L09 Destructors and Dynamic Memory Allocation of Arrays of Objects

#ece244

when is the destructor called?

building on to where we left off in L08:

• if an object is created normally (i.e., not by dynamically allocating memory), then the constructor is called upon creation, and the destructor is called when the object goes out of scope (i.e., the end of the function that the object resides in)
  • e.g. Student s; where Student is the class
• if an object is created via dynamically allocated memory, then the constructor is called when the object is created, but the destructor is never called unless the object is freed from memory (i.e., through delete)
  • e.g. Student* s = new Student where Student is the class

#include <iostream>
using namespace std;

class Student {
	private:
		string id;
	public:
		Student();
		~Student();
};

Student::Student() {
	// constructor
	cout << "constructor called" << endl;
}
Student::~Student() {
	// destructor
	cout << "destructor called" << endl;
}

int main(void) {
	Student x;
	Student *y = new Student;
	delete y; // if i comment this out the destructor will only be called for x
}

linked list of objects

this is a linked list of objects, each object links to another object

class ComplexNum {
	public:
		int real, img;
		ComplexNum* next;
		ComplexNum() { real = 0; img = 0; next = NULL;}
		~ComplexNum() {
			delete next; // destroy the 
		}
};

int main() {
	ComplexNum* p = new ComplexNum;
	(*p).real = 10;
	p->real = 10; // this and the above line are the same
	
	p->next = new ComplexNum; // creates a new object, next points to it
	p->next->real = 9;
	
	delete p;
}

We use a trick to avoid memory leaks here

if we deleted the top pointer without anything in the destructor, then it would leave the other pointer created inside next, without any reference to it.

So, we specify in the destructor delete next. This does the following:

• delete p
  • calls destructor on p
    • calls delete next;
      • calls destructor of next
        • calls delete next, where next is the next object, which is null.
        • thus, nothing happens
      • destroys next
  • destroys p