Creating and assigning objects

The following is a listing of a program that allows the user to move a label around on the screen by entering the X and Y properties of a point. The label moves when that point is assigned as the Location property of the label.

Dim ptLabelloc As Point
    
Private Sub btnMove_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnMove.Click
    ptLabelloc = New Point
    ptLabelloc.X = CInt(txtX.Text)
    ptLabelloc.Y = CInt(txtY.Text)
    lblmoves.Location = ptLabelloc
    lblmoves.Text = ptLabelloc.ToString()
End Sub

The Dim statement at the start of the program creates an object variable called ptLabelloc. Object variables are like the scalar variables we have used previously, but their values are objects rather than single numbers, strings or booleans.

Let's look more carefully at the event handler:

1. The first instruction of the event handler constructs a new Point, and assigns it to be the value of ptLabelloc. In more concrete terms, it sets aside memory to hold the values of the properties (X and Y) of the new point, and assigns them the default value of zero. It puts the address of the block of memory that holds the properties in ptLabelloc.
2. The next two instructions set the values of the X and Y properties of ptLabelloc using input supplied by the user.
3. We move the label by assigning ptLabelloc as the new value of the Location property of the label.
4. The ToString method of a Point returns a string containing the X and Y values in human-readable form. The last instruction assigns that as value of the Text property of the label.

Visualizing memory allocation may clarify the difference between object variables and scalar variables. With scalar variables, we have seen that the compiler sets aside a place in memory to hold one value, perhaps an integer, and gives it a name so it can later be retrieved or changed.

Memory allocation is a little more complex with object variables. In our example, the execution of the Dim statement would have set aside a place in memory and given it the name ptLabelloc. Instead of holding a value, that space holds the address of the properties of the object.

When the computer executes the assignment statement, it constructs the new Point by setting aside memory to hold its property values and putting the address of the block of memory that holds those values in ptLabelloc. Since it has the address of the properties, the computer can find them whenever the program refers to them. Note that it assigned the properties default values of 0.

Programmers would call the address a pointer because it points to the place in memory where the property values are stored.

We will add one more facet to the idea of an object variable in a subsequent note, but for now, this should make it concrete.