注意
本文最后更新于 2024-02-01，文中内容可能已过时。
一、导言
引用
模块包含不应该用作函数调用，因为模块可能被包含多次。本篇，我们将编写我们自己的包含保护机制，如果多次包含一个模块，将触发警告。内置的include_guard命令从3.10版开始可以使用，对于C/C++头文件，它的行为就像#pragma一样。对于当前版本的CMake，我们将演示如何重新定义函数和宏，并且展示如何检查CMake版本，对于低于3.10的版本，我们将使用定制的包含保护机制。
二、项目结构
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| .
├── cmake
│ ├── custom.cmake
│ └── include_guard.cmake
└── CMakeLists.txt
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项目地址：
https://gitee.com/jiangli01/tutorials/tree/master/cmake-tutorial/chapter7/05
cmake/include_guard.cmake
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| macro(include_guard)
if (CMAKE_VERSION VERSION_LESS "3.10")
# for CMake below 3.10 we define our
# own include_guard(GLOBAL)
message(STATUS "calling our custom include_guard")
# if this macro is called the first time
# we start with an empty list
if(NOT DEFINED included_modules)
set(included_modules)
endif()
if ("${CMAKE_CURRENT_LIST_FILE}" IN_LIST included_modules)
message(WARNING "module ${CMAKE_CURRENT_LIST_FILE} processed more than once")
endif()
list(APPEND included_modules ${CMAKE_CURRENT_LIST_FILE})
else()
# for CMake 3.10 or higher we augment
# the built-in include_guard
message(STATUS "calling the built-in include_guard")
_include_guard(${ARGV})
endif()
endmacro()
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引用
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| macro(include_guard)
if (CMAKE_VERSION VERSION_LESS "3.10")
# ...
else()
# ...
endif()
endmacro()
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include_guard宏包含两个分支，一个用于CMake低于3.10，另一个用于CMake高于3.10。
引用
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| message(STATUS "calling our custom include_guard")
# if this macro is called the first time
# we start with an empty list
if(NOT DEFINED included_modules)
set(included_modules)
endif()
if ("${CMAKE_CURRENT_LIST_FILE}" IN_LIST included_modules)
message(WARNING "module ${CMAKE_CURRENT_LIST_FILE} processed more than once")
endif()
list(APPEND included_modules ${CMAKE_CURRENT_LIST_FILE})
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如果CMake版本低于3.10，进入第一个分支，并且内置的include_guard不可用，所以我们自定义了一个。
如果第一次调用宏，则included_modules变量没有定义，因此我们将其设置为空列表。然后检查${CMAKE_CURRENT_LIST_FILE}是否是included_modules列表中的元素。如果是，则会发出警告；如果没有，我们将${CMAKE_CURRENT_LIST_FILE}追加到这个列表。CMake输出中，我们可以验证自定义模块的第二个包含确实会导致警告。
引用
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| macro(include_guard)
if (CMAKE_VERSION VERSION_LESS "3.10")
# ...
else()
message(STATUS "calling the built-in include_guard")
_include_guard(${ARGV})
endif()
endmacro()
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CMake 3.10及更高版本的情况有所不同；在这种情况下，存在一个内置的include_guard，我们用自己的宏接收到参数并调用它。
这里，_include_guard(${ARGV})指向内置的include_guard。这里，我们使用自定义消息(调用内置的include_guard)进行了扩展。这种模式为我们提供了一种机制，来重新定义自己的或内置的函数和宏，这对于调试或记录日志来说非常有用。
cmake/custom.cmake
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| include_guard(GLOBAL)
message(STATUS "custom.cmake is included and processed")
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CMakeLists.txt
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| cmake_minimum_required(VERSION 3.10 FATAL_ERROR)
project(example LANGUAGES NONE)
# (re)defines include_guard
include(cmake/include_guard.cmake)
# this is some custom module
include(cmake/custom.cmake)
# we simulate that we accidentally include the module a second time
include(cmake/custom.cmake)
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三、结果展示
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| $ mkdir -p build
$ cd build
$ cmake ..
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CMake 3.10及更高版本的结果如下:
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| -- calling the built-in include_guard
-- custom.cmake is included and processed
-- calling the built-in include_guard
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CMake 3.10以下的结果如下:
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| - calling our custom include_guard
-- custom.cmake is included and processed
-- calling our custom include_guard
CMake Warning at cmake/include_guard.cmake:7 (message):
module
/home/user/example/cmake/custom.cmake
processed more than once
Call Stack (most recent call first):
cmake/custom.cmake:1 (include_guard)
CMakeLists.txt:12 (include)
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