Understanding this code involving pointers, arrays, and the new operator

Question

So this is the original code:

class IntegerArray { 
public:   
   int *data;   
   int size; 
};  
int main() {   
   IntegerArray arr;   
   arr.size = 2;   
   arr.data = new int[arr.size];   
   arr.data[0] = 4; arr.data[1] = 5;   
   delete[] a.data; 
}

After moving arr.data = new int[arr.size] to a constructor, it becomes

class IntegerArray { 
public:   
   int *data;   
   int size;   
   IntegerArray(int size) {     
      data = new int[size];     
      this->size = size;   
      } 
   };  
int main() {   
   IntegerArray arr(2);   
   arr.data[0] = 4; arr.data[1] = 5;      
   delete[] arr.data; 
}

I'm fairly lost on what the code is trying to do. For

IntegerArray(int size) {     
   data = new int[size];     
   this->size = size;   
   } 

Is int size just the same as the int size that was declared in the class IntegerArray?

Does data = new int[size] just tell us that data is pointing to the output of the array at int size, with new saying that the size of the array is variable?

Is this-> size = size just a pointer that tells us that the size value of the constructor is just equal to the size parameter of the class?

Why are arr.data[0] and arr.data[1] even mentioned after IntegerArray arr(2)? They don't seem to follow the constructor, but I'm probably just too tired to comprehend that part.

Answer 1

IntegerArray(int size) {     
    data = new int[size];     
    this->size = size;   
} 

Is int size just the same as ...

This int size:

IntegerArray(int size) {
             ^^^^^^^^

is an argument to the constructor

Does data = new int[size] just tell us ...

new int[size] dynamically allocates an array that contains size number of int objects. data pointer is then assigned to point to that newly created array.

Is this-> size = size just a pointer ...

No. this is a special pointer, that within the constructor points to the object that is being constructed. this->size is the member variable that was declared here:

class IntegerArray { 
public:   
   int *data;   
   int size;
   ^^^^^^^^

The complete expression this->size = size assigns the value size that is the constructor argument, to the member variable size.

Why are arr.data[0] and arr.data[1] even mentioned after IntegerArray arr(2)?

The constructor does not initialize the contents of the array (the integer objects within the array). The mentioned code does assigns a values to them.

Answer 2

this->size = size //the fist size is your member variable which is inside IntegerArray. the 2nd size is the size you give over to the Method
data = new int[size];  // here you are creating a new array with the size of "size"

the method IntegerArray(int size) is a constructor method, you only can call it once per object (even at the time when you create the object)

int main() //the startpoint of your program
{   
    IntegerArray arr(2);   //you are creating a new object with a size of 2
    arr.data[0] = 4; arr.data[1] = 5;   //because of your array is public, you can call it direct from outside (you sould define it as private ore protected). arr.data[0] is the first item of the array, arr.data[1] is the 2nd item
    delete[] arr.data; 
} 

delete[] arr.data; should be inside the destructor of your class...