如何让FetchContent依赖的cpprestsdk在CMake配置阶段完成构建

问题分析 #

核心问题在于FetchContent_MakeAvailable默认将依赖构建延迟到项目构建阶段，但配置阶段CMake需要解析cpprestsdk::cpprest目标，此时该目标尚未生成，导致报错。另外你额外添加的find_package属于冗余操作，OVERRIDE_FIND_PACKAGE已让find_package复用FetchContent的结果，无需重复调用。

修正步骤 #

1. 移除冗余的find_package调用：FetchContent_MakeAvailable已负责导入cpprestsdk的CMake目标，无需额外执行find_package。
2. 强制依赖在配置阶段完成构建：通过手动触发CMake的配置与构建命令，在项目配置阶段就完成cpprestsdk的编译，确保目标在配置阶段就存在。

修正后的CMakeLists.txt #

cmake_minimum_required(VERSION 3.5)

project(myproject)
include(FetchContent)

# 关闭cpprestsdk的WERROR选项
set(ENABLE_WERROR OFF CACHE BOOL "Turn off Treat warnings as errors" FORCE)
set(WERROR OFF CACHE BOOL "Turn off Treat warnings as errors" FORCE)

FetchContent_Declare(
    cpprestsdk
    GIT_REPOSITORY https://github.com/microsoft/cpprestsdk.git
    GIT_TAG v2.10.17
    OVERRIDE_FIND_PACKAGE
)

# 检查cpprestsdk是否已被填充
FetchContent_GetProperties(cpprestsdk)
if(NOT cpprestsdk_POPULATED)
    # 填充依赖（下载、检出源码）
    FetchContent_Populate(cpprestsdk)
    
    # 在配置阶段手动构建cpprestsdk
    set(cpprest_build_dir "${cpprestsdk_BINARY_DIR}")
    file(MAKE_DIRECTORY "${cpprest_build_dir}")
    
    # 执行cmake配置
    execute_process(
        COMMAND ${CMAKE_COMMAND}
            -S "${cpprestsdk_SOURCE_DIR}"
            -B "${cpprest_build_dir}"
            -DENABLE_WERROR=OFF
            -DWERROR=OFF
            -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
            -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
            -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
        RESULT_VARIABLE cpprest_config_result
        OUTPUT_VARIABLE cpprest_config_output
        ERROR_VARIABLE cpprest_config_error
    )
    if(NOT cpprest_config_result EQUAL 0)
        message(FATAL_ERROR "Failed to configure cpprestsdk:\n${cpprest_config_error}")
    endif()
    
    # 执行cmake构建
    execute_process(
        COMMAND ${CMAKE_COMMAND} --build "${cpprest_build_dir}" --config ${CMAKE_BUILD_TYPE}
        RESULT_VARIABLE cpprest_build_result
        OUTPUT_VARIABLE cpprest_build_output
        ERROR_VARIABLE cpprest_build_error
    )
    if(NOT cpprest_build_result EQUAL 0)
        message(FATAL_ERROR "Failed to build cpprestsdk:\n${cpprest_build_error}")
    endif()
    
    # 导入cpprestsdk的目标
    add_subdirectory("${cpprestsdk_SOURCE_DIR}" "${cpprest_build_dir}" EXCLUDE_FROM_ALL)
endif()

add_executable(myproject svr.cc)
target_link_libraries(myproject 
    cpprestsdk::cpprest
)

关键说明 #

• 手动调用execute_process执行CMake配置和构建命令，强制在项目配置阶段完成cpprestsdk的编译，确保cpprestsdk::cpprest目标在配置阶段已生成。
• 通过FetchContent_GetProperties和NOT cpprestsdk_POPULATED判断依赖是否已处理，避免重复执行构建步骤。
• 传递当前项目的编译器、构建类型等参数给cpprestsdk的构建，保证构建环境一致。

内容的提问来源于stack exchange，提问作者Michaelzh