Data Structures: Stack and Memory Management

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Questions and Answers

What is the purpose of the 'push' function in the stack class?

To add an element onto the stack (correct)

To check if the stack is empty

To retrieve the value of the top element from the stack

To remove the top element from the stack

The 'pop' function can be called on an empty stack without causing an error.

False

What does the 'top' function return in the stack class?

the value of the top most element

In a linked list, each node is represented by a struct called ______.

lnode Signup and view all the answers

Match the functions with their descriptions:

push = Add an element to the stack pop = Remove the top element from the stack top = Return the value of the top element empty = Check if the stack has no elements Signup and view all the answers

What happens when you dereference a dangling pointer?

It results in an error. Signup and view all the answers

Releasing an object more than once using delete is a safe practice.

False Signup and view all the answers

What is a dangling pointer?

A pointer that points to a memory location that has been released. Signup and view all the answers

The function `pop()` in a stack implementation removes the ______ item from the top of the stack.

top Signup and view all the answers

Match the following operations with their effects:

new = Allocates memory for a new object delete = Releases memory occupied by an object push = Adds an item to the top of the stack pop = Removes the top item from the stack Signup and view all the answers

What is the purpose of the assert statement in the `pop()` function?

To ensure the stack is not empty before popping. Signup and view all the answers

What will happen to the variable 'topn' when the `pop()` function is called?

It will point to the next item in the stack. Signup and view all the answers

The statement `int nominator = (*t).denominator();` is valid after `delete s;`.

False Signup and view all the answers

What happens to objects created with new when delete is called?

They are deconstructed and memory is released. Signup and view all the answers

Each new statement must have a matching delete statement to prevent memory leakage.

True Signup and view all the answers

What is the consequence of not pairing every new with a delete statement?

Memory leaks occur. Signup and view all the answers

The delete operator does not set the pointer to ________ after deletion.

nullptr Signup and view all the answers

Match the following outcomes to their corresponding actions:

new = Allocates memory delete = Releases memory memory leak = Unused objects in memory heap overflow = Exhaustion of memory Signup and view all the answers

What might happen if a pointer pointing to a deleted object is accessed?

It may cause undefined behavior. Signup and view all the answers

After a delete operation, all pointers to the deleted object become invalid.

False Signup and view all the answers

What is the guideline for dynamic memory management regarding new and delete?

For each new, there must be a matching delete. Signup and view all the answers

What does the `empty()` function in the stack class indicate?

The stack is empty. Signup and view all the answers

The `top()` function can be called even if the stack is empty.

False Signup and view all the answers

What keyword is used to ensure that the stack is not empty in the `top()` method?

assert Signup and view all the answers

In the default constructor for the stack class, `topn` is initialized to ____.

nullptr Signup and view all the answers

Match the stack methods with their descriptions:

push = Adds an element to the top of the stack pop = Removes the top element from the stack top = Returns the top element without removing it empty = Checks if the stack has no elements Signup and view all the answers

What is the return type of the `top()` method?

int Signup and view all the answers

The `topn` variable in the stack class points to the last element added to the stack.

False Signup and view all the answers

What happens when the 'push' method is called with an integer value?

A new node is allocated in memory. Signup and view all the answers

The 'delete' operator releases memory for objects created with 'new'.

True Signup and view all the answers

What does the 'new' operator do when creating an object of type T?

Allocates memory and initializes the object using the matching constructor. Signup and view all the answers

What is the effect of calling 'delete' on an object created with 'new'?

The object is deconstructed and memory is released. Signup and view all the answers

The top of the stack will contain the value __ after calling 'push(4)'.

4 Signup and view all the answers

Match the following expressions with their effects:

push(4) = Allocates new node and sets it as top. delete expr = Deconstructs the object and releases memory. new lnode(value, topn) = Creates a new node with the given value. topn = References the current top of the stack. Signup and view all the answers

What type of storage duration do objects created with 'new' have?

Dynamic storage duration Signup and view all the answers

The 'topn' pointer remains the same after a 'push' operation.

False Signup and view all the answers

What does the 'matching constructor' do when an object is created?

Initializes the new object with the provided value. Signup and view all the answers

Study Notes

Memory Management

Dynamic data structures, like our_list, were allocated memory last week, but not released. This resulted in the absence of functions to remove elements.
Today's focus is correcting memory management using new and delete, aided by the dynamic data structure stack.

Goal: Class Stack with Memory Management

The goal is to create a stack class that manages memory.
- push(int value): Pushes an element onto the stack.
- pop(): Removes the top element from the stack (presuming it's not empty).
- top() const: Returns the value of the top element (presuming the stack is not empty).
- empty() const: Returns true if the stack is empty, false otherwise.

Recall the Linked List

Review the lnode structure, which represents an element in a linked list with an integer value and a pointer to the next node.
lnode(int v, lnode* n) is a constructor definition.

Stack = Pointer to the Top Element

The stack now utilizes a pointer (topn) to the top element of the linked list.

Empty Stack

The default constructor for the stack class, stack() : topn(nullptr) { }, initializes the top pointer to nullptr.
The empty() function returns true when topn is nullptr, and false otherwise.
The top() function asserts (!empty()) before returning the current top node's value via topn->value.

The new Expression

The new operator allocates memory for a new object of type T.
The effect is allocating new memory and initializes the object using the matching constructor.
The value returned is the address of the new object of type T (type: pointer T*).

The delete Expression

Objects allocated with new have dynamic storage duration and exist until explicitly deleted using delete.
The delete operator deallocates the memory associated with a pointer of type T* that was previously created with new. It deconstructs (cleans up) the object and does not set the pointer to nullptr. A common programming error involves reusing a pointer that has already been deleted.

Who is Born Must Die...

An important guideline for dynamic memory management: every new should have a corresponding delete.
Non-compliance can lead to memory leaks by leaving objects unused but in memory.

Careful with new and delete!

Be mindful of using new and delete to prevent memory leaks. Especially, avoid referencing a previously deleted object, otherwise it can lead to dangling pointers.

Stack Continued: pop()

The pop() function removes and deletes the top element by updating the topn pointer and handling the possible nullptr case.

Zombie Elements

Local stack variables (s1) can potentially exist even after leaving or exiting scope.
Memory that those stack variables referenced may not be deallocated immediately after exiting the scope of use, causing a memory leak.

The Destructor

The destructor (~T()) is a unique class member function that runs automatically when the memory duration of a an object ends: explicitly when delete is called on an object of type T* or when the enclosing scope finishes.
If no destructor is defined, one is automatically generated, handling the deallocation of members, like pointers in our stack.

Using a Destructor, It Works

The destructor is called when a stack is about to be deallocated, deleting each object. This avoids memory leaks that could arise with a stack variable from going out of scope.

Stack Done?

Demonstrates basic stack operation, but if a copy of a stack is made, both stacks point to the same memory.
This means modifying one might cause modifications in the other as well.

What Has Gone Wrong?

Copy initialization makes both stacks point to the same memory block. Subsequent actions like pop() on one stack will unintentionally affect the other.

The Actual Problem

Copying a stack with dynamic memory allocation copies only the pointer to the top node, not the node's content. This means both stacks point to the same memory location. This leads to unpredictable result as either variable can modify node data directly.

Possible Solutions

Deep copy: Create a complete duplicate of the stack data.
No copies: Prohibit the copying of the stack, thereby eliminating the shared memory issue.
Shared data structure: Employ a solution that manages the shared data structure properly, thereby preventing shared access to memory.

The Copy Constructor

A class's copy constructor is invoked when creating a new object by initializing it with an existing object of the same type.
It automatically duplicates the data of an object to avoid accidental modification of the original.
If no copy constructor is defined, the compiler will generate one, implementing member-wise initialization (copying of the individual elements).

It Works with a Copy Constructor

This provides a solution for creating true copies of the stacks, resulting in a separate copy and preventing overlapping memory usage.
The copy constructor is a method that correctly handles copying the dynamic memory from one object to another.

Initialization ≠ Assignment!

Initialization (e.g., stack s2 = s1;) and assignment (e.g., s2 = s1;) of a stack object behave differently. The first is the copy constructor and the second necessitates an assignment operator.

The Assignment Operator

The assignment operator operator= provides an efficient mechanism to assign one stack (RHS) to another (LHS).
It's a member function handling the proper assignment of a stack to another.
The operator copies data and handles self-assignment, ensuring memory is correctly allocated and freed.

Smart Pointers

Smart pointers are pointers that, with additional routines, also take care of memory management.
std::shared_ptr: It counts references to an object. It deletes the object only when the number of references drops to zero. So, if several pointers refer to the same object, they manage the memory automatically to avoid accidental memory leaks.

Shared Pointers: Example

Demonstrates use of std::make_shared and handling shared pointers similarly to regular pointers in code.
A shared_ptr is used to create objects that are shared amongst several variables. This ensures that the memory is released properly when the last variable referencing that memory is deallocated.

Application: Workshop

Workers share tools in a workshop, and the tools should be automatically returned when not in use.

Workshop: 1st Try

tool struct: Holds a tool's name.
worker struct: Manages tools ("hammer", etc.), with a use method for acquiring them and a release method for returning them (sharing is not correct).

Workshop: Attempt with Pointer

Using pointers (tool_ptr) improves sharing but introduces manual deallocation issues.
Incorrect sharing leads to multiple copies.

Workshop: Solution with Shared Pointers

Implement tool sharing via std::shared_ptr from the C++ standard library.
This approach ensures correct memory management by removing a particular pointer from the reference count when no longer in use. This automatically frees memory when no one holds the pointer.

Memory management independent of C++

Garbage collection (GC) in other languages automatically reclaims unused memory.
C++ provides explicit dynamic memory management.
C++'s way of managing memory provides more control, but requires developers to be mindful in handling pointers to avoid memory leaks and potential crashes.

Studying That Suits You

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Description

Test your knowledge on stack operations, memory management, and pointer concepts in data structures. This quiz covers essential functions such as 'push', 'pop', and the importance of memory release in programming. Perfect for students studying data structures and algorithms.

Data Structures: Stack and Memory Management

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Podcast

Questions and Answers

What is the purpose of the 'push' function in the stack class?

The 'pop' function can be called on an empty stack without causing an error.

What does the 'top' function return in the stack class?

In a linked list, each node is represented by a struct called ______.

Match the functions with their descriptions:

What happens when you dereference a dangling pointer?

Releasing an object more than once using delete is a safe practice.

What is a dangling pointer?

The function pop() in a stack implementation removes the ______ item from the top of the stack.

Match the following operations with their effects:

What is the purpose of the assert statement in the pop() function?

What will happen to the variable 'topn' when the pop() function is called?

The statement int nominator = (*t).denominator(); is valid after delete s;.

What happens to objects created with new when delete is called?

Each new statement must have a matching delete statement to prevent memory leakage.

What is the consequence of not pairing every new with a delete statement?

The delete operator does not set the pointer to ________ after deletion.

Match the following outcomes to their corresponding actions:

What might happen if a pointer pointing to a deleted object is accessed?

After a delete operation, all pointers to the deleted object become invalid.

What is the guideline for dynamic memory management regarding new and delete?

What does the empty() function in the stack class indicate?

The top() function can be called even if the stack is empty.

What keyword is used to ensure that the stack is not empty in the top() method?

In the default constructor for the stack class, topn is initialized to ____.

Match the stack methods with their descriptions:

What is the return type of the top() method?

The topn variable in the stack class points to the last element added to the stack.

What happens when the 'push' method is called with an integer value?

The 'delete' operator releases memory for objects created with 'new'.

What does the 'new' operator do when creating an object of type T?

What is the effect of calling 'delete' on an object created with 'new'?

The top of the stack will contain the value __ after calling 'push(4)'.

Match the following expressions with their effects:

What type of storage duration do objects created with 'new' have?

The 'topn' pointer remains the same after a 'push' operation.

What does the 'matching constructor' do when an object is created?

Study Notes

Memory Management

Goal: Class Stack with Memory Management

Recall the Linked List

Stack = Pointer to the Top Element

Empty Stack

The new Expression

The delete Expression

Who is Born Must Die...

Careful with new and delete!

Stack Continued: pop()

Zombie Elements

The Destructor

Using a Destructor, It Works

Stack Done?

What Has Gone Wrong?

The Actual Problem

Possible Solutions

The Copy Constructor

It Works with a Copy Constructor

Initialization ≠ Assignment!

The Assignment Operator

Smart Pointers

Shared Pointers: Example

Application: Workshop

Workshop: 1st Try

Workshop: Attempt with Pointer

Workshop: Solution with Shared Pointers

Memory management independent of C++

Studying That Suits You

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The function `pop()` in a stack implementation removes the ______ item from the top of the stack.

What is the purpose of the assert statement in the `pop()` function?

What will happen to the variable 'topn' when the `pop()` function is called?

The statement `int nominator = (*t).denominator();` is valid after `delete s;`.

What does the `empty()` function in the stack class indicate?

The `top()` function can be called even if the stack is empty.

What keyword is used to ensure that the stack is not empty in the `top()` method?

In the default constructor for the stack class, `topn` is initialized to ____.

What is the return type of the `top()` method?

The `topn` variable in the stack class points to the last element added to the stack.