Introduction to classes


A class is a template which defines the entities we want to model by combining data representation and methods for manipulating that data into one package.

We can create many different instances of a class which have property values which are different from each other even though they come from the same template.

A class can be instantiated both on the stack and the heap, but they most commonly instantiated on the heap.


Classes on stack vs heap


class Colour {
    private:
        string name;
        int r, g, b;
    public:
       Colour() { 
          name = ""; 
          r = 0;
          g = 0;
          b = 0; 
       }
}
                    


    //declare and create instance
0x0061ff00
0x0061ff04 "" Colour favColour; string name;
0x0061ff08
0x0061ff0c 0 int r;
0x0061ff10 0 int g;
0x0061ff14 0 int b;
0x0061ff18 0x0061ff2d rtn addr
0x0061ff1c 0 rtn val
Stack 

class Colour {
    private:
       string name;
       int r, g, b;
    public:
       Colour() { 
          name = ""; 
          r = 0;
          g = 0;
          b = 0; 
       }
}
                    


    //declare pointer and create instance
0x0061ff0c
0x0061ff10 0x001d54f4 Colour *favCol;
0x0061ff14
0x0061ff18 0x0061ff2d rtn addr
0x0061ff1c 0 rtn val
Stack
Heap
0x001d54f0 memory leak!
0x001d54f4 "" string name;
0x001d54f8
0x001d54fc 0 int r;
0x001d5500 0 int g;
0x001d5504 0 int b;
0x001d5508

Classes on stack vs heap - delete


class Person {
    private:
        string name;
        int age;
    public:
        Person() {
            name = "";
            age = 0;
        }
}
0x0061ff00
0x0061ff04 0x001d54f4 Person *jack;
0x0061ff08
0x0061ff0c "" Person jill; string name;
0x0061ff10
0x0061ff14 0 int age;
0x0061ff18 0x0061ff2d rtn addr
0x0061ff1c 0 rtn val
Stack
Heap
0x001d54f0
0x001d54f4 "" string name;
0x001d54f8
0x001d54fc 0 int age;
0x001d5500
0x001d5504
0x001d5508 

class Person {
    private:
        string name;
        int age;
    public:
        Person() {
            name = "";
            age = 0;
        }
}
0x0061ff0c
0x0061ff10 0x001d54f0 Person *jack;
0x0061ff14
0x0061ff18 0x0061ff2d rtn addr
0x0061ff1c 0 rtn val
Stack
Heap
0x001d54f0 "" String name;
0x001d54f4
0x001d54f8 0 int age;
0x001d54fc
0x001d5500 "" String name;
0x001d5504
0x001d5508 0 int age;
0x001d550c
0x001d5510

Default vs Explicit constructors


class Person {
    private:
        string name;
        int age;
    public:
        Person() {
            name = "";
            age = 0;
        }
        Person(const string &name, int age) {
            this->name = name;
            this->age = age;
        }
}
0x0061feec
0x0061fef0 0x001d5508 Person *bart;
0x0061fef4
0x0061fef8 "Lisa" Person lisa; string name;
0x0061fefc
0x0061ff00 8 int age;
0x0061ff04 0x001d54f4 Person *marge;
0x0061ff08
0x0061ff0c "" Person homer; string name;
0x0061ff10
0x0061ff14 0 int age;
0x0061ff18 0x0061ff2d rtn addr
0x0061ff1c 0 rtn val
Stack
Heap
0x001d54f0
0x001d54f4 "" string name;
0x001d54f8
0x001d54fc 0 int age;
0x001d5500
0x001d5504
0x001d5508 "Bart" string name;
0x001d550c
0x001d5510 10 int age;
0x001d5514