CMake SYSTEM标志无法屏蔽Boost仅头文件库编译警告

问题场景 #

编译依赖多个仅头文件外部库的CMake项目时，已在target_include_directories中通过SYSTEM标志标记Boost、fftw、cairo等外部库的头文件路径，配置如下：

target_include_directories(cartogram
  PRIVATE
    ${PROJECT_SOURCE_DIR}/include
  SYSTEM
    ${Boost_INCLUDE_DIRS}
    PkgConfig::fftw
    PkgConfig::cairo
    $<$<BOOL:${APPLE}>:/opt/homebrew/include>
)

但编译时仍收到来自Boost头文件的警告，示例警告：

In file included from /Users/adi/Desktop/github/cartogram-cpp/src/inset_state/write_inset_to_geojson.cpp:1:
In file included from /Users/adi/Desktop/github/cartogram-cpp/include/inset_state.hpp:6:
In file included from /Users/adi/Desktop/github/cartogram-cpp/include/geo_div.hpp:4:
In file included from /Users/adi/Desktop/github/cartogram-cpp/include/ellipse.hpp:4:
In file included from /Users/adi/Desktop/github/cartogram-cpp/include/cgal_typedef.hpp:11:
In file included from /opt/homebrew/include/CGAL/Polyline_simplification_2/simplify.h:29:
In file included from /opt/homebrew/include/CGAL/Modifiable_priority_queue.h:20:
In file included from /opt/homebrew/include/boost/heap/pairing_heap.hpp:21:
/opt/homebrew/include/boost/heap/detail/stable_heap.hpp:100:32: warning: unused parameter 'rhs' [-Wunused-parameter]
  100 |     size_holder(size_holder && rhs) BOOST_NOEXCEPT

临时有效方案 #

手动添加编译选项，直接通过-isystem告诉编译器将Boost头文件目录视为系统头文件：

target_compile_options(cartogram PRIVATE -isystem ${Boost_INCLUDE_DIRS})

添加后Boost的警告被成功屏蔽。

原因解析 #

1. 导入目标的SYSTEM属性处理差异：PkgConfig导出的PkgConfig::fftw、PkgConfig::cairo这类目标，其包含路径已被CMake正确标记为SYSTEM属性，编译器会自动屏蔽这些目录的警告；但Boost通过Config模式查找时，${Boost_INCLUDE_DIRS}对应的路径可能未被CMake完全标记为SYSTEM，导致编译器仍会检查该目录下的代码并输出警告。
2. 直接使用-isystem绕过CMake的属性传递：手动添加-isystem选项，直接向编译器传递指令，强制将Boost头文件目录当作系统头文件处理，从而跳过警告检查。

更规范的CMake写法 #

避免直接使用编译器特定选项，改用Boost提供的导入目标Boost::headers（该目标已将所有Boost头文件路径标记为SYSTEM）：

target_include_directories(cartogram
  PRIVATE
    ${PROJECT_SOURCE_DIR}/include
  SYSTEM
    PkgConfig::fftw
    PkgConfig::cairo
    $<$<BOOL:${APPLE}>:/opt/homebrew/include>
)

# 链接Boost::headers目标，自动引入带SYSTEM属性的头文件路径
target_link_libraries(cartogram PRIVATE Boost::headers)

这种方式符合CMake的跨平台规范，无需依赖编译器特定参数，同时能正确屏蔽Boost的头文件警告。

内容的提问来源于stack exchange，提问作者adisidev