What are the benefits and considerations of using the PIMPL idiom in C ?

Benefits of the "PIMPL" Idiom

In the "Pointer to Implementation" (PIMPL) idiom, a public class encapsulates an internal structure or class (the "implementation"), providing implementation hiding from external users. This technique offers several advantages:

• Protection of Internal State: PIMPL hides implementation details and data from library users, ensuring their confidentiality and protection from external tampering.
• Simplified Interface: The public class presents a straightforward interface to users, without exposing unnecessary implementation complexity. This simplifies code interaction and reduces dependency on implementation specifics.
• Enhanced Flexibility: Changes to the implementation (e.g., method optimizations, bug fixes) can be isolated within the PIMPL class. This minimizes impact on the public class and prevents the need for widespread recompilation of code depending on it.

Design Considerations for "Pimpl Class"

While PIMPL isolates implementation, placing public methods on the PIMPL class instead of the public class may seem counterintuitive. However, this approach ensures that:

• Interface Stability: The public class interface remains unchanged, even if the implementation undergoes significant modifications. This stability enhances usability for external users.
• Efficient Compilation: PIMPL class methods are compiled into the library, reducing the number of files users need to include. This improves compilation performance and reduces code dependency.
• Header File Optimization: The public class header file becomes more concise and focused, containing only the essential interface declarations. This simplifies its maintenance and reduces the burden on compiler preprocessor directives.

Example Implementation with Isolated Implementation

Consider the example code below, where the "Purr()" implementation is contained within the PIMPL class:

// header file:
class Cat {
  private:
    class CatImpl;  // PIMPL class
    CatImpl* cat_; // Pointer to PIMPL

  public:
    Cat();            // Constructor
    ~Cat();           // Destructor
    Purr();           // Public method
};

// CPP file:
#include "cat.h"

class Cat::CatImpl {
  public:
    Purr();
};

Cat::Cat() {
    cat_ = new CatImpl;
}

Cat::~Cat() {
    delete cat_;
}

void Cat::Purr() {
    cat_->Purr();
}

void CatImpl::Purr() {
    // Actual implementation of "Purr()"
}

In this example, the public class "Cat" handles the creation and destruction of the "CatImpl" object but delegates the implementation of "Purr()" to the PIMPL class. This approach ensures that the details of the "Purr()" implementation are hidden from external users while providing a stable interface for interacting with the "Cat" class.

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