Contribution guide


All contributed code must be compatible with the Apache 2 license, preferably by being contributed under the Apache 2 license. Code contributed with another license will need the license reviewed by Intel before it can be accepted.

Code formatting

All C/C++ source code in the repository, including the test code, must adhere to the source-code formatting and style guidelines described here. The coding style described here applies to the nGraph repository. Related repositories may make adjustments to better match the coding styles of libraries they are using.

Adding ops to nGraph Core

Our design philosophy is that the graph is not a script for running optimized kernels; rather, the graph is a specification for a computation composed of basic building blocks which we call ops. Compilation should match groups of ops to appropriate optimal semantically equivalent groups of kernels for the backend(s) in use. Thus, we expect that adding of new Core ops should be infrequent and that most functionality instead gets added with new functions that build sub-graphs from existing core ops.

Coding style

We have a coding standard to help us to get development done. If part of the standard is impeding progress, we either adjust that part or remove it. To this end, we employ coding standards that facilitate understanding of what nGraph components are doing. Programs are easiest to understand when they can be understood locally; if most local changes have local impact, you do not need to dig through multiple files to understand what something does and if it is safe to modify.


Names should briefly describe the thing being named and follow these casing standards:

  • Define C++ class or type names with CamelCase.
  • Assign template parameters with UPPER_SNAKE_CASE.
  • Case variable and function names with lower_snake_case.

Method names for basic accessors are prefixed by get_, is_, or set_ and should have simple \(\mathcal{O}(1)\) implementations:

  • A get_ method should be externally idempotent. It may perform some simple initialization and cache the result for later use. Trivial get_ methods can be defined in a header file. If a method is non-trivial, that is often a sign that it is not a basic accessor.
  • An is_ may be used instead of get_ for boolean accessors.
  • A set_ method should change the value returned by the corresponding get_ method.
    • Use set_is_ if using is_ to get a value.
    • Trivial set_ methods may be defined in a header file.
  • Names of variables should indicate the use of the variable.
    • Member variables should be prefixed with m_.
    • Static member variables should be rare and be prefixed with s_.
  • Do not use using to define a type alias at top-level in header file. If the abstraction is useful, give it a class.
    • C++ does not enforce the abstraction. For example if X and Y are aliases for the same type, you can pass an X to something expecting a Y.
    • If one of the aliases were later changed, or turned into a real type, many callers could require changes.


  • ngraph is for the public API, although this is not currently enforced.
    • Use a nested namespace for implementation classes.
    • Use an unnamed namespace or static for file-local names. This helps prevent unintended name collisions during linking and when using shared and dynamically-loaded libraries.
    • Never use using at top-level in a header file.
      • Doing so leaks the alias into users of the header, including headers that follow.
      • It is okay to use using with local scope, such as inside a class definiton.
    • Be careful of C++’s implicit namespace inclusions. For example, if a parameter’s type is from another namespace, that namespace can be visible in the body.
    • Only use using std and/or using ngraph in .cpp files. using a nested namespace has can result in unexpected behavior.

File Names

  • Do not use the same file name in multiple directories. At least one IDE/debugger ignores the directory name when setting breakpoints.

  • Use .hpp for headers and .cpp for implementation.

  • Reflect the namespace nesting in the directory hierarchy.

  • Unit test files are in the tests directory.

    • Transformer-dependent tests are tests running on the default transformer or specifying a transformer. For these, use the form

      TEST(file_name, test_name)
    • Transformer-independent tests:

      • File name is file_name.in.cpp

      • Add #include "test_control.hpp" to the file’s includes

      • Add the line static std::string s_manifest = "${MANIFEST}"; to the top of the file.

      • Use

        NGRAPH_TEST(${BACKEND_NAME}, test_name)

        for each test. Files are generated for each transformer and the ${BACKEND_NAME} is replaced with the transformer name.

        Individual unit tests may be disabled by adding the name of the test to the unit_test.manifest file found in the transformer’s source file directory.


Things that look different should look different because they are different. We use clang format to enforce certain formatting. Although not always ideal, it is automatically enforced and reduces merge conflicts.

  • The .clang-format file located in the root of the project specifies our format. Simply run:

    $ make style-check
    $ make style-apply
  • Formatting with #include files:

    • Put headers in groups separated by a blank line. Logically order the groups downward from system-level to 3rd-party to ngraph.

    • Formatting will keep the files in each group in alphabetic order.

    • Use this syntax for files that do not change during nGraph development; they will not be checked for changes during builds. Normally this will be everything but the ngraph files:

      #include <file>
    • Use this syntax for files that are changing during nGraph development; they will be checked for changes during builds. Normally this will be ngraph headers:

      #include "file"
    • Use this syntax for system C headers with C++ wrappers:

      #include <c...>
  • To guard against multiple inclusion, use:

    #pragma once
    • The syntax is a compiler extension that has been adopted by all supported compilers.
  • The initialization

    Foo x{4, 5};

    is preferred over

    Foo x(4, 5);
  • Indentation should be accompanied by braces; this includes single-line bodies for conditionals and loops.

  • Exception checking:

    • Throw an exception to report a problem.
    • Nothing that calls abort, exit or terminate should be used. Remember that ngraph is a guest of the framework.
    • Do not use exclamation points in messages!
    • Be as specific as practical. Keep in mind that the person who sees the error is likely to be on the other side of the framework and the message might be the only information they see about the problem.
  • If you use auto, know what you are doing. auto uses the same type-stripping rules as template parameters. If something returns a reference, auto will strip the reference unless you use auto&:

    • Don’t do things like

      auto s = Shape{2,3};

      Instead, use

      Shape s{2, 3};
    • Indicate the type in the variable name.

  • One variable declaration/definition per line

    • Don’t use the C-style

      int x, y, *z;

      Instead, use:

      int x;
      int y;
      int* z;

To contribute documentation for your code, please see the Contributing to documentation.

Contributing to documentation


Tips for contributors who are new to the highly-dynamic environment of documentation in AI software:

  • A good place to start is “document something you figured out how to get working”. Content changes and additions should be targeted at something more specific than “developers”. If you don’t understand how varied and wide the audience is, you’ll inadvertently break or block things.
  • There are experts who work on all parts of the stack; try asking how documentation changes ought to be made in their respective sections.
  • Start with something small. It is okay to add a “patch” to fix a typo or suggest a word change; larger changes to files or structure require research and testing first, as well as some logic for why you think something needs changed.
  • Most documentation should wrap at about 80. We do our best to help authors source-link and maintain their own code and contributions; overwriting something already documented doesn’t always improve it.
  • Be careful editing files with links already present in them; deleting links to papers, citations, or sources is discouraged.
  • Please do not submit Jupyter* notebook code to the nGraph Library or core repos; best practice is to maintain any project-specific examples, tests, or walk-throughs in a separate repository and to link back to the stable op or Ops that you use in your project.

For updates within the nGraph Library /doc repo, please submit a PR with any changes or ideas you’d like integrated. This helps us maintain trackability with respect to changes made, additions, deletions, and feature requests.

If you prefer to use a containerized application, like Jupyter* notebooks, Google Docs*, the GitHub* GUI, or MS Word* to explain, write, or share documentation contributions, you can convert the doc/sphinx/source/*.rst files to another format with a tool like pypandoc and share a link to your efforts on our wiki.

Another option is to fork the ngraph repo, essentially snapshotting it at that point in time, and to build a Jupyter* notebook or other set of docs around it for a specific use case. Add a note on our wiki to show us what you did; new and novel applications may have their projects highlighted on an upcoming ngraph.ai release.


Please do not submit Jupyter* notebook code to the nGraph Library or core repos; best practice is to maintain any project-specific examples, tests, or walk-throughs in a separate repository.

Documenting source code examples

When verbosely documenting functionality of specific sections of code – whether they are entire code blocks within a file, or code strings that are outside the nGraph Library’s documentation repo, here is an example of best practice:

Say a file has some interesting functionality that could benefit from more explanation about one or more of the pieces in context. To keep the “in context” navigable, write something like the following in your .rst documentation source file:

.. literalinclude:: ../../../examples/abc/abc.cpp
   :language: cpp
   :lines: 20-31

And the raw code will render as follows

using namespace ngraph;

int main()
    // Build the graph
    Shape s{2, 3};
    auto a = std::make_shared<op::Parameter>(element::f32, s);
    auto b = std::make_shared<op::Parameter>(element::f32, s);
    auto c = std::make_shared<op::Parameter>(element::f32, s);

    auto t0 = std::make_shared<op::Add>(a, b);

You can now verbosely explain the code block without worrying about breaking the code. The trick here is to add the file you want to reference relative to the folder where the Makefile is that generates the documentation you’re writing.

See the note at the bottom of this page for more detail about how this works in the current 0.26 version of nGraph Library documentation.

Adding captions to code blocks

One more trick to helping users understand exactly what you mean with a section of code is to add a caption with content that describes your parsing logic. To build on the previous example, let’s take a bigger chunk of code, add some line numbers, and add a caption:

.. literalinclude:: ../../../examples/abc/abc.cpp
   :language: cpp
   :lines: 48-56
   :caption: "caption for a block of code that initializes tensors"

and the generated output will show readers of your helpful documentation

“caption for a block of code that initializes tensors”
    // Initialize tensors
    float v_a[2][3] = {{1, 2, 3}, {4, 5, 6}};
    float v_b[2][3] = {{7, 8, 9}, {10, 11, 12}};
    float v_c[2][3] = {{1, 0, -1}, {-1, 1, 2}};

    t_a->write(&v_a, 0, sizeof(v_a));
    t_b->write(&v_b, 0, sizeof(v_b));
    t_c->write(&v_c, 0, sizeof(v_c));

Our documentation practices are designed around “write once, reuse” that we can use to prevent code bloat. See the Contribution guide for our code style guide.

How to build the documentation


Stuck on how to generate the html? Run these commands; they assume you start at a command line running within a clone (or a cloned fork) of the ngraph repo. You do not need to run a virtual environment to create documentation if you don’t want; running $ make clean in the doc/sphinx folder removes any generated files.

Right now the minimal version of Sphinx needed to build the documentation is Sphinx v. 1.7.5. This can be installed with pip3, either to a virtual environment, or to your base system if you plan to contribute much core code or documentation. For C++ API docs that contain inheritance diagrams and collaboration diagrams which are helpful for framework integratons, building bridge code, or creating a backend UI for your own custom framework, be sure you have a system capable of running doxygen.

To build documentation locally, run:

$ sudo apt-get install python3-sphinx
$ pip3 install Sphinx==1.7.5
$ pip3 install breathe numpy
$ cd doc/sphinx/
$ make html
$ cd build/html
$ python3 -m http.server 8000

Then point your browser at localhost:8000.

To build documentation in a python3 virtualenv, try:

$ python3 -m venv py3doc
$ . py3doc/bin/activate
(py3doc)$ pip install Sphinx breathe numpy
(py3doc)$ cd doc/sphinx
(py3doc)$ make html
(py3doc)$ cd build/html
(py3doc)$ python -m http.server 8000

Then point your browser at localhost:8000.


For docs built in a virtual env, Sphinx latest changes may break documentation; try building with a specific version of Sphinx.

For tips on writing reStructuredText-formatted documentation, see the sphinx stable reST documentation.