12: Memory Management
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Questions and Answers

What must be done in memory management for every new allocation?

  • Allocate additional memory
  • Copy the allocated memory
  • Free the allocated memory (correct)
  • Move the allocated memory
  • A stack allows insertion and removal of elements only at the bottom.

    False (B)

    What is the primary function of the 'pop' method in a stack?

    To delete the top-most element from the stack.

    The data structure stack is essentially a linked list where elements can be inserted and removed only from the ______.

    <p>top</p> Signup and view all the answers

    Match the following methods with their descriptions:

    <p>push = Adds an element to the top of the stack pop = Removes the top-most element from the stack top = Returns the value of the top-most element empty = Checks if the stack is empty</p> Signup and view all the answers

    In the struct 'lnode', what type is the 'value' field?

    <p>int (D)</p> Signup and view all the answers

    What is the purpose of the 'empty' method in the stack class?

    <p>To determine whether the stack has any elements.</p> Signup and view all the answers

    What is the purpose of the assignment operator in the stack class implementation?

    <p>To create a copy of another stack instance (A)</p> Signup and view all the answers

    The Rule of Three states that if one of the constructor, destructor, or assignment operator is implemented, all three must be implemented.

    <p>True (A)</p> Signup and view all the answers

    What happens to the local variable 'copy' when it goes out of scope after the swap in the assignment operator?

    <p>It is cleaned up, deleting the stack associated with 'copy'.</p> Signup and view all the answers

    The stack class needs a __________ to destruct instances that are not used anymore.

    <p>destructor</p> Signup and view all the answers

    Match the member functions of a C++ class with their primary function:

    <p>Constructor = Creates a new instance Destructor = Destroys an instance Copy Constructor = Initializes a new instance as a copy Assignment Operator = Assigns a copy of one instance to another</p> Signup and view all the answers

    What does the default constructor stack() : topn(nullptr) {} do?

    <p>Initializes topn with the null pointer (C)</p> Signup and view all the answers

    The function stack::empty() returns true if the stack has at least one element.

    <p>False (B)</p> Signup and view all the answers

    What is the purpose of the top() function in the stack implementation?

    <p>To return the top value of the stack while ensuring the stack is not empty.</p> Signup and view all the answers

    When calling push(4), a new node is created with value __ and the next pointer to the current top node.

    <p>4</p> Signup and view all the answers

    What happens when the pop() function is called on an empty stack?

    <p>It throws an assertion error. (B)</p> Signup and view all the answers

    The topn pointer is redirected to the next node after a pop() operation.

    <p>True (A)</p> Signup and view all the answers

    What is the consequence of dereferencing a null pointer while using the top() function?

    <p>It leads to undefined behavior.</p> Signup and view all the answers

    The operation topn = new lnode(value, topn); in the push() method creates a new node with the value of __.

    <p>value</p> Signup and view all the answers

    What does copy construction in C++ refer to?

    <p>Initializing an object with another object of the same type (B)</p> Signup and view all the answers

    If a copy constructor is not defined, the automatically-generated copy constructor will not be called.

    <p>False (B)</p> Signup and view all the answers

    What is the signature of the copy assignment operator in C++?

    <p>T&amp; T::operator=(const T&amp; t)</p> Signup and view all the answers

    The copy constructor of a class T has the declaration T (const T& ______).

    <p>t</p> Signup and view all the answers

    Match the following concepts with their definitions:

    <p>Copy Constructor = Called when an object is initialized using another object of the same type Assignment Operator = Overloaded function for assigning values to an already declared object Deep Copy = Creating a new copy of an object including all its dynamic resources Shallow Copy = Copying the object's bit pattern without creating duplicates of dynamic resources</p> Signup and view all the answers

    When is the copy constructor called for a class object?

    <p>When an object is declared and initialized with another object of the same class (A)</p> Signup and view all the answers

    What is the purpose of the clean_up_nodes function?

    <p>To clean up the nodes and prevent memory leaks (B)</p> Signup and view all the answers

    The assignment operator should return a reference to the right-hand side object.

    <p>False (B)</p> Signup and view all the answers

    What is the purpose of defining our own copy constructor?

    <p>To control how an object is copied, especially for dynamic resources.</p> Signup and view all the answers

    It is valid to directly call the destructor to clean up a stack's data.

    <p>False (B)</p> Signup and view all the answers

    What does the assignment operator do in the context of stacks?

    <p>It makes the left operand a copy of the right operand, managing memory by cleaning up old data.</p> Signup and view all the answers

    The automatically-generated assignment operator corresponds to a member-wise ______.

    <p>assignment</p> Signup and view all the answers

    The copy-and-swap idiom is an elegant strategy to implement the assignment operator using the ________ constructor.

    <p>copy</p> Signup and view all the answers

    What happens if no assignment operator is declared for a class?

    <p>A default assignment operator is automatically generated (A)</p> Signup and view all the answers

    Match the following functions with their description:

    <p>clean_up_nodes = Cleans up the nodes to prevent memory leaks ~stack = Destructor that calls clean_up_nodes operator= = Assignment operator that makes a copy of another stack copy_nodes = Creates a copy of nodes from another stack</p> Signup and view all the answers

    What does std::swap do in the context of the copy-and-swap idiom?

    <p>It exchanges the top node pointers of two stacks. (B)</p> Signup and view all the answers

    The assignment operator requires a conditional check to avoid self-assignment.

    <p>True (A)</p> Signup and view all the answers

    What happens to the local copy of a stack when it goes out of scope in the copy-and-swap idiom?

    <p>Its destructor is called, which cleans up its data.</p> Signup and view all the answers

    In the assignment operator, after cleaning up old data, the new data is copied from the right operand to the left operand using the ________ function.

    <p>copy_nodes</p> Signup and view all the answers

    What is the primary effect of using the delete operator on a dynamically allocated object?

    <p>The object is deconstructed and memory is released (C)</p> Signup and view all the answers

    Every new allocation in dynamic memory management must have a corresponding delete operation.

    <p>True (A)</p> Signup and view all the answers

    What is meant by 'dangling pointers'?

    <p>Pointers that reference memory locations that have been released.</p> Signup and view all the answers

    The function to remove the top element from a stack is called __.

    <p>pop</p> Signup and view all the answers

    Match each term with its correct definition:

    <p>new = Allocates memory for a new object delete = Deallocates memory for an object stack = A data structure that follows LIFO principle lnode = Node structure used in linked list implementation</p> Signup and view all the answers

    What may happen if you release an object more than once using delete?

    <p>It can lead to undefined behavior (D)</p> Signup and view all the answers

    The stack’s destructor automatically deletes all its elements upon release.

    <p>True (A)</p> Signup and view all the answers

    What is the role of the 'push' method in the stack implementation?

    <p>To add a new element to the top of the stack.</p> Signup and view all the answers

    The expression 'delete p;' in the pop() method is used to __.

    <p>release the memory for the top node</p> Signup and view all the answers

    When deleting an object, what does the delete operator NOT do?

    <p>Set the pointer to nullptr (B)</p> Signup and view all the answers

    What is the role of the 'push' method in the stack class?

    <p>Adds an element to the top of the stack (B)</p> Signup and view all the answers

    In the stack class, the 'empty()' method returns true if there are elements in the stack.

    <p>False (B)</p> Signup and view all the answers

    What is the default value of the 'topn' pointer in the stack's constructor?

    <p>nullptr</p> Signup and view all the answers

    The ______ method in the stack class asserts that the stack is not empty before returning the top value.

    <p>top</p> Signup and view all the answers

    Match the stack methods with their descriptions:

    <p>push = Adds an element to the stack pop = Removes the top element from the stack top = Returns the top element of the stack empty = Checks if the stack has any elements</p> Signup and view all the answers

    Which statement about the 'new' operator in C++ is true?

    <p>It allocates memory for a new object and returns its pointer. (D)</p> Signup and view all the answers

    A stack implemented with a linked list allows elements to be accessed from both ends.

    <p>False (B)</p> Signup and view all the answers

    What happens in the stack when the 'pop()' method is called on an empty stack?

    <p>An error occurs or an assertion fails</p> Signup and view all the answers

    The 'next' pointer in the 'lnode' struct points to the ______ node in the linked list.

    <p>next</p> Signup and view all the answers

    In the given stack implementation, which operator will not function correctly if no assignment operator is defined?

    <p>The assignment operator (D)</p> Signup and view all the answers

    Which of the following describes the purpose of the '+ operator=' in a class?

    <p>Allows assigning values to an already initialized object (A)</p> Signup and view all the answers

    A default constructor is called only when a new object is created.

    <p>True (A)</p> Signup and view all the answers

    What happens to dynamically allocated memory if delete is not called?

    <p>Memory leak occurs.</p> Signup and view all the answers

    The ________ function is called automatically when the lifetime of an object ends.

    <p>destructor</p> Signup and view all the answers

    Which statement about the destructor is true?

    <p>It is automatically called when the object's scope ends. (B)</p> Signup and view all the answers

    Why should pointers to deleted objects be set to nullptr?

    <p>To prevent dangling pointers.</p> Signup and view all the answers

    The copy constructor and the assignment operator serve the same purpose in class management.

    <p>False (B)</p> Signup and view all the answers

    When implementing a class that uses pointers, it is important to follow the Rule of ______.

    <p>Three</p> Signup and view all the answers

    What happens when a class does not declare a destructor?

    <p>An automatically generated destructor will be created. (D)</p> Signup and view all the answers

    What is the primary purpose of the operator= function in the stack class?

    <p>To assign one stack object to another (D)</p> Signup and view all the answers

    The Rule of Three states that if a class defines a destructor, it is optional to define a copy constructor and an assignment operator.

    <p>False (B)</p> Signup and view all the answers

    What occurs when the number of std::shared_ptr references to an object reaches zero?

    <p>The object is automatically deleted.</p> Signup and view all the answers

    In the assignment operator, the method used to swap pointers is called ________.

    <p>std::swap</p> Signup and view all the answers

    Match the following components with their description:

    <p>Destructors = Responsible for cleanup of resources Copy Constructors = Create a copy of an object Assignment Operators = Assign values from one object to another Smart Pointers = Manage dynamic memory automatically</p> Signup and view all the answers

    What happens to an object when its shared_ptr count decreases to zero?

    <p>It is deleted automatically. (D)</p> Signup and view all the answers

    The statement if (this != &s) checks for self-assignment in the assignment operator.

    <p>True (A)</p> Signup and view all the answers

    What mechanism does C++ provide for memory management when using std::shared_ptr?

    <p>Reference counting.</p> Signup and view all the answers

    The ________ mechanism ensures that all necessary resource management functions are implemented for a class in C++.

    <p>Rule of Three</p> Signup and view all the answers

    In the assignment operator for the stack, what does the line stack copy = s; effectively do?

    <p>It initializes a new stack with existing elements. (C)</p> Signup and view all the answers

    Flashcards

    Stack Data Structure

    A dynamic data structure where elements can only be added or removed from the top, like a stack of books.

    Lnode (Linked Node)

    A struct that holds a value (data) and a pointer to the next node in the linked list. It is the building block of a stack.

    Topn

    A pointer that points to the top-most node of the stack, allowing access to the entire structure.

    push(value)

    Adds a new element to the top of the stack.

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    pop()

    Removes and returns the top element from the stack.

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    top()

    Returns the value of the top element on the stack without removing it.

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    empty()

    Checks whether the stack is empty or not.

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    Stack

    A data structure that allows elements to be added and removed only from the top, following the LIFO (Last-In, First-Out) principle.

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    Default constructor

    A constructor that initializes a stack by setting the topn pointer to nullptr, making the stack empty. This ensures a clean slate.

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    Dynamic allocation

    A memory allocation technique that dynamically allocates memory for a new node when an element is pushed onto the stack. This allows the stack to grow as needed.

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    Undefined behavior

    An error that occurs when a program attempts to access or modify memory that it is not allowed to. This can happen if a program tries to dereference a null pointer.

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    stack::clean_up_nodes()

    A member function of a class that cleans up the nodes of the stack by deleting each node starting from the top until the top becomes nullptr. It iterates through the linked list, freeing memory for each node.

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    Stack Destructor (~stack())

    The function that is called automatically when an object of the stack class goes out of scope. It is responsible for releasing the memory allocated to the stack's nodes to avoid memory leaks.

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    Stack Assignment Operator (operator=)

    An operator that allows assigning one stack object to another. It ensures efficient copying by first cleaning up the left operand's existing nodes, then copying the nodes from the right operand (source stack) to the left operand (destination stack).

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    Explicit Destructor Call

    Refers to the practice of avoiding direct calls to the destructor within the assignment operator's implementation to prevent potential issues with object state.

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    Copy-and-Swap Idiom

    A technique for implementing the assignment operator that leverages the copy constructor and the swap function. It creates a local copy of the right operand, swaps the nodes between the original and the copy, and then lets the local copy's destructor handle the cleanup.

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    Copy constructor

    The process of creating a new instance of a class using an existing instance as a template. It involves copying the data from the existing instance to the new instance, but not referencing the same memory locations. This ensures that changes to the existing instance don't affect the newly created one.

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    Destructor

    A special function in a class that is automatically called when an object of that class is being destroyed. This ensures that resources allocated to the object are properly deallocated, like releasing memory and closing files.

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    Assignment operator

    An operation that allows you to create a new instance of a class using the data from an existing instance. It involves copying relevant data from the existing object while ensuring that any modifications to the copied object don't affect the original object.

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    Rule of Three

    A design principle that suggests that if you need to manually implement either a destructor, copy constructor, or assignment operator, then all three should be implemented to ensure proper object management, even if they seem like obvious operations.

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    T(const T& t)

    A special type of constructor that takes a constant reference to an object of the same class as its parameter. It's used for copying data from one object to another.

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    Automatically-generated copy constructor

    When copy constructors are not explicitly defined for a class, the compiler generates a default copy constructor. This constructor performs a shallow copy, where the data members of the new object are simply copied from the original object. This can lead to problems if the original object contains pointers.

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    Deep Copy Constructor

    A copy constructor that copies the data members of the original object, but instead of just copying the pointers, it allocates new memory for the data pointed to by the pointers and copies the data into that new memory. This ensures that both objects have independent copies of the data.

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    Automatically Generated Assignment Operator

    The assignment operator that is automatically generated by the compiler. This operator, just like the automatically generated copy constructor, performs a shallow copy. As a result, issues can arise if any data members are pointers.

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    User-defined Assignment Operator

    When we overload the assignment operator, we provide our own implementation to handle the assignment operation. This allows for custom behavior, such as deep copies of data, which is essential for avoiding unintended side effects.

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    Memory Management

    This refers to the process of allocating and deallocating memory for objects. It usually involves managing the lifetime of dynamically allocated memory to prevent leaks or corruption.

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    Dynamic Memory Allocation

    This describes the process of allocating memory for objects during program execution, which is used for creating objects within the program's runtime environment.

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    Garbage Collection

    A strategy for managing memory that involves tracking and releasing unused memory to prevent leaks. This approach allows for efficient and safe memory usage by freeing up resources for future allocation.

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    String

    A data type that holds a sequence of characters. It can be visualized as a container that stores individual characters, allowing you to access and manipulate them.

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    Dangling Pointer

    A type of error that occurs when a program tries to access data through a pointer that is no longer valid. Imagine trying to open a door that has disappeared!

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    delete Operator

    The process of releasing memory that was dynamically allocated previously. It eliminates the need for unnecessary space, preventing memory leaks.

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    Heap Overflow

    A memory allocation error that occurs when a program requests more memory than is available in the heap. It's like trying to store too many packages in a small box.

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    Matching 'new' and 'delete'

    A fundamental programming principle that suggests that every dynamic memory allocation (using 'new') should have a matching deallocation (using 'delete'). It's like keeping your space tidy by returning what you borrow.

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    Topn Pointer

    A pointer that points to the top node of the stack which allows you to access the entire stack structure. Similar to the head pointer in a linked list.

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    new Operator

    A function in C++ that creates a new object of a given data type (in this case, a linked node) in memory. You need this to dynamically make new nodes in the stack.

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    top() Function

    This function lets you peek at the very top element of the stack without removing it. This function is useful to check what element is currently at the top. It is defined with the const keyword to ensure that the top value is not changed.

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    pop() Function

    A function that removes the top element from a non-empty stack. It returns the value that was at the top of the stack. In the stack implementation, it updates 'topn', the pointer to the top of the stack.

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    push() Function

    A function that adds a new element to the top of the stack. This function creates a new node (using the 'new' operator) and inserts it at the top of the stack, updating the 'topn' pointer appropriately.

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    empty() Function

    This function checks if the stack is empty (no elements). It returns true if the stack is empty or false if the stack has at least one element. It checks if the topn pointer has been initialized (if it is null)

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    stack::clean_up_nodes() Function

    If the stack is not cleared before a new stack is assigned, it results in dangling pointers and memory leaks. This function cleans up by delete-ing all of the lnode objects using a loop, setting topn to NULL to signify the end of the list.

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    Stack Cleanup (clean_up_nodes)

    A process that ensures all nodes in a stack are deleted, releasing the memory they were holding.

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    What are new and delete operators in C++?

    In C++, new allocates memory for an object and calls the constructor to initialize it, while delete calls the destructor and frees the allocated memory. Think of new as reserving a table at a restaurant and delete as leaving the table and releasing it for others.

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    What is a destructor in C++?

    A destructor is a special member function that's automatically called when an object goes out of scope (e.g., when it's deleted or leaves the scope). It's like cleaning up your room before leaving, ensuring everything is in order.

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    In C++, what does operator= represent?

    The operator= is used to copy data from one object to another after initialization. Consider it as copying recipes from one cookbook to another - the content is copied, but the source remains unchanged.

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    What is a copy constructor in C++?

    A copy constructor is invoked when a new object is initialized using an existing object. Think of it as making a duplicate of a recipe - both the original and the copy exist, allowing you to modify one without affecting the other.

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    What is the fundamental difference between a copy constructor and the assignment operator?

    The copy constructor is called during object initialization, while the assignment operator is used after initialisation. Think of the difference between taking a photo immediately after buying a camera (copy constructor) and taking a photo later after the camera is set up (assignment operator).

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    What is dynamic memory allocation?

    Dynamic memory allocation involves allocating memory during runtime. Think of it as reserving a parking spot for your car at a busy parking lot.

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    What is a memory leak in dynamic memory allocation?

    A memory leak occurs when dynamically allocated memory isn't properly freed. Imagine leaving a parking spot occupied even after your car is gone - it's wasted space.

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    How does the delete operator help in memory management?

    The delete operator releases the memory allocated with new, preventing memory leaks. Think of it as returning a rented car to the rental company.

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    Why is it important to match every new with a corresponding delete?

    It's crucial to ensure that every new operation has a corresponding delete to avoid leaks. This ensures resources are freed up and prevents memory exhaustion. Think of it as balancing your bank account by spending and earning in equal amounts.

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    What is the Rule of Three in C++?

    The Rule of Three indicates that if you need to explicitly define a destructor, copy constructor, or assignment operator for a class, you should implement all three to maintain proper object management. Think of it as a set of instructions for maintaining a car: If you need to change the tires, you should also check the engine and car fluids.

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    Study Notes

    Introduction to Computer Science

    • Course offered by the Department of Computer Science, ETH Zurich
    • Course code: 252-0032, 252-0047, 252-0058
    • Fall 2024
    • Authors: Manuela Fischer and Felix Friedrich

    Memory Management

    • Dynamic memory allocation is discussed.
    • Dynamic data structures (like stacks) require explicit memory management.
    • Correct memory management involves a delete for every new operation.
    • A stack is a fundamental data structure that allows insertion and removal of elements only from the top (LIFO – Last-In, First-Out).
    • Stacks are implemented as linked lists.

    Basic Stack Functionality

    • A stack's functionality includes:
      • push: Adds an element to the top.
      • pop: Removes the top element.
      • top(): Returns the top element (without removing it).
      • empty(): Checks if the stack is empty.

    Stack Implementation Details (Data Structure)

    • lnode struct (used to represent nodes in a stack):
      • Contains value (data held by the node)
      • Contains next (pointer to the next lnode in the sequence).
    • topn is a pointer to the top node in the stack; it is a member variable of the stack class

    Removing Elements from Stack

    • pop() function removes the top element:
      • A temporary pointer old_topn stores the top pointer
      • topn is re-directed to the next element
      • old_topn is deleted

    Dynamic Memory Management

    • For every new, there must be a matching delete
    • Ignoring this rule leads to memory leaks.

    Avoiding Memory Leaks in Stacks

    • A push() function allocates memory when an element is pushed onto the stack.
    • The pop() function should deallocate memory when an element is removed from the stack.
    • A temporary variable old_topn is used to store the top node before it's removed; allowing correct deletion
    • The destructor for the stack class should release allocated memory during destruction. This involves iterating through all nodes and deallocating each node.

    Destructors

    • The destructor of the stack class is essential for releasing dynamically allocated memory when the stack goes out of scope.
    • It will loop through all the remaining nodes and recursively call delete on every remaining node.

    Copy Assignment Operator (Overloading)

    • Using stack s2 = s1 creates a copy of the stack s1 in stack s2.
    • Overloading the = operator is crucial to make sure copies are independent, and memory leaks don't occur.
    • Deep-copy mechanisms are needed to create true independent copies, using the copy_nodes() function.
    • The clean_up_nodes() function is crucial to free nodes that are now orphaned by the assignment operator.

    Rule of Three

    • The rule of three states that if a class requires a custom constructor, a destructor, or a copy assignment operator, it should also implement the other two.
    • Often, providing a custom copy constructor or assignment operator requires modifying the rest of the special member functions (the Rule of Three).

    Shared Pointers

    • Shared pointers automatically manage memory.
    • They maintain a reference count to track how many pointers refer to an object.
    • When the reference count reaches zero, the object is automatically deleted/freed.
    • Use std::make_shared<T> to create shared pointers to custom data types (e.g., rational).
    • Shared pointers alleviate memory management by automatically deleting an object when the last shared pointer goes out of scope.
    • Example use (std::shared_ptr) given: std::shared_ptr<rational> sp1 = std::make_shared<rational>(3, 4);
    • Proper deletion is crucial with shared pointers (e.g., sp1 = nullptr;) to prevent lingering references and possible memory leaks when several shared pointers exist for the same object.

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    Description

    This quiz covers memory management concepts in computer science, focusing on dynamic memory allocation and the fundamentals of stack data structures. Participants will explore stack operations such as push and pop, along with the implementation details pertaining to memory management. Assess your understanding of these crucial programming concepts.

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