---
icon: cpu
title: Write C++ Bindings
order: 17
---

## Quick Start

pkpy provides a [pybind11](https://pybind11.readthedocs.io/en/stable/) compatible layer which allows users to do convenient bindings.
Header files are located in the `include/pybind11` directory. Make sure you have added `-Iinclude` to your compiler flags.

To begin with, use `py::scoped_interpreter guard{}` to start the interpreter before using any Python objects.
Or explicitly call `py::interpreter::initialize()` and `py::interpreter::finalize()`.

### module

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

PYBIND11_EMBEDDED_MODULE(example, m) {
    m.def("add", [](int a, int b) {
        return a + b;
    });

    auto math = m.def_submodule("math");
}
```

### function

```cpp
int add(int a, int b) { return a + b; }

int add(int a, int b, int c) { return a + b + c; }

void register_function(py::module_& m)
{
    m.def("add", py::overload_cast<int, int>(&add));

    // support function overload
    m.def("add", py::overload_cast<int, int, int>(&add));

    // bind with default arguments
    m.def("sub", [](int a, int b) { 
        return a - b; 
    }, py::arg("a") = 1, py::arg("b") = 2);

    // bind *args
    m.def("add", [](py::args args) {
        int sum = 0;
        for (auto& arg : args) {
            sum += arg.cast<int>();
        }
        return sum;
    });

    // bind **kwargs
    m.def("add", [](py::kwargs kwargs) {
        int sum = 0;
        for (auto item : kwargs) {
            sum += item.second.cast<int>();
        }
        return sum;
    });
}
```

### class

```cpp
struct Point
{
    const int x;
    int y;

public:
    Point() : x(0), y(0) {}

    Point(int x, int y) : x(x), y(y) {}

    Point(const Point& p) : x(p.x), y(p.y) {}

    std::string stringfy() const { 
        return "(" + std::to_string(x) + ", " + std::to_string(y) + ")"; 
    }
};

struct Point3D : Point
{
private:
    int z;

public:
    Point3D(int x, int y, int z) : Point(x, y), z(z) {}

    int get_z() const { return z; }

    void set_z(int z) { this->z = z; }
};

void bind_class(py::module_& m)
{
    py::class_<Point>(m, "Point")
        .def(py::init<>())
        .def(py::init<int, int>())
        .def(py::init<const Point&>())
        .def_readonly("x", &Point::x)
        .def_readwrite("y", &Point::y)
        .def("__str__", &Point::stringfy);

    // only support single inheritance
    py::class_<Point3D, Point>(m, "Point3D", py::dynamic_attr())
        .def(py::init<int, int, int>())
        .def_property("z", &Point3D::get_z, &Point3D::set_z);

    // dynamic_attr will enable the dict of bound class
}
```

### operators

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

struct Int {
    int value;

    Int(int value) : value(value) {}

    Int operator+(const Int& other) const {
        return Int(value + other.value);
    }

    Int operator-(const Int& other) const {
        return Int(value - other.value);
    }

    bool operator==(const Int& other) const {
        return value == other.value;
    }

    bool operator!=(const Int& other) const {
        return value != other.value;
    }
};

void bind_operators(py::module_& m)
{
    py::class_<Int>(m, "Int")
        .def(py::init<int>())
        .def(py::self + py::self)
        .def(py::self - py::self)
        .def(py::self == py::self)
        .def(py::self != py::self);
        // other operators are similar
}
```

### py::object

`py::object` is just simple wrapper around `PyVar`. It supports some convenient methods to interact with Python objects.


here are some common methods:

```cpp
obj.attr("x"); // access attribute
obj[1]; // access item

obj.is_none(); // same as obj is None in Python
obj.is(obj2); // same as obj is obj2 in Python

// operators
obj + obj2; // same as obj + obj2 in Python
// ...
obj == obj2; // same as obj == obj2 in Python
// ...

obj(...); // same as obj.__call__(...)

py::cast(obj); // cast to Python object
obj.cast<T>; // cast to C++ type

py::type::of(obj); // get type of obj
py::type::of<T>(); // get type of T, if T is registered
```

you can also create some builtin objects with their according wrappers:

```cpp
py::bool_ b = {true};
py::int_ i = {1};
py::float_ f = {1.0};
py::str s = {"hello"};
py::list l = {1, 2, 3};
py::tuple t = {1, 2, 3};
// ...
```



## More Examples

More examples please see the test [folder](https://github.com/pocketpy/gsoc-2024-dev/tree/main/pybind11/tests) in the GSoC repository. All tested features are supported.

## Limits and Comparison

This is a feature list of pybind11 for pocketpy. It lists all completed and pending features. It also lists the features that cannot be implemented in the current version of pocketpy.

### [Function](https://pybind11.readthedocs.io/en/stable/advanced/functions.html)

- [x] Function overloading
- [x] Return value policy
- [x] is_prepend
- [x] `*args` and `**kwargs`
- [ ] Keep-alive
- [ ] Call Guard
- [x] Default arguments
- [ ] Keyword-Only arguments
- [ ] Positional-Only arguments
- [ ] Allow/Prohibiting None arguments

### [Class](https://pybind11.readthedocs.io/en/stable/classes.html)

- [x] Creating bindings for a custom type
- [x] Binding lambda functions
- [x] Dynamic attributes
- [x] Inheritance and automatic downcasting
- [x] Enumerations and internal types
- [ ] Instance and static fields

> Binding static fields may never be implemented in pocketpy because it requires a metaclass, which is a heavy and infrequently used feature.

### [Exceptions](https://pybind11.readthedocs.io/en/stable/advanced/exceptions.html)

Need further discussion.

### [Smart pointers](https://pybind11.readthedocs.io/en/stable/advanced/smart_ptrs.html)

- [ ] std::shared_ptr
- [ ] std::unique_ptr
- [ ] Custom smart pointers

### [Type conversions](https://pybind11.readthedocs.io/en/stable/advanced/cast/index.html)

- [x] Python built-in types
- [x] STL Containers
- [ ] Functional
- [ ] Chrono

### [Python C++ interface](https://pybind11.readthedocs.io/en/stable/advanced/pycpp/object.html)

Need further discussion.

- [x] `object`
- [x] `none`
- [x] `type`
- [x] `bool_`
- [x] `int_`
- [x] `float_`
- [x] `str`
- [ ] `bytes`
- [ ] `bytearray`
- [x] `tuple`
- [x] `list`
- [ ] `set`
- [x] `dict`
- [ ] `slice`
- [x] `iterable`
- [x] `iterator`
- [ ] `function`
- [ ] `buffer`
- [ ] `memoryview`
- [x] `capsule`

### [Miscellaneous](https://pybind11.readthedocs.io/en/stable/advanced/misc.html)

- [ ] Global Interpreter Lock (GIL)
- [ ] Binding sequence data types, iterators, the slicing protocol, etc.
- [x] Convenient operators binding

### Differences between CPython and pocketpy

- only `add`, `sub` and `mul` have corresponding right versions in pocketpy. So if you bind `int() >> py::self`, it will has no effect in pocketpy.

- `__new__` and `__del__` are not supported in pocketpy.

- in-place operators, such as `+=`, `-=`, `*=`, etc., are not supported in pocketpy.

- the return value of `globals` is immutable in pocketpy.