如何让CMake FetchContent拉取的C++第三方库不显示编译警告？

问题背景 #

使用CMake的FetchContent模块拉取wxWidgets（v3.1.4）开发C++20项目，静态链接依赖可正常编译运行，但编译时wxWidgets自身的警告与项目代码警告混杂（VS Code中超过一千条），难以排查自有代码的问题。

当前CMake脚本 #

cmake_minimum_required(VERSION 3.11 FATAL_ERROR)
project(my_project LANGUAGES CXX)

# Using C++20
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

# Use wxWidgets static libraries
set(wxWidgets_USE_STATIC 1)
set(wxBUILD_SHARED OFF)

# Include FetchContent module
include(FetchContent)

# Fetch wxWidgets dependency statement declaration
FetchContent_Declare(
    wxWidgets
    GIT_REPOSITORY
        https://github.com/wxWidgets/wxWidgets.git
    GIT_TAG
        v3.1.4
)

# Message to inform user that script is not bugged, just busy resolving dependecy
message(STATUS "Baixando wxWidgets e incluindo-o no projeto")

# Effectively fetch wxWidgets dependency
FetchContent_GetProperties(wxWidgets)

if(NOT wxWidgets_POPULATED)
    FetchContent_Populate(wxWidgets)
    add_subdirectory(${wxwidgets_SOURCE_DIR} ${wxwidgets_BUILD_DIR})
endif()

# Include all of the program's header and source files
file(GLOB_RECURSE HEADERS ${CMAKE_CURRENT_LIST_DIR}/include/*.h)
file(GLOB_RECURSE SOURCES ${CMAKE_CURRENT_LIST_DIR}/source/*.cpp)

# Create program executable and link against all of the program's header and source files
add_executable(${PROJECT_NAME} ${SOURCES} ${HEADERS})

# Link wxWidgets libraries against program executable
target_include_directories(${PROJECT_NAME} PRIVATE ${wxwidgets_SOURCE_DIR}/include)
target_link_libraries(${PROJECT_NAME} PRIVATE wx::base wx::core)

# Copy all assets to build directory to make them available to the program
add_custom_target(copy_assets
    COMMAND ${CMAKE_COMMAND} -E copy_directory ${CMAKE_CURRENT_LIST_DIR}/assets ${CMAKE_CURRENT_BINARY_DIR}/assets
)
add_dependencies(${PROJECT_NAME} copy_assets)

解决方案 #

1. 修正FetchContent变量大小写错误 #

FetchContent_Declare指定的名称是wxWidgets，因此生成的变量名应为大写开头：wxWidgets_SOURCE_DIR（源目录）和wxWidgets_BINARY_DIR（构建目录）。原脚本中的小写变量名在大小写敏感的系统（如Linux）会导致路径错误，需修正：

if(NOT wxWidgets_POPULATED)
    FetchContent_Populate(wxWidgets)
    add_subdirectory(${wxWidgets_SOURCE_DIR} ${wxWidgets_BINARY_DIR})
endif()

2. 隔离警告：仅对自有代码启用警告 #

通过以下三步实现警告隔离：

• 编译wxWidgets时临时关闭所有警告
• 将wxWidgets头文件标记为系统头文件，避免编译器触发包含警告
• 为自有代码明确设置警告规则

修改后的完整CMake脚本

cmake_minimum_required(VERSION 3.11 FATAL_ERROR)
project(my_project LANGUAGES CXX)

# Using C++20
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

# Use wxWidgets static libraries
set(wxWidgets_USE_STATIC 1)
set(wxBUILD_SHARED OFF)

# Include FetchContent module
include(FetchContent)

# Fetch wxWidgets dependency statement declaration
FetchContent_Declare(
    wxWidgets
    GIT_REPOSITORY
        https://github.com/wxWidgets/wxWidgets.git
    GIT_TAG
        v3.1.4
)

# Message to inform user that script is not bugged, just busy resolving dependecy
message(STATUS "Baixando wxWidgets e incluindo-o no projeto")

# Effectively fetch wxWidgets dependency
FetchContent_GetProperties(wxWidgets)

if(NOT wxWidgets_POPULATED)
    FetchContent_Populate(wxWidgets)
    
    # 保存当前警告设置，临时关闭wxWidgets编译警告
    set(OLD_CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
    set(OLD_CMAKE_CXX_WARNING_LEVEL ${CMAKE_CXX_WARNING_LEVEL})
    
    if(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
        # MSVC: 关闭警告
        string(REPLACE "/W4" "/W0" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
    else()
        # GCC/Clang: 关闭所有警告
        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -w")
    endif()
    set(CMAKE_CXX_WARNING_LEVEL 0)
    
    add_subdirectory(${wxWidgets_SOURCE_DIR} ${wxWidgets_BINARY_DIR})
    
    # 恢复原有警告设置
    set(CMAKE_CXX_FLAGS ${OLD_CMAKE_CXX_FLAGS})
    set(CMAKE_CXX_WARNING_LEVEL ${OLD_CMAKE_CXX_WARNING_LEVEL})
endif()

# Include all of the program's header and source files
file(GLOB_RECURSE HEADERS ${CMAKE_CURRENT_LIST_DIR}/include/*.h)
file(GLOB_RECURSE SOURCES ${CMAKE_CURRENT_LIST_DIR}/source/*.cpp)

# Create program executable and link against all of the program's header and source files
add_executable(${PROJECT_NAME} ${SOURCES} ${HEADERS})

# 为自有代码启用严格警告
target_compile_options(${PROJECT_NAME} PRIVATE
    $<$<CXX_COMPILER_ID:GNU>:-Wall -Wextra -Wpedantic>
    $<$<CXX_COMPILER_ID:Clang>:-Wall -Wextra -Wpedantic>
    $<$<CXX_COMPILER_ID:MSVC>:/W4>
)

# Link wxWidgets libraries against program executable
# 将wxWidgets头文件标记为系统头文件，忽略包含时的警告
target_include_directories(${PROJECT_NAME} PRIVATE SYSTEM ${wxWidgets_SOURCE_DIR}/include)
target_link_libraries(${PROJECT_NAME} PRIVATE wx::base wx::core)

# Copy all assets to build directory to make them available to the program
add_custom_target(copy_assets
    COMMAND ${CMAKE_COMMAND} -E copy_directory ${CMAKE_CURRENT_LIST_DIR}/assets ${CMAKE_CURRENT_BINARY_DIR}/assets
)
add_dependencies(${PROJECT_NAME} copy_assets)

3. 确认静态链接设置的正确性 #

你当前的静态链接配置是正确的：

• set(wxWidgets_USE_STATIC 1) 和 set(wxBUILD_SHARED OFF) 会告诉wxWidgets的CMake脚本构建静态库
• 链接wx::base和wx::core目标时，CMake会自动使用静态库版本

内容的提问来源于stack exchange，提问作者Lucas Yotsui