Why Use the PIMPL Idiom Instead of Directly Implementing Methods in the Public Class?

PIMPL Idiom: Benefits of Hiding Implementation

The PIMPL (Pointer to Implementation) idiom is a powerful technique for concealing implementation details in C . However, a question arises: why not directly implement methods on the public class rather than the private implementation class?

In the provided code example, the Purr method is defined in the CatImpl class, which is then wrapped by the public Cat class. This approach follows the "Handle Body" idiom, where the public interface serves as a pointer to the actual implementation.

By separating the interface from the implementation, several benefits emerge:

• Information Hiding: The internal data and details of the implementation remain hidden from the user of the library. This enables the implementation to be modified or replaced without affecting the public interface.
• Flexibility: Changes to the implementation can be made without recompiling code that uses the interface. This flexibility allows for bug fixes, performance optimizations, or new feature additions without disrupting the stability of the product.
• Modular Development: Decoupling the interface from the implementation facilitates parallel development of both aspects. The implementation can be iterated on separately, while the public interface remains intact.

In commercial software products, this idiom is particularly valuable. It allows libraries to be shipped with a public API, while keeping the implementation confidential. Users can access the library's functionality without having to know its inner workings.

Furthermore, PIMPL facilitates design by contract, a methodology where the interface and implementation adhere to a specification. By keeping the implementation hidden, the focus is shifted to the adherence to the specification rather than the specific implementation.

In summary, the PIMPL idiom offers significant advantages by providing information hiding, flexibility, modular development, and support for design by contract. By leveraging this technique, developers can create stable, maintainable, and extensible software architectures.

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