✅ Definition

RAII is a C++ programming idiom where resources (like memory, file handles, locks, sockets) are tied to the lifetime of objects.

The resource is:

So when an object goes out of scope, its destructor is automatically called, releasing the resource.

// MyClass.h
class MyClassImpl; // Forward declaration

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

    void doSomething();

private:
    MyClassImpl* impl; // Pointer to implementation
};

// MyClass.cpp
#include "MyClass.h"

class MyClassImpl {
public:
    void doSomethingImpl() {
        std::cout << "Doing real work...\\n";
    }
};

MyClass::MyClass() : impl(new MyClassImpl) {}

MyClass::~MyClass() { delete impl; }

void MyClass::doSomething() {
    impl->doSomethingImpl();
}

🔍 Key Ideas

Feature Purpose
Forward Declaration Avoids including the full definition of MyClassImpl in header
Private Pointer Actual implementation is hidden from users of the class
Dynamic Allocation Impl object is created in .cpp file, not exposed outside

🎯 Use Cases

Use Case Description
Library design You can change implementation without recompiling users
Reducing build times Prevents cascading recompilation on internal changes
Encapsulation Completely hides private data and types from public headers
Stable APIs ABI doesn’t change when internal members change

⚠️ Downsides