是否可以使用 cmake 的 FetchContent 包含 protobuf？

Question

I want to use protobuf in my C++ library. All dependencies so far are included using cmake's FetchContent module. I want to do the same with protobuf . However, I run into the following problem: Unknown CMake command "protobuf_generate_cpp". Any hints on how to solve this?

Excerpt of my CMakeLists.txt :

FetchContent_Declare(fmt
        GIT_REPOSITORY https://github.com/fmtlib/fmt.git
        GIT_TAG 9.0.0)

FetchContent_Declare(protobuf
        GIT_REPOSITORY https://github.com/protocolbuffers/protobuf.git
        GIT_TAG v21.4)

FetchContent_MakeAvailable(fmt protobuf)

include_directories(${Protobuf_INCLUDE_DIRS})
include_directories(${CMAKE_CURRENT_BINARY_DIR})

protobuf_generate_cpp(PROTO_SRCS PROTO_HDRS message.proto)

Answer 1

This works for me:

FetchContent_Declare(protobuf
  GIT_REPOSITORY https://github.com/protocolbuffers/protobuf.git
  GIT_TAG        v21.4
  SOURCE_SUBDIR  cmake
  FIND_PACKAGE_ARGS NAMES protobuf
)
FetchContent_MakeAvailable(protobuf)

On the consumer end do this

include(FindProtobuf)
find_package(protobuf CONFIG REQUIRED)

Note: This has only been tested on CMake v3.25

Answer 2

protobuf_generate_cpp is from FindProtobuf . It doesn't seem to work with a protoc that was build in the project with FetchContent . You'll have to call the protoc binary explicitly.

set(GENERATED_CODE_DIR ${CMAKE_CURRENT_BINARY_DIR}/generated)
set(PROTO_SRCS ${GENERATED_CODE_DIR}/message.pb.cc)
set(PROTO_HDRS ${GENERATED_CODE_DIR}/message.pb.h)
set(PROTOC ${protobuf_BINARY_DIR}/protoc)
set(PROTO_DIR ${CMAKE_CURRENT_SOURCE_DIR})

add_custom_command(
    OUTPUT ${PROTO_SRCS} ${PROTO_HDRS}
    COMMAND ${PROTOC} --proto_path ${PROTO_DIR} message.proto --cpp_out ${GENERATED_CODE_DIR}
    DEPENDS ${PROTOC} ${PROTO_DIR}/message.proto
    )