Smart pointers & py::class_#

The binding generator for classes, py::class_, can be passed a template type that denotes a special holder type that is used to manage references to the object. If no such holder type template argument is given, the default for a type T is std::unique_ptr<T>.

Note

A py::class_ for a given C++ type T — and all its derived types — can only use a single holder type.

py::smart_holder#

Starting with pybind11v3, py::smart_holder is built into pybind11. It is the recommended py::class_ holder for most situations. However, for backward compatibility it is not the default holder, and there are no plans to make it the default holder in the future.

It is extremely easy to use the safer and more versatile py::smart_holder: simply add py::smart_holder to py::class_:

  • py::class_<T> to

  • py::class_<T, py::smart_holder>.

Note

A shorthand, py::classh<T>, is provided for py::class_<T, py::smart_holder>. The h in py::classh stands for smart_holder but is shortened for brevity, ensuring it has the same number of characters as py::class_. This design choice facilitates easy experimentation with py::smart_holder without introducing distracting whitespace noise in diffs.

The py::smart_holder functionality includes the following:

  • Support for two-way Python/C++ conversions for both std::unique_ptr<T> and std::shared_ptr<T> simultaneously.

  • Passing a Python object back to C++ via std::unique_ptr<T>, safely disowning the Python object.

  • Safely passing “trampoline” objects (objects with C++ virtual function overrides implemented in Python, see Overriding virtual functions in Python) via std::unique_ptr<T> or std::shared_ptr<T> back to C++: associated Python objects are automatically kept alive for the lifetime of the smart-pointer.

  • Full support for std::enable_shared_from_this (cppreference).

std::unique_ptr#

This is the default py::class_ holder and works as expected in most situations. However, handling base-and-derived classes involves a reinterpret_cast, which is, strictly speaking, undefined behavior. Also note that the std::unique_ptr holder only supports passing a std::unique_ptr from C++ to Python, but not the other way around. For example, the following code works as expected with py::class_<Example>:

std::unique_ptr<Example> create_example() { return std::unique_ptr<Example>(new Example()); }

m.def("create_example", &create_example);

However, this will fail with py::class_<Example> (but works with py::class_<Example, py::smart_holder>):

void do_something_with_example(std::unique_ptr<Example> ex) { ... }

Note

The reinterpret_cast mentioned above is here. For completeness: The same cast is also applied to py::smart_holder, but that is safe, because py::smart_holder is not templated.

std::shared_ptr#

It is possible to use std::shared_ptr as the holder, for example:

py::class_<Example, std::shared_ptr<Example> /* <- holder type */>(m, "Example");

Compared to using py::class_<Example, py::smart_holder>, there are two noteworthy disadvantages:

  • Because a py::class_ for a given C++ type T can only use a single holder type, std::unique_ptr<T> cannot even be passed from C++ to Python. This will become apparent only at runtime, often through a segmentation fault.

  • Similar to the std::unique_ptr holder, the handling of base-and-derived classes involves a reinterpret_cast that has strictly speaking undefined behavior, although it works as expected in most situations.

Custom smart pointers#

For custom smart pointers (e.g. c10::intrusive_ptr in pytorch), transparent conversions can be enabled using a macro invocation similar to the following. It must be declared at the top namespace level before any binding code:

PYBIND11_DECLARE_HOLDER_TYPE(T, SmartPtr<T>)

The first argument of PYBIND11_DECLARE_HOLDER_TYPE() should be a placeholder name that is used as a template parameter of the second argument. Thus, feel free to use any identifier, but use it consistently on both sides; also, don’t use the name of a type that already exists in your codebase.

The macro also accepts a third optional boolean parameter that is set to false by default. Specify

PYBIND11_DECLARE_HOLDER_TYPE(T, SmartPtr<T>, true)

if SmartPtr<T> can always be initialized from a T* pointer without the risk of inconsistencies (such as multiple independent SmartPtr instances believing that they are the sole owner of the T* pointer). A common situation where true should be passed is when the T instances use intrusive reference counting.

Please take a look at the General notes regarding convenience macros before using this feature.

By default, pybind11 assumes that your custom smart pointer has a standard interface, i.e. provides a .get() member function to access the underlying raw pointer. If this is not the case, pybind11’s holder_helper must be specialized:

// Always needed for custom holder types
PYBIND11_DECLARE_HOLDER_TYPE(T, SmartPtr<T>)

// Only needed if the type's `.get()` goes by another name
namespace PYBIND11_NAMESPACE { namespace detail {
    template <typename T>
    struct holder_helper<SmartPtr<T>> { // <-- specialization
        static const T *get(const SmartPtr<T> &p) { return p.getPointer(); }
    };
}}

The above specialization informs pybind11 that the custom SmartPtr class provides .get() functionality via .getPointer().

Note

The two noteworthy disadvantages mentioned under the std::shared_ptr section apply similarly to custom smart pointer holders, but there is no established safe alternative in this case.

See also

The file tests/test_smart_ptr.cpp contains a complete example that demonstrates how to work with custom reference-counting holder types in more detail.