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# 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.