Pybind11 callback

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pybind11 callback

Pybind11 is a lightweight header-only library that exposes C++ types in Python and vice versa. It enables seamless interoperability between Python and C++, allowing you to call C++ code from Python and vice versa.

A callback in pybind11 refers to the ability to pass a Python function as an argument to a C++ function and later invoke that function from within the C++ code.

Here’s an example to illustrate how to use callback functions in pybind11:

#include <pybind11/pybind11.h>
#include <iostream>

int CallbackExample(int x, int y, pybind11::function callback) {
    if (callback) {
        int result = callback(x, y);
        return result;
    }
    
    return x + y;
}

namespace py = pybind11;

PYBIND11_MODULE(callback_example, m) {
    m.def("callback_func", [](int a, int b){ 
        std::cout << "Callback Function: " << a + b << std::endl;
        return a + b;
    });

    m.def("call_callback_func", &CallbackExample, py::arg("x"), py::arg("y"), py::arg("callback") = nullptr);
}

Explanation:

  • The above C++ code defines a function CallbackExample that takes two integers x and y as arguments, along with a callback function.
  • If a valid callback function is provided, the code invokes that callback function with x and y as arguments and returns the result.
  • If no callback function is provided, it simply returns the sum of x and y.
  • In the Python code, callback_func is defined as a lambda function that takes two integers a and b, prints the sum, and returns it.
  • The function call_callback_func is exposed to Python using pybind11, and a call to this function in Python will pass the callback_func as the third argument.

Here’s how you can use the code in Python:

import callback_example

result = callback_example.call_callback_func(5, 3, callback_example.callback_func)
print("Result:", result)

Output:

Callback Function: 8
Result: 8

As you can see, the Python code calls the C++ function call_callback_func with arguments 5 and 3, along with the callback function callback_example.callback_func. The C++ code invokes the callback function with the provided arguments, prints the sum, and returns it to Python.

This is a basic example of using callbacks in pybind11. You can customize the callback function to perform any desired operations within your C++ code.

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