cmake target_compile_options 工作，但 add_compile_options 没有

Question

我有 make (gnu make) 背景，正在为我的 c++ 项目学习 cmake。

我的系统是 ubuntu 虚拟机：Linux osboxes 4.15.0-46-generic #49-Ubuntu SMP Wed Feb 6 09:33:07 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

我对这个答案here 有点困惑。它说我可以做以下任何一项：

• target_compile_options(${PROJ_NAME} PRIVATE -w)
• add_compile_options(${PROJ_NAME} PRIVATE -w)

我正在编译 log4cpp 代码，当使用标准警告级别编译时会发出大量警告。它的第 3 方代码，所以我不想听到它们。因此我使用 gcc/g++ -w 标志。

当我使用上面的target_compile_options 时它工作正常（没有看到警告）但是当我使用add_compile_options 时它对我不起作用（即我看到所有错误，好像-w 是未应用）。我不确定我在这里做错了什么（但可能是什么！）。

这是我的 CMakeLists.txt 文件供参考：

cmake_minimum_required(VERSION 3.10.2)

# Set the project name
set(PROJ_NAME log4cpp)
project (${PROJ_NAME})

# Set release build type
#set(CMAKE_BUILD_TYPE release)

# Use c++11 standard
set (CMAKE_CXX_STANDARD 11)

# Include path
include_directories(
    inc
    inc/log4cpp
)

# Include source files by wild card
file(GLOB SOURCES "src/log4cpp/*.cpp")

# The executable file
#add_executable(${PROJ_NAME} ${SOURCES})
#add_library(${PROJ_NAME} STATIC ${SOURCES})
add_library(${PROJ_NAME} SHARED ${SOURCES})

# Set Warning flags - disable
#target_compile_options(${PROJ_NAME} PRIVATE -w)
add_compile_options(${PROJ_NAME} PRIVATE -w)

# Need threads lib (need to be specified after ${PROJ_NAME} executable)
find_package(Threads REQUIRED)
target_link_libraries(${PROJ_NAME} Threads::Threads)

# From: http://derekmolloy.ie/hello-world-introductions-to-cmake/
# GOT TO: LISTING 5

add_compile_options 的输出：

[  2%] Building CXX object CMakeFiles/log4cpp.dir/src/log4cpp/AbortAppender.cpp.o /home/admin/dev/dl-cmake/log4cpp/src/log4cpp/AbortAppender.cpp:46:10: warning: ‘template<class> class std::auto_ptr’ is deprecated [-Wdeprecated-declarations]
     std::auto_ptr<Appender> create_abort_appender(const FactoryParams& params)
          ^~~~~~~~ In file included from /usr/include/c++/7/memory:80:0,
                 from /home/admin/dev/dl-cmake/log4cpp/src/log4cpp/AbortAppender.cpp:14: /usr/include/c++/7/bits/unique_ptr.h:51:28: note: declared here    template<typename> class auto_ptr;
                            ^~~~~~~~ /home/admin/dev/dl-cmake/log4cpp/src/log4cpp/AbortAppender.cpp: In function ‘std::auto_ptr<log4cpp::Appender> log4cpp::create_abort_appender(const log4cpp::FactoryParams&)’: /home/admin/dev/dl-cmake/log4cpp/src/log4cpp/AbortAppender.cpp:50:20: warning: ‘template<class> class std::auto_ptr’ is deprecated [-Wdeprecated-declarations]
        return std::auto_ptr<Appender>(new AbortAppender(name));
                    ^~~~~~~~ In file included from /usr/include/c++/7/memory:80:0,
                 from /home/admin/dev/dl-cmake/log4cpp/src/log4cpp/AbortAppender.cpp:14: /usr/include/c++/7/bits/unique_ptr.h:51:28: note: declared here    template<typename> class auto_ptr;
                            ^~~~~~~~

target_compile_options 的输出：

[  2%] Building CXX object CMakeFiles/log4cpp.dir/src/log4cpp/AbortAppender.cpp.o
[  4%] Building CXX object CMakeFiles/log4cpp.dir/src/log4cpp/Appender.cpp.o
[  6%] Building CXX object CMakeFiles/log4cpp.dir/src/log4cpp/AppenderSkeleton.cpp.o
[  8%] Building CXX object CMakeFiles/log4cpp.dir/src/log4cpp/AppendersFactory.cpp.o
[ 10%] Building CXX object CMakeFiles/log4cpp.dir/src/log4cpp/BasicConfigurator.cpp.o
[ 12%] Building CXX object CMakeFiles/log4cpp.dir/src/log4cpp/BasicLayout.cpp.o

Answer 1

命令add_compile_options 仅影响目标，这些目标是在调用命令之后创建的。当前command documentation 说：

向COMPILE_OPTIONS 目录属性添加选项。

（进一步创建的目标使用目录的属性初始化它们的属性，但之前创建的目标不受此目录属性的影响）。

因为你在add_compile_options之前调用add_library，所以这个库的选项没有改变。

---

但是add_definitions 命令也会影响之前创建的目标。来自its documentation：

为当前目录及以下目录中的目标添加定义到编译器命令行（无论是在调用此命令之前还是之后添加）。