pybind11-2.10.4/docs/advanced/smart_ptrs.rst

Smart pointers

std::unique_ptr

Given a class Example with Python bindings, it's possible to return instances wrapped in C++11 unique pointers, like so

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

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

In other words, there is nothing special that needs to be done. While returning unique pointers in this way is allowed, it is illegal to use them as function arguments. For instance, the following function signature cannot be processed by pybind11.

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

The above signature would imply that Python needs to give up ownership of an object that is passed to this function, which is generally not possible (for instance, the object might be referenced elsewhere).

std::shared_ptr

The binding generator for classes, 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 named Type is std::unique_ptr<Type>, which means that the object is deallocated when Python's reference count goes to zero.

It is possible to switch to other types of reference counting wrappers or smart pointers, which is useful in codebases that rely on them. For instance, the following snippet causes std::shared_ptr to be used instead.

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

Note that any particular class can only be associated with a single holder type.

One potential stumbling block when using holder types is that they need to be applied consistently. Can you guess what's broken about the following binding code?

class Child { };

class Parent {
public:
   Parent() : child(std::make_shared<Child>()) { }
   Child *get_child() { return child.get(); }  /* Hint: ** DON'T DO THIS ** */
private:
    std::shared_ptr<Child> child;
};

PYBIND11_MODULE(example, m) {
    py::class_<Child, std::shared_ptr<Child>>(m, "Child");

    py::class_<Parent, std::shared_ptr<Parent>>(m, "Parent")
       .def(py::init<>())
       .def("get_child", &Parent::get_child);
}

The following Python code will cause undefined behavior (and likely a segmentation fault).

from example import Parent

print(Parent().get_child())

The problem is that Parent::get_child() returns a pointer to an instance of Child, but the fact that this instance is already managed by std::shared_ptr<...> is lost when passing raw pointers. In this case, pybind11 will create a second independent std::shared_ptr<...> that also claims ownership of the pointer. In the end, the object will be freed twice since these shared pointers have no way of knowing about each other.

There are two ways to resolve this issue:

1. For types that are managed by a smart pointer class, never use raw pointers in function arguments or return values. In other words: always consistently wrap pointers into their designated holder types (such as std::shared_ptr<...>). In this case, the signature of get_child() should be modified as follows:

std::shared_ptr<Child> get_child() { return child; }

2. Adjust the definition of Child by specifying std::enable_shared_from_this<T> (see cppreference for details) as a base class. This adds a small bit of information to Child that allows pybind11 to realize that there is already an existing std::shared_ptr<...> and communicate with it. In this case, the declaration of Child should look as follows:

class Child : public std::enable_shared_from_this<Child> { };

Custom smart pointers

pybind11 supports std::unique_ptr and std::shared_ptr right out of the box. For any other custom smart pointer, 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 macro_notes 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().

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.