CMake中的include命令用于从文件或模块(file or module)加载并运行CMake code。其格式如下：

include(<file|module> [OPTIONAL] [RESULT_VARIABLE <var>]
                      [NO_POLICY_SCOPE])

      从给定的文件加载并运行CMake code。变量读写访问调用者的范围(Variable reads and writes access the scope of the caller (dynamic scoping))。
      如果指定了OPTIONAL，即使文件不存在也不会触发error。

include(xxxx.cmake OPTIONAL) # xxxx.cmake不存在也不会触发warning或error
include(xxxx.cmake) # xxxx.cmake不存在,会触发error
                    # CMake Error at test_include.cmake:9 (include):
                    #   include could not find requested file: xxxx.cmake

      如果给定了RESULT_VARIABLE，变量<var>将被设置为已包含的完整的文件名，如果没有找到且指定了OPTIONAL则为NOTFOUND。

include(test_project.cmake RESULT_VARIABLE var)
message("var: ${var}") # var: /home/spring/GitHub/Linux_Code_Test/Samples_CMake/messy_usage/test_project.cmake

include(xxxx.cmake OPTIONAL RESULT_VARIABLE var) # xxxx.cmake不存在
message("var: ${var}") # var: NOTFOUND

include(xxxx.cmake RESULT_VARIABLE var) # xxxx.cmake不存在,触发error
                                        # CMake Error at test_include.cmake:20 (include):
                                        #   include could not find requested file: xxxx.cmake

      如果指定了module而不是file，则首先在CMAKE_MODULE_PATH中搜索名为<modulename>.cmake的文件，然后在CMake module目录(如/usr/share/cmake-3.22/Modules/)中搜索。有一个例外，如果调用include命令的文件本身位于CMake内置模块目录(builtin module directory)中，则首先搜索CMake内置模块目录，然后搜索CMAKE_MODULE_PATH。

include(FindCUDA) # 在/usr/share/cmake-3.22/Modules/目录下查找到

message("cmake module path: ${CMAKE_MODULE_PATH}") # cmake module path:
include(FindCUDA OPTIONAL RESULT_VARIABLE var)
message("var: ${var}") # var: /usr/share/cmake-3.22/Modules/FindCUDA.cmake

# 下载opencv 4.6.0并将其解压缩到/opt/opencv-4.6.0目录下
set(CMAKE_MODULE_PATH /opt/opencv-4.6.0/cmake)
message("cmake module path: ${CMAKE_MODULE_PATH}") # cmake module path: /opt/opencv-4.6.0/cmake
include(FindCUDA OPTIONAL RESULT_VARIABLE var)
message("var: ${var}") # var: /opt/opencv-4.6.0/cmake/FindCUDA.cmake

      如果include命令读取的文件包含对cmake_policy命令的使用，则默认情况下该policy的设置不会影响调用者，可通过include命令中可选的NO_POLICY_SCOPE关键字来控制此行为。

cmake_policy(GET CMP0065 var) # 3.4
message("var: ${var}") # var: NEW

# 临时调整test_cmake_policy.cmake中的第3测试段，使其显示设置CMP0065为OLD
include(test_cmake_policy.cmake) # 不带NO_POLICY_SCOPE,在test_cmake_policy.cmake中对CMP0065设置为OLD时,在test_include.cmake中不起作用

cmake_policy(GET CMP0065 var) # 3.4
message("var: ${var}") # var: NEW

# 临时调整test_cmake_policy.cmake中的第3测试段，使其显示设置CMP0065为OLD
include(test_cmake_policy.cmake NO_POLICY_SCOPE) # 带NO_POLICY_SCOPE后,在test_cmake_policy.cmake中对CMP0065设置为OLD时,在test_include.cmake中也生效

cmake_policy(GET CMP0065 var) # 3.4
message("var: ${var}") # var: OLD

       执行上述测试代码需要3个文件：build.sh, CMakeLists.txt, test_include.cmake

      build.sh内容如下：

#! /bin/bash

# supported input parameters(cmake commands)
params=(function macro cmake_parse_arguments \
		find_library find_path find_file find_program find_package \
		cmake_policy cmake_minimum_required project include \
		string)

usage()
{
	echo "Error: $0 needs to have an input parameter"

	echo "supported input parameters:"
	for param in ${params[@]}; do
		echo "  $0 ${param}"
	done

	exit -1
}

if [ $# != 1 ]; then
	usage
fi

flag=0
for param in ${params[@]}; do
	if [ $1 == ${param} ]; then
		flag=1
		break
	fi
done

if [ ${flag} == 0 ]; then
	echo "Error: parameter \"$1\" is not supported"
	usage
	exit -1
fi

if [[ ! -d "build" ]]; then
	mkdir build
	cd build
else
	cd build
fi

echo "==== test $1 ===="
cmake -DTEST_CMAKE_FEATURE=$1 ..

      CMakeLists.txt内容如下：

cmake_minimum_required(VERSION 3.22)
project(cmake_feature_usage)

message("#### current cmake version: ${CMAKE_MAJOR_VERSION}.${CMAKE_MINOR_VERSION}.${CMAKE_PATCH_VERSION}")
include(test_${TEST_CMAKE_FEATURE}.cmake)
message("==== test finish ====")

      test_include.cmake:为上面的所有示例代码

      可能的执行结果如下图所示：

      GitHub： https://github.com/fengbingchun/Linux_Code_Test