Solving the Python and C+ + Class Inheritance Issue with Pybind11

Discover how to properly declare C+ + classes in `Pybind11` to resolve inheritance issues between `Python` and `C+ + `.
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Solving the Python and C+ + Class Inheritance Issue with Pybind11

When using Pybind11 to interface between C+ + and Python, one common issue developers might face is the recognition of derived classes created in Python when they are passed to C+ + functions. In this guide, we'll explore this problem and provide detailed steps to resolve it.

The Problem: Derived Classes Not Recognized

Imagine you've defined a C+ + class hierarchy with a base class and a derived class. You want to call a C+ + function from your Python code, passing an instance of the derived class. However, C+ + functions expect an object of the base class type, leading to confusion and errors.

The Minimum Code to Reproduce the Issue

To demonstrate the problem, consider the following simplified code:

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The Error Message

When you attempt to execute the code above, you might encounter the following error:

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This error indicates that the function fff_float is expecting an object of BaseClass<float>, but it's receiving an object of DerivedClass<float> instead.

The Solution: Explicit Declaration of Base Class

To resolve this issue, you need to inform Pybind11 that DerivedClass_float extends BaseClass_float. Here’s how to do this:

Step-by-Step Solution

Declare the Base Class in Python:
Create an exposed class for the base class in your Pybind11 module. This is crucial because it will make the base class accessible in Python even if it cannot be instantiated directly.

Specify Inheritance:
When declaring the derived class in Python, you need to explicitly state that it extends the base class.

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Explanation of Changes

The addition of py::class_<BaseClass<float>>(m, "BaseClass_float"); exposes the base class to Python. This is necessary because even though you can't instantiate it directly (since it lacks a constructor), you need it recognized for type relationships.

By modifying the declaration for the derived class to py::class_<DerivedClass<float>, BaseClass<float>>(...), Pybind11 is properly informed of the inheritance, hence resolving the issue of DerivedClass_float being viewed as a different type when passed to fff_float.

Conclusion

By modifying the way we declare classes in our Pybind11 module, we can effectively solve the problem of base and derived class recognition. This solution not only clarifies the relationship between the classes but also allows for smoother interactions between Python and C+ + .

With these adjustments, you're now equipped to handle class inheritance issues in Pybind11, enhancing your integration of Python and C+ + . Happy coding!