.. _building:

Setting up a build system
#########################

This section assumes that you have followed the instructions to :ref:`install
<installing>` nanobind. The easiest way to compile a nanobind-based extension
involves a CMake-based build system. Other build systems can likely be used as
well, but they are not officially supported.
(The first section of the :ref:`CMake API reference <api_cmake>` mentions
some alternatives.)

Here, we will create a new package from scratch. If you already have an
existing CMake build system, it should be straightforward to merge some of the
following snippets into it.

Preliminaries
-------------

Begin by creating a new file named ``CMakeLists.txt`` in the root directory of
your project. It should start with the following lines that declare a project
name and tested CMake version range. The third line line searches for Python >=
3.9 including the ``Development.Module`` component required by nanobind. The
name of this module changed across CMake versions, hence the additional
conditional check.

.. code-block:: cmake

    cmake_minimum_required(VERSION 3.15...3.27)
    project(my_project) # Replace 'my_project' with the name of your project

    if (CMAKE_VERSION VERSION_LESS 3.18)
      set(DEV_MODULE Development)
    else()
      set(DEV_MODULE Development.Module)
    endif()

    find_package(Python 3.9 COMPONENTS Interpreter ${DEV_MODULE} REQUIRED)

Add the following lines below. They configure CMake to perform an optimized
*release* build by default unless another build type is specified. Without this
addition, binding code may run slowly and produce large binaries.

.. code-block:: cmake

   if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
     set(CMAKE_BUILD_TYPE Release CACHE STRING "Choose the type of build." FORCE)
     set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS "Debug" "Release" "MinSizeRel" "RelWithDebInfo")
   endif()

Finding nanobind
----------------

Next, we must inform CMake about the presence of nanobind so that it can load
the functionality needed to compile extension modules. The details of this
step depend on *how you installed* nanobind, in the :ref:`previous section
<installing>`.

1. If you installed nanobind as a Pip or Conda package, append the following
   lines at the end of ``CMakeLists.txt``. They query the package to determine
   its installation path and then import it.

   .. code-block:: cmake

       # Detect the installed nanobind package and import it into CMake
       execute_process(
         COMMAND "${Python_EXECUTABLE}" -m nanobind --cmake_dir
         OUTPUT_STRIP_TRAILING_WHITESPACE OUTPUT_VARIABLE nanobind_ROOT)
       find_package(nanobind CONFIG REQUIRED)

2. If you installed nanobind as a `Git submodule
   <https://git-scm.com/book/en/v2/Git-Tools-Submodules>`_, append the
   following lines at the end of ``CMakeLists.txt`` to point CMake to the
   directory where nanobind is checked out.

   .. code-block:: cmake

       add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/ext/nanobind)

Building an extension
---------------------

Finally, we are ready to build an extension! Append the following line at the end of
``CMakeLists.txt``. It will compile a new extension named ``my_ext`` from the
source code contained in the file ``my_ext.cpp``.

.. code-block:: cmake

   nanobind_add_module(my_ext my_ext.cpp)

:cmake:command:`nanobind_add_module` resembles standard CMake commands like
``add_executable()`` and ``add_library()``. Any number of source code and
header files can be declared when the extension is more complex and spread out
over multiple files.

.. note::

   One opinionated choice of :cmake:command:`nanobind_add_module` is that it
   optimizes the *size* of the extension by default (i.e., ``-Os`` is passed to
   the compiler regardless of the project-wide settings). You must specify the
   ``NOMINSIZE`` parameter to the command to disable this behavior and, e.g.,
   optimize extension code for speed (i.e., ``-O3``):

   .. code-block:: cmake

      nanobind_add_module(my_ext NOMINSIZE my_ext.cpp)

   The default is chosen this way since extension code usually wraps existing
   C++ libraries, in which the main computation takes place. Optimizing the
   bindings for speed does not measurably improve performance, but it does make
   the bindings *significantly* larger.

   If you observe slowdowns when porting a pybind11 extension, or if your
   extension performs significant amounts of work within the binding layer,
   then you may want to experiment with passing the ``NOMINSIZE`` parameter.

The :ref:`next section <basics>` will review the contents of example module
implementation in ``my_ext.cpp``.
