Standalone C++ example: build, save and load an ONNX model with only `lib_onnx_proto`#

This page documents examples/build_save_load_onnx_proto (view on GitHub), a self-contained CMake project that builds a tiny ONNX onnx::ModelProto entirely in C++, serializes it to disk, parses it back, and checks the round-trip.

The example purposely links against onnx_light::lib_onnx_proto only: no operator schemas, no shape inference, no checker, and no backend kernels are needed. It is the minimum surface required to produce and consume an .onnx file from C++.

Step 1 – Install the C++ library#

From the onnx-light repository root, build and install the static library and its public headers. The Python extension is not required:

cmake -S . -B build-install \
      -DCMAKE_BUILD_TYPE=Release \
      -DONNX_LIGHT_BUILD_PYTHON=OFF \
      -DCMAKE_INSTALL_PREFIX=/usr/local
cmake --build  build-install
cmake --install build-install

Step 2 – Build the example#

Point CMAKE_PREFIX_PATH at the install prefix chosen above:

cmake -S examples/build_save_load_onnx_proto -B build-build-save-load \
      -DCMAKE_BUILD_TYPE=Release \
      -DCMAKE_PREFIX_PATH=/usr/local
cmake --build build-build-save-load

Step 3 – Run the example#

./build-build-save-load/build_save_load_onnx_proto /tmp/example_add.onnx

Example output:

Saved: /tmp/example_add.onnx
  IR version       : 7
  Producer name    : build_save_load_onnx_proto
  Graph name       : add_graph
  Nodes            : 1
  Inputs           : 1
  Outputs          : 1
  Initializers     : 1
Loaded: /tmp/example_add.onnx
  ...
  Initializer B    : [1, 2, 3]
Round-trip OK

The generated file is a valid ONNX model and can be opened with the reference onnx Python package (onnx.load(...) followed by onnx.checker.check_model(...)).

CMakeLists.txt#

The example CMake project uses find_package to locate the installed library and links against the exported onnx_light::lib_onnx_proto target only:

cmake_minimum_required(VERSION 3.15)
project(build_save_load_onnx_proto LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(onnx_light REQUIRED)

add_executable(build_save_load_onnx_proto main.cc)
target_link_libraries(build_save_load_onnx_proto PRIVATE onnx_light::lib_onnx_proto)

Key API types#

onnx::ModelProto, onnx::GraphProto, onnx::NodeProto, onnx::TensorProto, onnx::ValueInfoProto: Plain proto-message classes used to assemble the model in memory.
onnx::utils::FileWriteStream: Buffered binary output stream used to write the serialized model.
onnx::utils::FileStream: Buffered binary input stream used to read the model back from disk.
onnx::SerializeModelProtoToStream(): Writes a onnx::ModelProto to a binary stream using the configured onnx::SerializeOptions.
onnx::ParseModelProtoFromStream(): Parses a binary protobuf stream into a onnx::ModelProto, optionally honouring onnx::ParseOptions (threading, no-copy, etc.).

Standalone C++ example: build, save and load an ONNX model with only lib_onnx_proto#