Step One (Basic examples)

We connect c++ code with python calling through a lib: pybind11. Before you start, make sure it's installed correctly

Simple way:

pip install pybind11

Case1: How to call function.

Related files in "basics" file:

• “functions.cc". Your c++ function implementation.
• ”function_call.py“. A demo shows call the c++ functions.

pybind11 provides a way connect c++ with python:

The bind operations are all in a head file, it does not relay on any other lib. Using syntax is as follows:

Include head file and refer namespace:

#include <pybind11/pybind11.h>
namespace py = pybind11;

Create glue func/cls:

PYBIND11_MODULE(functions, m) {
    m.doc() = "pybind11 example plugin";  // optional module docstring. Could be printed by python help().
    m.def("add", &add, "A function that adds two numbers");
}

parameter explain:

• PYBIND11_MODULE() macro: Create functions and classes for python calling.
• functions: The module created to import in python env.
• m: A variable of type py::module_ which is the main interface for creating bindings.
• .doc : Define module docstring.
• .def : Define a python func for calling.
  • parameter-"add": The function name in python, can be changed to any others;
  • parameter-&add: The c++ pointer of target function.
  • parameter-"...": A description of this function.

Running the snippet: 1 Use c++ compiling a python lib. functions.

# Compiler: g++/gcc.
g++ -O3 -Wall -shared -std=c++11 -fPIC $(python3 -m pybind11 --includes) functions.cc -o functions.so

2 To run ”function_call.py“, you'll see info below:

Add called, input numbers: i=3 j=4
7

Note: pybind11 python pkg provides binding libs. You can see them in its lib.

echo $(python3 -m pybind11 --includes)

Printing info on console likes:

-I/home/kaiyuan/anaconda3/envs/py3.9/include/python3.9 -I/home/kaiyuan/anaconda3/envs/py3.9/lib/python3.9/site-packages/pybind11/include

Case2: How to call objects.

This example shows c++ class basic calling and how to deal with overload and inheritance.

Related files in "step_one file":

• “classes.cc". Your c++ classes implementation.
• ”classes_call.py“. A demo shows call the c++ obj.

1. Compile to get .so:

g++ -O3 -Wall -shared -std=c++11 -fPIC $(python3 -m pybind11 --includes) classes.cc -o classes.so

2. To run ”classes_call.py“, you'll see info below:

The profile of this shape:
Desription:Basic example of c++ class
Num:1
Area:10.000000
The profile of this shape:
Desription:Basic example of c++ class
Num:2
Area:20.000000
100.0

Advanced Practice

Building

Both python setuptools and cmake are available, which deeps on your need.

There are two good examples on pybind github.

• python example: https://github.com/pybind/python_example
• cmake example: https://github.com/pybind/cmake_example