Nanobind C++: Passing Python Type to Cpp Function and Using it for Casting – A Comprehensive Guide

Introduction

In the world of programming, interoperability between languages is a vital aspect. One such challenge is passing data between Python and C++. With the help of Nanobind, a high-performance binding library, we can seamlessly integrate Python and C++ code. In this article, we’ll delve into the process of passing Python types to C++ functions and using them for casting. Get ready to unlock the full potential of Nanobind C++!

Prerequisites

Before we dive into the meat of the article, make sure you have the following installed:

• Python 3.7 or later
• C++ compiler (e.g., GCC)
• Nanobind installed via pip: pip install nanobind

Understanding Nanobind

Nanobind is a header-only library that provides a simple and efficient way to bind Python and C++ code. It’s designed to be easy to use, flexible, and highly performant. With Nanobind, you can:

• Pass Python objects to C++ functions
• Return C++ objects to Python
• Use C++ functions as Python callables
• Expose C++ classes to Python

Passing Python Types to C++ Functions

Now, let’s explore how to pass Python types to C++ functions using Nanobind. We’ll use a simple example to illustrate the process.

Example: Passing a Python String to a C++ Function

Suppose we have a C++ function that takes a string as an argument:

void print_string(const std::string& str) {
  std::cout << "C++ received: " << str << std::endl;
}

To pass a Python string to this function, we'll use Nanobind's py::object type:

#include <nanobind/nanobind.h>

void print_string(const py::object& obj) {
  if (py::isinstance<py::str>(obj)) {
    std::string str = obj.cast<py::str>().c_str();
    std::cout << "C++ received: " << str << std::endl;
  } else {
    throw std::runtime_error("Expected a string");
  }
}

In this example, we use Nanobind's py::object type to pass a Python object to the C++ function. We then use py::isinstance to check if the object is a Python string, and if so, cast it to a C++ std::string using obj.cast<py::str>().

Example: Passing a Python List to a C++ Function

Let's consider a C++ function that takes a vector of integers:

void process_vector(const std::vector<int>& vec) {
  for (int val : vec) {
    std::cout << "C++ received: " << val << std::endl;
  }
}

To pass a Python list to this function, we'll use Nanobind's py::list type:

#include <nanobind/nanobind.h>

void process_vector(const py::object& obj) {
  if (py::isinstance<py::list>(obj)) {
    py::list list = obj.cast<py::list>();
    std::vector<int> vec;
    for (auto item : list) {
      vec.push_back(item.cast<int>());
    }
    process_vector(vec);
  } else {
    throw std::runtime_error("Expected a list of integers");
  }
}

In this example, we use Nanobind's py::list type to pass a Python list to the C++ function. We then iterate over the list, casting each item to a C++ int using item.cast<int>(), and store them in a C++ std::vector.

Using Python Types for Casting in C++

Now that we've seen how to pass Python types to C++ functions, let's explore how to use these types for casting in C++.

Casting Python Objects to C++ Types

Nanobind provides various casting methods to convert Python objects to C++ types. Here are a few examples:

Python Type	C++ Type	Casting Method
int	int	obj.cast<int>()
float	double	obj.cast<double>()
str	std::string	obj.cast<py::str>().c_str()
list	std::vector<T>	obj.cast<py::list>()

These casting methods allow you to convert Python objects to their corresponding C++ types. Note that the casting method depends on the Python type and the desired C++ type.

Example: Casting a Python Integer to a C++ Integer

py::object obj = py::int_(10);
int val = obj.cast<int>();
std::cout << "C++ value: " << val << std::endl;

In this example, we create a Python integer object using py::int_(10), and then cast it to a C++ int using obj.cast<int>().

Conclusion

In this comprehensive guide, we've explored the process of passing Python types to C++ functions using Nanobind, and how to use these types for casting in C++. With Nanobind, you can seamlessly integrate Python and C++ code, unlocking new possibilities for your projects.

Additional Resources

For further learning, we recommend:

• The official Nanobind documentation: https://nanobind.org/docs/
• Nanobind tutorials and examples: https://nanobind.org/examples/
• Python-C++ integration using Nanobind (YouTube tutorial): https://www.youtube.com/watch?v=dQw4w9WgXcQ

Happy coding!

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