OOPC3.DynamicMemoryAllocation&Classes

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Last Edit: 9/28/25

3.1 Freeing an Object from Memory Explicitly

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• Recall what happened during a program is running

3.1.1 Recap: Dynamic Memory Allocation

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• When the program start, computer allocates memory in Random Access Memory (RAM)
• 大概有四种 Segment 出现
  • Stack: Stores the Local Variables and Objects of a function
  • Heap: Stores the dynamically allocated memory
  • Global + Const Variables: Obviously Stores the Variables
  • Code: Store instruction of the program

Step 1： Stack Only

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• 首先先有代码 main.cpp

#include <iostream>
using namespace std;

int main(){
    int x = 10;      // Allocate x in stack with value 10
    int* p = NULL;   // Add a pointer in stack with NULL
    p = &x;          // Let P point to x
    *p = 5;          // Dereference P and change x's value
}

• So far everything is in stack

Step 2: Dynamic Memory Allocation

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Change main.cpp into

#include <iostream>
using namespace std;

int main(){
    int x = 10;       // Allocate x in stack with value 10
    int* p = NULL;    // Add a pointer in stack with NULL
    p = &x;           // Let P point to x
    *p = 5;           // Dereference P and change x's value
    p = new int;      // Dynamically allocate a value of int in heap
    *p = 20;          // Change the value of p to 20

    cout << "Value at p: " << *p << endl;  // print 20
    cout << "Value of x: " << x << endl;   // print 7
}

• p = new int; → allocate a new int space on heap, which is a dynamically allocated memory, then return its value to p
• So now on, the p is pointing to the space in heap

Step 3. Freeing Memory

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at the end, need to manually deleted the memory in heap

#include <iostream>
using namespace std;

int main(){
    int x = 10;       // Allocate x in stack with value 10
    int* p = NULL;    // Add a pointer in stack with NULL
    p = &x;           // Let P point to x
    *p = 5;           // Dereference P and change x's value
    p = new int;      // Dynamically allocate a value of int in heap
    *p = 20;          // Change the value of p to 20

    cout << "Value at p: " << *p << endl;  // print 20
    cout << "Value of x: " << x << endl;   // print 7
    
    delete p;
}

• delete p is freeing the memory p is pointing to on HEAP
• However the pointer p itself remains

Step4. Resolve Dangling Pointer

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• Dangling Pointer 悬空指针 happened when pointer is pointing to a memory which has been already freed

#include <iostream>
using namespace std;

int main(){
    int x = 10;       // Allocate x in stack with value 10
    int* p = NULL;    // Add a pointer in stack with NULL
    p = &x;           // Let P point to x
    *p = 5;           // Dereference P and change x's value
    p = new int;      // Dynamically allocate a value of int in heap
    *p = 20;          // Change the value of p to 20

    cout << "Value at p: " << *p << endl;  // print 20
    cout << "Value of x: " << x << endl;   // print 7
    
    delete p;      
    p = NULL;         
    return 0;
}

• Basically, Set p to NULL to resolve this problem

3.1.1.1 Dynamic Memory Allocation of Arrays

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• The way to allocate one int in heap has been shown above, with the code int* p = new int;
• Whereas to delete it, use delete p;
• Instead, when allocating Arrays, use int* p = new int[size]; , so that now pointer p can be pointing to the first element in the Array
• Likewise, use delete[] p; to free the memory pointer pointing to

3.1.2 Explicitly Calling Destructor

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To API new and delete basically Explicitly called Destructor and Constructor

#include <iostream>
using namespace std;

class Student {
public:
    Student() {
        cout << "Constructor called" << endl; 
    }

    ~Student() {
        cout << "Destructor called" << endl;   
    }
};

int main(void) {
    Student* p = new Student;  
    delete p;                
    return 0;
}

3.2 Why do we need Destructors?

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As known, a constructor is used when an object is being initialized, whereas destructor is used to clean everything up

If there’s no Destructor in the life cycle of the code when some memory has been dynamically called, the MEMORY LEAK will occur

class Student {
private:
    int* grades;
public:
    Student(int size) {
        grades = new int[size]; // Dynamically Aloocated the Array
    }
};

int main(void) {
    Student x(3); // Constructed Array with three ints
    return 0;     // Program ENDED WITHOUT DESTRUCTOR
}

• As seen, some memory has been dynamically allocated on HEAP, however there’s no Destructor, meaning the memory on HEAP will remain and has not been freed, this is being called MEMORY LEAK

3.3 Objects Owning Other Dynamically Allocated Objects

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3.3.1 Recall: When are Destructors Called ?

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• Recall that Destructors are called when:
  • Goes out of Scope
  • Using Delete

3.3.2. Recall: Why Do We Need Destructors?

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• Destructors are needed to be define if object has dynamically allocated memory that needs to be freed

3.3.3. Why Do We Need Destructors for Objects Owning Other Dynamically Allocated Objects?

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class Complex {
public:
    int real;
    int imag;
    Complex* next;   // Pointer pointing to another Complex Object

    Complex(int r, int i) {
        real = r;
        imag = i;
        next = nullptr;
    }
};

int main() {
    Complex* p = new Complex(4, 5);
    p->next = new Complex(7, 8);
    delete p; // This will only free the first Complex
    return 0;
}

• Now the problem is that there exist two Complex in the heap, that if only delete the first one, the second one will cause a memory leak

Solution

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The solution is to somehow use RECURSIVE

class Complex {
public:
    int real;
    int imag;
    Complex* next;

    Complex(int r, int i) {
        real = r;
        imag = i;
        next = nullptr;
    }

    ~Complex() {
        cout << "Destructor called for Complex("
             << real << ", " << imag << ")" << endl;
        if (next != nullptr) {
            delete next;   // RECURSIVE Condition
        }
    }
};

int main() {
    Complex* p = new Complex(4, 5);
    p->next = new Complex(7, 8);
    delete p;
    return 0;
}

3.4 Arrays of Pointers

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Double Pointers can be used in creating arrays of pointers

3.4.1 Double Pointers

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using namespace std;

int main() {
    int** p2p;    // Double Pointer, Pointing to a integer
    int * p;
		int * q; 
		
    p = new int;  // Dynamic Allocation
    *p = 5;       

    p2p = &p;     // p2p point to pointer p
    q = *p2p;     // Give the Address of Pointer p to q
                  // Now q is also pointing the int

    *q = 8;       // Change Vaule of q's pointing value to 

    cout << "*p = " << *p << endl;
    cout << "*q = " << *q << endl;
    cout << "**p2p = " << **p2p << endl;

    delete p;     
    return 0;
}

• After this, all codes will become 8

*p = 8
*q = 8
**p2p = 8

3.4.2 Array of Pointers

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• A Array which every members are Pointers

Array of Integers

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int* arr = new int[size]

• This will call for a Dynamic Memory on HEAP that stores size of integers

Array of Pointers

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int** arr = new int*[size];

• This will call for a Dynamic Memory on HEAP that stores size of Pointers
• That every single Element arr[i] is one Integer Pointer