CMake find_package와 외부 의존성 — Module·Config·FetchContent

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# Linux
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CFLAGS += -I/usr/include/openssl
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LDFLAGS += -L/usr/lib -lssl -lcrypto -lz
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# macOS (Homebrew Apple Silicon)
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CFLAGS += -I/opt/homebrew/opt/openssl/include
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LDFLAGS += -L/opt/homebrew/opt/openssl/lib -lssl -lcrypto -lz
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# macOS (Homebrew Intel) — 또 다름
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CFLAGS += -I/usr/local/opt/openssl/include
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# ...
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# Windows + vcpkg — 또 다름
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# ...

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find_package(OpenSSL REQUIRED)
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find_package(ZLIB REQUIRED)
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add_executable(myapp main.cpp)
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target_link_libraries(myapp PRIVATE
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    OpenSSL::SSL
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    OpenSSL::Crypto
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    ZLIB::ZLIB
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)

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find_package(ZLIB REQUIRED)
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add_executable(myapp main.cpp)
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target_link_libraries(myapp PRIVATE ZLIB::ZLIB)

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$CMAKE/Modules/
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├── FindZLIB.cmake
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├── FindOpenSSL.cmake
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├── FindThreads.cmake
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├── FindJPEG.cmake
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└── ... (수십 개)

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/usr/lib/cmake/fmt/
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├── fmt-config.cmake          ← find_package(fmt)가 찾는 파일
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├── fmt-config-version.cmake  ← 버전 정보
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└── fmt-targets.cmake         ← imported 타겟 정의

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# Config 모드 강제 — Find 스크립트는 무시
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find_package(fmt REQUIRED CONFIG)
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# Module 모드 강제 — Config 파일은 무시
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find_package(ZLIB REQUIRED MODULE)

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find_package(ZLIB)  # 없어도 진행
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if(ZLIB_FOUND)
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    target_link_libraries(myapp PRIVATE ZLIB::ZLIB)
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    target_compile_definitions(myapp PRIVATE HAS_ZLIB)
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endif()

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# 1.70 이상
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find_package(Boost 1.70 REQUIRED)
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# 정확히 4.0
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find_package(OpenCV 4.0 EXACT REQUIRED)
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# 범위
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find_package(fmt 10.0...<12.0 REQUIRED)  # CMake 3.19+

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# Boost의 filesystem과 system만 필요
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find_package(Boost 1.70 REQUIRED COMPONENTS filesystem system)
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# Qt6의 특정 모듈만
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find_package(Qt6 REQUIRED COMPONENTS Widgets Gui Core)

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find_package(OpenSSL REQUIRED)
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target_link_libraries(myapp PRIVATE OpenSSL::SSL OpenSSL::Crypto)
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find_package(Threads REQUIRED)
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target_link_libraries(myapp PRIVATE Threads::Threads)
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find_package(CURL REQUIRED)
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target_link_libraries(myapp PRIVATE CURL::libcurl)

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find_package(JPEG REQUIRED)
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# 레거시 방식
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target_include_directories(myapp PRIVATE ${JPEG_INCLUDE_DIRS})
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target_link_libraries(myapp PRIVATE ${JPEG_LIBRARIES})

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find_package(Threads REQUIRED)
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target_link_libraries(myapp PRIVATE Threads::Threads)

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find_package(Boost 1.70 REQUIRED COMPONENTS filesystem regex)
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target_link_libraries(myapp PRIVATE
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    Boost::filesystem
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    Boost::regex
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)
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# 헤더 전용 Boost 라이브러리
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find_package(Boost REQUIRED)
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target_link_libraries(myapp PRIVATE Boost::headers)

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find_package(OpenSSL REQUIRED)
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target_link_libraries(myapp PRIVATE OpenSSL::SSL OpenSSL::Crypto)

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find_package(CURL REQUIRED)
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target_link_libraries(myapp PRIVATE CURL::libcurl)

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find_package(ZLIB REQUIRED)
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target_link_libraries(myapp PRIVATE ZLIB::ZLIB)

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find_package(fmt REQUIRED)
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target_link_libraries(myapp PRIVATE fmt::fmt)

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find_package(nlohmann_json REQUIRED)
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target_link_libraries(myapp PRIVATE nlohmann_json::nlohmann_json)

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find_package(PkgConfig REQUIRED)
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pkg_check_modules(LIBUSB REQUIRED IMPORTED_TARGET libusb-1.0)
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target_link_libraries(myapp PRIVATE PkgConfig::LIBUSB)

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# 버전 요구
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pkg_check_modules(FOO REQUIRED foo>=1.0)
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# 조용히 (오류 메시지 없이)
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pkg_check_modules(FOO QUIET foo)
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# Imported 타겟 생성 (권장)
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pkg_check_modules(FOO REQUIRED IMPORTED_TARGET foo)

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include(FetchContent)
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FetchContent_Declare(
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    fmt
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    GIT_REPOSITORY https://github.com/fmtlib/fmt.git
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    GIT_TAG        11.0.2
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)
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FetchContent_MakeAvailable(fmt)
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target_link_libraries(myapp PRIVATE fmt::fmt)

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include(FetchContent)
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FetchContent_Declare(
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    fmt
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    GIT_REPOSITORY https://github.com/fmtlib/fmt.git
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    GIT_TAG        11.0.2
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)
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FetchContent_Declare(
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    spdlog
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    GIT_REPOSITORY https://github.com/gabime/spdlog.git
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    GIT_TAG        v1.15.0
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)
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FetchContent_Declare(
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    googletest
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    GIT_REPOSITORY https://github.com/google/googletest.git
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    GIT_TAG        v1.15.2
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)
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# 한 번에 모두 가져오기
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FetchContent_MakeAvailable(fmt spdlog googletest)

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FetchContent_Declare(
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    json
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    URL https://github.com/nlohmann/json/releases/download/v3.11.3/json.tar.xz
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    URL_HASH SHA256=8c4b26bf4b422252e13f332bc5e388ec0ab5c3443d24f...
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)
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FetchContent_MakeAvailable(json)

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include(FetchContent)
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# googletest 옵션 설정 (FetchContent_MakeAvailable 전에!)
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set(BUILD_GMOCK OFF CACHE BOOL "" FORCE)
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set(INSTALL_GTEST OFF CACHE BOOL "" FORCE)
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FetchContent_Declare(
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    googletest
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    GIT_REPOSITORY https://github.com/google/googletest.git
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    GIT_TAG        v1.15.2
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)
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FetchContent_MakeAvailable(googletest)

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find_package(fmt QUIET)
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if(NOT fmt_FOUND)
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    message(STATUS "fmt not found, fetching from GitHub...")
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    include(FetchContent)
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    FetchContent_Declare(
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        fmt
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        GIT_REPOSITORY https://github.com/fmtlib/fmt.git
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        GIT_TAG        11.0.2
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    )
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    FetchContent_MakeAvailable(fmt)
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endif()
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target_link_libraries(myapp PRIVATE fmt::fmt)

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include(FetchContent)
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FetchContent_Declare(
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    fmt
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    GIT_REPOSITORY https://github.com/fmtlib/fmt.git
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    GIT_TAG        11.0.2
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    FIND_PACKAGE_ARGS  # 먼저 find_package(fmt) 시도
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)
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FetchContent_MakeAvailable(fmt)
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# 시스템에 있으면 시스템 것, 없으면 다운로드한 것 사용
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target_link_libraries(myapp PRIVATE fmt::fmt)

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include(ExternalProject)
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ExternalProject_Add(
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    external_zlib
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    URL https://zlib.net/zlib-1.3.tar.gz
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    URL_HASH SHA256=...
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    PREFIX ${CMAKE_BINARY_DIR}/external
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    CMAKE_ARGS
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        -DCMAKE_INSTALL_PREFIX=${CMAKE_BINARY_DIR}/external/install
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)

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list(APPEND CMAKE_PREFIX_PATH "/opt/mylibs")

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cmake_minimum_required(VERSION 3.20)
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project(MyServer VERSION 1.0.0 LANGUAGES CXX)
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set(CMAKE_CXX_STANDARD 17)
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set(CMAKE_CXX_STANDARD_REQUIRED ON)
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include(FetchContent)
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# === 필수 시스템 의존성 ===
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find_package(Threads REQUIRED)
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find_package(OpenSSL REQUIRED)
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# === 선택적 의존성 (시스템 또는 FetchContent) ===
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# fmt: 시스템에 있으면 사용, 없으면 다운로드
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find_package(fmt 10.0 QUIET)
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if(NOT fmt_FOUND)
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    message(STATUS "fmt not found, fetching...")
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    FetchContent_Declare(fmt
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        GIT_REPOSITORY https://github.com/fmtlib/fmt.git
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        GIT_TAG        11.0.2
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    )
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    FetchContent_MakeAvailable(fmt)
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endif()
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# spdlog: 위와 같은 패턴
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find_package(spdlog 1.12 QUIET)
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if(NOT spdlog_FOUND)
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    message(STATUS "spdlog not found, fetching...")
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    set(SPDLOG_FMT_EXTERNAL ON CACHE BOOL "" FORCE)
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    FetchContent_Declare(spdlog
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        GIT_REPOSITORY https://github.com/gabime/spdlog.git
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        GIT_TAG        v1.15.0
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    )
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    FetchContent_MakeAvailable(spdlog)
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endif()
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# === 실행 파일 ===
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add_executable(myserver
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    src/main.cpp
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    src/server.cpp
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)
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target_link_libraries(myserver PRIVATE
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    Threads::Threads
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    OpenSSL::SSL
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    OpenSSL::Crypto
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    fmt::fmt
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    spdlog::spdlog
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)

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# 회피: 레거시 방식
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find_package(ZLIB REQUIRED)
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target_include_directories(myapp PRIVATE ${ZLIB_INCLUDE_DIRS})
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target_link_libraries(myapp PRIVATE ${ZLIB_LIBRARIES})
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# Good: Imported 타겟
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find_package(ZLIB REQUIRED)
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target_link_libraries(myapp PRIVATE ZLIB::ZLIB)

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# 회피: 옵션 설정이 MakeAvailable 뒤에
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FetchContent_MakeAvailable(googletest)
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set(BUILD_GMOCK OFF CACHE BOOL "" FORCE)  # 너무 늦음!
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# Good: MakeAvailable 전에 설정
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set(BUILD_GMOCK OFF CACHE BOOL "" FORCE)
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FetchContent_MakeAvailable(googletest)

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# 회피: 찾지 못해도 사용 시도
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find_package(ZLIB)
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target_link_libraries(myapp PRIVATE ZLIB::ZLIB)  # ZLIB 없으면 오류
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# Good: REQUIRED 사용
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find_package(ZLIB REQUIRED)
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target_link_libraries(myapp PRIVATE ZLIB::ZLIB)
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# 또는: _FOUND 체크
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find_package(ZLIB)
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if(ZLIB_FOUND)
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    target_link_libraries(myapp PRIVATE ZLIB::ZLIB)
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endif()

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# 회피: 브랜치는 변할 수 있음
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FetchContent_Declare(fmt
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    GIT_REPOSITORY https://github.com/fmtlib/fmt.git
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    GIT_TAG        master  # 위험! 내일 다른 코드
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)
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# Good: 고정된 태그나 커밋 해시
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FetchContent_Declare(fmt
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    GIT_REPOSITORY https://github.com/fmtlib/fmt.git
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    GIT_TAG        11.0.2  # 버전 태그
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)