Pimpl idiom C++ (Pointer to Implementation)

The Pimpl idiom (short for Pointer to Implementation) is a design pattern in C++ that helps to hide implementation details of a class from its users. The goal is to reduce the dependencies between the class’s interface and its implementation, leading to better encapsulation, reduced compilation times, and sometimes better binary compatibility.

Table of Contents

Why Use Pimpl?

Decoupling Interface and Implementation: Changes to the implementation (e.g., adding member variables) don’t require recompiling code that uses the class.
Binary Compatibility: The layout of the class’s implementation can change without affecting the ABI (Application Binary Interface), which is helpful when maintaining libraries.
Encapsulation: It hides the details of the class implementation from the header file, reducing exposure to clients.

How Pimpl Works

You define a class that exposes a public interface and hides the implementation details in a separate class, often defined in a source file (.cpp).

Example:

MyClass.h (Header file – Interface):

#ifndef MYCLASS_H
#define MYCLASS_H

#include <memory> // For std::unique_ptr

class MyClassImpl;  // Forward declaration of the implementation class

class MyClass {
public:
    MyClass();
    ~MyClass();
    void doSomething();

private:
    std::unique_ptr<MyClassImpl> pImpl;  // Pointer to the implementation
};

#endif

MyClass.cpp (Source file – Implementation):

#include "MyClass.h"
#include <iostream>

class MyClassImpl {
public:
    void doSomethingInternal() {
        std::cout << "Doing something in the implementation!" << std::endl;
    }
};

MyClass::MyClass() : pImpl(std::make_unique<MyClassImpl>()) {}

MyClass::~MyClass() = default;

void MyClass::doSomething() {
    pImpl->doSomethingInternal();  // Call the implementation
}

Key Points:

Forward Declaration: The MyClassImpl class is only forward-declared in the header, so it doesn’t need to be fully defined there.
std::unique_ptr: The implementation is stored in a std::unique_ptr (or another smart pointer like std::shared_ptr). This takes care of memory management automatically.
Encapsulation: The details of MyClassImpl are completely hidden from the users of MyClass.

Pros:

Faster Compilation: Changes to the implementation do not require recompiling files that depend on the header.
Better ABI Compatibility: You can change the private data members of MyClassImpl without affecting the interface or any code that uses MyClass.
Cleaner Interface: You reduce the amount of exposed implementation details in the header.

Cons:

More Indirection: Every time you access data or functions from MyClass, it requires an extra pointer dereference.
Memory Overhead: Using a pointer (like std::unique_ptr) adds a small memory overhead.

Overall, the Pimpl idiom is a powerful technique for managing complexity in large codebases, especially when dealing with libraries or systems that evolve over time.

Advanced Example with Multiple Functions

Here’s a more complex example where the implementation involves multiple functions and types:

MyClass.h:

#ifndef MYCLASS_H
#define MYCLASS_H

#include <memory>
#include <string>

class MyClassImpl;  // Forward declaration of the implementation class

class MyClass {
public:
    MyClass();
    ~MyClass();
    void doSomething() const;
    void setName(const std::string& name);
    std::string getName() const;

private:
    std::unique_ptr<MyClassImpl> pImpl;  // Pointer to the implementation
};

#endif

MyClass.cpp:

#include "MyClass.h"
#include <iostream>

// Implementation details are in this private class
class MyClassImpl {
public:
    std::string name;

    void doSomethingInternal() const {
        std::cout << "Hello, " << name << "!" << std::endl;
    }
};

MyClass::MyClass() : pImpl(std::make_unique<MyClassImpl>()) {}

MyClass::~MyClass() = default;

void MyClass::doSomething() const {
    pImpl->doSomethingInternal();
}

void MyClass::setName(const std::string& name) {
    pImpl->name = name;
}

std::string MyClass::getName() const {
    return pImpl->name;
}

In this version:

We have added getter and setter methods for a std::string name field, which is part of the implementation.
The doSomethingInternal method in MyClassImpl now interacts with the name field.

By separating the interface from the implementation like this, if we need to modify the name handling in MyClassImpl, we don’t need to recompile code that uses MyClass, as long as the interface remains the same.

When to Avoid Pimpl

Although Pimpl has many advantages, it’s not always the best choice. Here are cases where you might want to avoid it:

Performance Concerns: The extra pointer indirection can slightly impact performance. For some performance-critical applications, such as real-time systems or games, this extra overhead might not be acceptable.
Simpler Designs: For smaller projects or classes where the implementation is unlikely to change significantly, Pimpl can introduce unnecessary complexity. If your class is small, the added complexity of managing pointers may not be justified.
Simplified Debugging: Since the implementation is hidden, it can sometimes be harder to debug issues, as stepping through the code involves navigating multiple layers (interface, pointer dereferencing, etc.).

Alternative: Inline Functions or Separate Headers

If the main goal is to reduce compilation times but you want to avoid pointer indirection, you could:

Inline Functions: You could define simple functions inline in the header file, which reduces the need to declare separate function signatures in the header and the implementation.
Separate Implementation Header: Instead of using a Pimpl, you could put your implementation in a .impl.h file, which is included only in the .cpp file. This way, you could still separate interface from implementation, but without the indirection of a pointer.

Binary Compatibility

One of the major advantages of using Pimpl is improving binary compatibility, especially in a library context. If you’re working on a library and want to ensure clients can continue to use it without recompiling their own code when your library’s internal implementation changes, Pimpl is very useful.

For example:

Adding New Members: If you add new members to MyClassImpl, the size and layout of MyClass does not change, so clients don’t need to recompile their code.
Changing the Implementation: If you change the internal implementation in MyClassImpl, clients that use MyClass won’t need to recompile either.

This approach is critical when designing long-term APIs or libraries where you want clients to be insulated from internal changes.

Potential Problems with Pimpl

Memory Allocation: Each instance of MyClass will require dynamic memory allocation for pImpl. This can be avoided with more sophisticated techniques like using custom allocators or keeping the object inline, but that’s an added complexity.
Large Header Files: If your interface is simple, and the only thing you’re hiding is a few member variables, the cost of using Pimpl may outweigh the benefits. In that case, traditional encapsulation (using private members directly in the class) might be simpler.
Code Clarity: Although Pimpl hides complexity from the interface, it also makes the code harder to understand at first glance. When you look at MyClass::doSomething(), you can’t see directly what it’s doing without tracing through the pointer to the implementation, which could be a minor inconvenience when debugging or reading the code.

Real-World Example

In libraries like Qt, the Pimpl idiom is widely used. The Qt framework has a very large codebase, and the Pimpl idiom helps mitigate the issues of binary compatibility and compilation overhead. By using Pimpl, Qt can frequently change the implementation of its internal classes without requiring all of the client code (which links against Qt) to recompile.

Conclusion

The Pimpl idiom is a great tool for managing large and evolving codebases, especially in libraries and frameworks where binary compatibility and faster compile times are important. However, like any tool, it should be used judiciously, especially in terms of performance and complexity trade-offs. If you’re working on a simple or small project, the overhead of Pimpl might not be worth it, and more traditional encapsulation techniques may be more suitable.