Is it possible to include protobuf using cmake's FetchContent? - cmake

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
)