slsDetectorPackage/libs/pybind/docs/advanced/cast/overview.rst

Overview

1. Native type in C++, wrapper in Python

Exposing a custom C++ type using py::class_ was covered in detail in the /classes section. There, the underlying data structure is always the original C++ class while the py::class_ wrapper provides a Python interface. Internally, when an object like this is sent from C++ to Python, pybind11 will just add the outer wrapper layer over the native C++ object. Getting it back from Python is just a matter of peeling off the wrapper.

2. Wrapper in C++, native type in Python

This is the exact opposite situation. Now, we have a type which is native to Python, like a tuple or a list. One way to get this data into C++ is with the py::object family of wrappers. These are explained in more detail in the /advanced/pycpp/object section. We'll just give a quick example here:

void print_list(py::list my_list) {
    for (auto item : my_list)
        std::cout << item << " ";
}

>>> print_list([1, 2, 3])
1 2 3

The Python list is not converted in any way -- it's just wrapped in a C++ py::list class. At its core it's still a Python object. Copying a py::list will do the usual reference-counting like in Python. Returning the object to Python will just remove the thin wrapper.

3. Converting between native C++ and Python types

In the previous two cases we had a native type in one language and a wrapper in the other. Now, we have native types on both sides and we convert between them.

void print_vector(const std::vector<int> &v) {
    for (auto item : v)
        std::cout << item << "\n";
}

>>> print_vector([1, 2, 3])
1 2 3

In this case, pybind11 will construct a new std::vector<int> and copy each element from the Python list. The newly constructed object will be passed to print_vector. The same thing happens in the other direction: a new list is made to match the value returned from C++.

Lots of these conversions are supported out of the box, as shown in the table below. They are very convenient, but keep in mind that these conversions are fundamentally based on copying data. This is perfectly fine for small immutable types but it may become quite expensive for large data structures. This can be avoided by overriding the automatic conversion with a custom wrapper (i.e. the above-mentioned approach 1). This requires some manual effort and more details are available in the opaque section.

List of all builtin conversions

The following basic data types are supported out of the box (some may require an additional extension header to be included). To pass other data structures as arguments and return values, refer to the section on binding classes.

Data type	Description	Header file
int8_t, uint8_t	8-bit integers	pybind11/pybind11.h
int16_t, uint16_t	16-bit integers	pybind11/pybind11.h
int32_t, uint32_t	32-bit integers	pybind11/pybind11.h
int64_t, uint64_t	64-bit integers	pybind11/pybind11.h
ssize_t, size_t	Platform-dependent size	pybind11/pybind11.h
float, double	Floating point types	pybind11/pybind11.h
bool	Two-state Boolean type	pybind11/pybind11.h
char	Character literal	pybind11/pybind11.h
char16_t	UTF-16 character literal	pybind11/pybind11.h
char32_t	UTF-32 character literal	pybind11/pybind11.h
wchar_t	Wide character literal	pybind11/pybind11.h
const char *	UTF-8 string literal	pybind11/pybind11.h
const char16_t *	UTF-16 string literal	pybind11/pybind11.h
const char32_t *	UTF-32 string literal	pybind11/pybind11.h
const wchar_t *	Wide string literal	pybind11/pybind11.h
std::string	STL dynamic UTF-8 string	pybind11/pybind11.h
std::u16string	STL dynamic UTF-16 string	pybind11/pybind11.h
std::u32string	STL dynamic UTF-32 string	pybind11/pybind11.h
std::wstring	STL dynamic wide string	pybind11/pybind11.h
std::string_view, std::u16string_view, etc.	STL C++17 string views	pybind11/pybind11.h
std::pair<T1, T2>	Pair of two custom types	pybind11/pybind11.h
std::tuple<...>	Arbitrary tuple of types	pybind11/pybind11.h
std::reference_wrapper<...>	Reference type wrapper	pybind11/pybind11.h
std::complex<T>	Complex numbers	pybind11/complex.h
std::array<T, Size>	STL static array	pybind11/stl.h
std::vector<T>	STL dynamic array	pybind11/stl.h
std::deque<T>	STL double-ended queue	pybind11/stl.h
std::valarray<T>	STL value array	pybind11/stl.h
std::list<T>	STL linked list	pybind11/stl.h
std::map<T1, T2>	STL ordered map	pybind11/stl.h
std::unordered_map<T1, T2>	STL unordered map	pybind11/stl.h
std::set<T>	STL ordered set	pybind11/stl.h
std::unordered_set<T>	STL unordered set	pybind11/stl.h
std::optional<T>	STL optional type (C++17)	pybind11/stl.h
std::experimental::optional<T>	STL optional type (exp.)	pybind11/stl.h
std::variant<...>	Type-safe union (C++17)	pybind11/stl.h
std::filesystem::path<T>	STL path (C++17)1	pybind11/stl/filesystem.h
std::function<...>	STL polymorphic function	pybind11/functional.h
std::chrono::duration<...>	STL time duration	pybind11/chrono.h
std::chrono::time_point<...>	STL date/time	pybind11/chrono.h
Eigen::Matrix<...>	Eigen: dense matrix	pybind11/eigen.h
Eigen::Map<...>	Eigen: mapped memory	pybind11/eigen.h
Eigen::SparseMatrix<...>	Eigen: sparse matrix	pybind11/eigen.h

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1. std::filesystem::path is converted to pathlib.Path and os.PathLike is converted to std::filesystem::path.↩︎