE})

message(STATUS "After setting - ${MY_VARIABLE}")

# second check
if(NOT DEFINED ${MY_VARIABLE})
  message(STATUS "What - ${MY_VARIABLE}")
endif(NOT DEFINED ${MY_VARIABLE})

CMake配置的输出是:

-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/local/bin/cc
-- Check for working C compiler: /usr/local/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/local/bin/c++
-- Check for working CXX compiler: /usr/local/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Before setting - 
-- After setting - true
-- What - true
-- Configuring done
-- Generating done
-- Build files have been written to: /tmp/build

问题:为什么第二次检查变量定义报告变量未定义,即使它已定义？令人惊讶的是,变量的值也正确打印!

Answer 1

if(NOT DEFINED VAR_NAME)和之间有区别if(NOT DEFINED ${VAR_NAME})

第一个引用变量而另一个引用其内容.