Smart Pointers in C++

Smart pointers are objects that manage dynamically allocated memory automatically. They are provided by the C++ Standard Library and help manage the lifetime of dynamically allocated objects.

• Automatically release memory when the managed object is no longer needed.
• Help reduce memory leaks and dangling pointer issues.
• The <memory> header provides smart pointers such as unique_ptr, shared_ptr, and weak_ptr.

Example: Problem with Raw Pointers (Memory Leak Example)

#include <iostream>
using namespace std;

int main() {
    int* ptr = new int(10);

    cout << *ptr;

    // delete ptr; is required to release the memory

    return 0;
} 

10

Explanation:

• new int(10) dynamically allocates memory and stores 10 in it.
• The memory remains allocated until it is explicitly released using delete.
• Forgetting to call delete can cause a memory leak.
• Smart pointers automatically manage this memory.

Syntax of Smart Pointers

Smart pointers are template classes, so the type of object they manage is specified inside angle brackets.

smart_pointer_type<data_type> pointer_name;

For example:

unique_ptr<int> ptr;
shared_ptr<int> ptr;
weak_ptr<int> ptr;

Types of Smart Pointers

C++ provides three commonly used smart pointers:

1. auto_ptr (Deprecated)

auto_ptr was an early smart pointer that automatically deleted the managed object when it went out of scope.

• Ownership is transferred, leaving the original pointer null (empty).
• Copy semantics are unsafe and error-prone.

#include <iostream>
#include <memory>
using namespace std;

int main() {
    auto_ptr<int> ptr1(new int(10));
    cout << *ptr1 << endl;

    auto_ptr<int> ptr2 = ptr1;  // ownership transfer
    cout << *ptr2;
    return 0;
}

10
10

Note: auto_ptr was deprecated in C++11 and removed in C++17.

2. unique_ptr

unique_ptr stores one pointer only at a time. We cannot copy unique_ptr, only transfer ownership of the object to another unique_ptr using the move() method.

• Only one unique_ptr can own an object at a time.
• Lightweight and efficient.
• Ideal for single ownership scenarios.
• make_unique() is the recommended and safer way to create a unique_ptr.

#include <iostream>
#include <memory>
using namespace std;

class Rectangle {
    int length, breadth;

public:
    Rectangle(int l, int b) : length(l), breadth(b) {}

    int area() {
        return length * breadth;
    }
};

int main() {
    unique_ptr<Rectangle> ptr1 = make_unique<Rectangle>(10, 5);

    cout << ptr1->area() << endl;

    unique_ptr<Rectangle> ptr2 = move(ptr1);

    cout << ptr2->area();

    return 0;
} 

50
50

Explanation: make_unique() creates a unique_ptr that exclusively owns the Rectangle object, while move() transfers ownership from ptr1 to ptr2. The object is automatically deleted when the owning unique_ptr goes out of scope.

3. shared_ptr

shared_ptr allows multiple pointers to share ownership of the same object. It uses reference counting to manage memory.

• Supports shared ownership.
• Uses reference counting to manage the object's lifetime.
• Multiple shared_ptr objects can own the same object.
• make_shared() is the recommended way to create a shared_ptr.

#include <iostream>
#include <memory>
using namespace std;

class Rectangle {
    int length, breadth;

public:
    Rectangle(int l, int b) : length(l), breadth(b) {}

    int area() {
        return length * breadth;
    }
};

int main() {
    shared_ptr<Rectangle> ptr1 = make_shared<Rectangle>(10, 5);
    shared_ptr<Rectangle> ptr2 = ptr1;

    cout << ptr1->area() << endl;
    cout << ptr2->area() << endl;
    cout << ptr1.use_count();

    return 0;
} 

50
50
2

Explanation: make_shared() creates a shared_ptr, and copying ptr1 to ptr2 makes them share ownership. use_count() shows the number of owners, and the object is destroyed when the last shared_ptr is gone.

4. weak_ptr

weak_ptr is a non-owning smart pointer used with shared_ptr. It provides access to an object without increasing its reference count. It is mainly used to prevent circular ownership between shared_ptr objects.

• Does not own the managed object.
• Does not increase the shared_ptr reference count.
• Helps prevent circular ownership.
• Uses lock() to obtain a temporary shared_ptr when the object is still alive.

#include <iostream>
#include <memory>
using namespace std;

class Rectangle {
    int length, breadth;

public:
    Rectangle(int l, int b) : length(l), breadth(b) {}

    int area() {
        return length * breadth;
    }
};

int main() {
    shared_ptr<Rectangle> ptr1 = make_shared<Rectangle>(10, 5);
    weak_ptr<Rectangle> ptr2 = ptr1;

    cout << ptr1->area() << endl;
    cout << ptr2.use_count();

    return 0;
} 

50
1

Explanation: ptr1 owns the Rectangle object, while ptr2 observes it without increasing the reference count. lock() can be used to obtain a shared_ptr if the object is still alive.

Problem: Circular Dependency with shared_ptr

If two objects hold shared_ptr to each other:

• Reference count never reaches zero
• Memory leak occurs

This is the reason we use weak pointers(weak_ptr) is a non-owning reference to an object managed by shared_ptr.

So, in the case of shared_ptr because of cyclic dependency use_count never reaches zero which is prevented by using weak_ptr, which removes this problem by declaring A_ptr as weak_ptr, thus class A does not own it, only has access to it and we also need to check the validity of object as it may go out of scope. In general, it is a design issue.

#include <iostream>
#include <memory>
using namespace std;

class Rectangle {
    int length, breadth;

public:
    Rectangle(int l, int b) : length(l), breadth(b) {}
    int area() { return length * breadth; }
};

int main() {
    shared_ptr<Rectangle> P1(new Rectangle(10, 5));
    weak_ptr<Rectangle> P2(P1);

    cout << P1->area() << endl;
    cout << P2.use_count();
    return 0;
}

50
1

Problems with Raw Pointers

Raw pointers provide direct control over memory but require careful manual management.

• Memory Leak: Occurs when dynamically allocated memory is not released after use.
• Dangling Pointer: Occurs when a pointer refers to memory that has already been deallocated.
• Wild Pointer: A pointer that has not been properly initialized and does not point to a valid memory location.
• Manual Memory Management: The programmer must explicitly manage allocation and deallocation using new and delete.

Pointers Vs Smart Pointers