Now we get down to designing the interface for the CCalculator class which will provide all the functionality for our calculator.

The class will accept numbers and operations (Add,Multiply,Divide and Subtract). On initialisation it will expect a number followed by an operation followed by a number etc.

This continues until either the class is destroyed or the Cancel method is called. At this point it starts again expecting a number,operation, number etc. Each time a valid result is produced it is placed into the mdblResult variable which can be accessed through the Result property. This way interim results to calculations can be obtained.

The first variable mdblResult holds the result of the calculations performed by our calculator class. Each time a number is added to the class we will check to see whether it is the first number entered since either the class was initialised or since the Cancel method was called. If it isn't then we can assume we have at least two numbers and we perform whatever operation the user requested on the numbers (Add,Subtract,Multiply or Divide)

The second variable mstrOperation holds the operation that was last selected ("+", "-", "*", or "/")

The next four variables (mflgAdd, mflgSubtract, mflgMultiply and mflgDivide) are flags to set which operation was last requested. Hopefully this will all make sense further down the line although I'm making no guarantees…J

 We then create an array (mdblNumbers) with two elements to initially hold the first and second numbers that the user inputs. After two numbers have been input then the second element holds the interim result of each calculation and new numbers are added to the first element. This way we can hold the running total generated by our object in the second element of the array and return it through the Result property whenever the user of our object requests it.

We have a couple of Enums. The first (CCalcEntryPoint) to hold the element number in our mbblNumbers variable and the second (CCalcErrors) to hold error numbers generated by our class. Notice that the numbers generated use the constant vbObjectError + 512 which guarantees that the number is in the range allowed for user defined errors. The first Enum is probably a bit over the top but it gives you a (rather useless) example of the things that Enums can be used for.

1. A number is expected first
2. The number input will be placed in the first element of the mdblNumbers array.

The class initialise routine is ran once when the class is first instantiated (created) by using either Dim/Private/Public myVariable as CCalculator or Dim/Private/Public myVariable as New CCalculator. The first method requires you to create an instance of the class using set myVariable = new Ccalculator. This is the preferred method since it gives you control over when your class is instantiated. The second method automatically creates an instance of the class when you call one of its properties or methods. This leaves you far less control over where in your code the class is created since it is dependant on the route taken through your code.

If the number is the first number input then the number is added to the first element of our mdblNumbers array. Then the first number flag is reset. It will only be reset again on Class initialise or if the Cancel method is called.

If the number is the second number input then the number is assigned to the second element in the mdblNumbers array and then since we now have at least two numbers the Calculate method is called

This property is read-only (i.e there is only a Property Get procedure) and it passes out the value of the private variable mdblResult.

The methods set the mstrOperation private string variable to either "+", "-", "*" or "/" depending on the operation to be performed and set the respective operation flag to TRUE depending on the operation being performed. They then set the flags which the object uses to determine whether it is expecting a number or an operation next.

The calculate method is called from the Number Property Let procedure when there are two numbers to work with and thereafter each time a new number is added until either the object is destroyed using set objCalc = Nothing or when the Cancel method is called. The value of the calculation is stored in the first element of the mdblNumbers array providing a running total of the current calculation which can be obtained by calling the Result property of the CCalculator object.