Create COM Object, Build COM Component, ATL Component with Visual C++

Writing a COM Object with VisualBasic 6.0
Building COM Objects with Java
Using Business Objects (COM Components) In Your ASP Code

Furthermore, this article makes no attempt to teach the C++ syntax and assumes the reader is already familiar with the C++ language. For this article I will be using Microsoft Visual C++ 6.0.

Now that we've got all of the disclaimers out of the way, let's start building our COM component! For this article we will be creating a very simple (but useful) COM component. This COM component will calculate a binomial coefficient, sometimes referred to as a combinatorial. A combinatorial, C(n,k), indicates how many ways n items can be arranged into groups of size k. For example, if we have four objects, a, b, c, and d, these can be arranged into four groups of size 3: a b c, a b d, a c d, and b c d. Note that order is not important; that is, a b d and d b a are considered equal.

In the above example, we showed that C(4,3) = 4. This would be pronounced "4 choose 3 equals 4," meaning that out of a pool of four objects, there are four ways to create groups of size three. Our COM component will be used to calculate n choose k. It will consist of a single method, Comb, that expects two parameters: n and k. The method returns the value of n choose k. The Comb method will use the formula for binomial coefficients, which is:

C(n,k) = n! / (k! * 
						(n-k)!)

The exclamation point is a short-hand way of denoting a factorial. Factorials are large numbers, computed by the formula:

n! = n * (n-1) * 
						(n-2) * ... * 3 * 2 * 1

Therefore 5! = 5 * 4 * 3 * 2 * 1, or 120. Now, you may be wondering why the hell anyone would be interested in these types of things. Believe it or not, but binomial coefficients have many practical uses in mathematics, especially in probability. Therefore, we can use combinatorials to determine gambling odds! For a look at how to calculate gambling odds with binomial coefficients check out the extended readings for this article.

Building the Component in Visual C++
If you've built a COM component in VisualBasic before, you may think that COM objects are quite simple and can be built in a relatively short amount of time. This is a misconception - COM is a very intricate technology that is anything but "simple." VisualBasic, kindly, hides the messy details, making it appear very straightforward and simple. COM components can be built from the ground up just using C or C++, but that is incredibly involved and difficult. To aid with developing COM components in Visual C++ Microsoft has provided ATL (Active Template Library). ATL allows the program to contain macro-like language which is converted into the more complex-COM code at compile time. Furthermore, ATL Wizards exist to make the process of creating a COM component that much easier. In this article we'll use those Wizard to greatly simplify development.

Since building a COM component is much more involved using Visual C++/ATL than using a language like VisualBasic or Java, one may wonder why anyone would wish to create a COM component using Visual C++. The decision to use Visual C++ is usually a performance-based one. Apps built using Visual C++ still boasts better performance than those built using VisualBasic; also, with Visual C++ you can perform lower-level functions, such as working with pointers, and make use of the standard template library (STL); finally, C++ is cool and is a very neat programming language. Of course, if you are wanting to prototype a COM component or are not overly concerned with performance, use VisualBasic or Java due to the time you'll be able to save during component development.

That being said, let's start building our COM component! Begin by starting up Visual C++ and selecting to create a New Project. You should be presented with the following dialog box on the right.

Select the create an ATL COM AppWizard Project and give it the Project name Math. Once you have selected this you will be taken to the ATL COM AppWizard. This Wizard, which we'll discuss in detail in Part 2, will write the vast majority of the code needed to build our COM component!

In Part 1 we discussed binomial coefficients and how to start the COM component building process in Visual C++. In this part we'll continue to work through the steps of building the COM component! (Interested in learning more about binomial coefficients? Be sure to visit the extended readings!)

Recall that in Part 1 we left off with selecting to create an ATL COM AppWizard Project named Math. From that first dialog box we are taken into the ATL COM AppWizard, which contains only one step. The Wizard can be seen to the right.

Make sure you choose to create a DLL and leave the three checkboxes unchecked. When you are ready, click on the Finish button. At that point you will be presented with a dialog box that lists the files the wizard will create. Click OK and the needed skeleton files will automatically be generated.

While the Wizard has created the skeleton files for the Math project, we still have to create a class in the project that contains the properties and methods we need. To do this, we will add a Simple ATL Object. To do this, go to the ClassView and right click on the Math classes text; select the option New ATL Object. At this point you should be presented with the following dialog box:

Add a simple ATL object to the Math project.

Choose to add a simple object and click Next. You will now be taken to a dialog box into which you can enter the properties for the new simple object, as shown below:

Simply enter the value Comb in the Short Name text box. The rest of the text boxes will be filled out automatically. Note that the ProgID text box has a value of Math.Comb. This is the ProgID we'll use in our ASP page to instantiate our COM component (Server.CreateObject("Math.Comb")). Take a moment to click on the Attributes tab in the above dialog box. This shows the threading model, interface, and aggregation types for the component. For this example, leave the default values selected.

By adding a new simple ATL object, a class, CComb, has been added to our project. Also the interface IComb has also been added. This interface determines how the outside world (an ASP page, for example) sees the COM component. To add properties and methods to our component we will add methods and properties to this interface. The interface's properties and methods are mapped to public member functions in the class CComb.

In Part 3 we'll look at how to add the Comb method to our IComb interface and how to tie a CComb member function to this interface method!

In Part 2 we looked at how to use the ATL COM AppWizard to build the skeleton files needed for our COM component. Also, we examined how to add a Simple ATL Object to the project. In this part we will look at how to add methods to the Simple ATL Object's interface.

As we discussed in Part 1, our COM component will have one method and no properties. This method,


									Comb

, will take two integer inputs,

and k, and return the value of


									C(n,k)

(which is also an integer). Therefore we will need to add one method to our interface IComb. To do this, right click on the interface name,


									IComb

, and select Add Method. You will be presented with the dialog box on the right.

Note that in the dialog box I entered Comb as the Method Name. The method expects two in parameters, n and k, and will return an integer, NchooseK. Note that to denote in parameters a [in] prefixes the variable name. This is similar to a ByVal parameter in VisualBasic. Note that to have a method return a value you need to specify it as a parameter in the function's parameter list, specifically denoted as [out, retval]. Furthermore, retval parameters must be pointers. Therefore, our Comb method has the parameter list:


											[in] 
											int n, [in] int k, [out, retval] int 
											* NchooseK

To add a property to a COM component, right click on the interface and select Add Property. Since this component doesn't require any properties, we won't be creating one in this article.

Once you have entered the Method Name and Parameters values in the Add Method to Interface dialog box, click OK. This will create a new method in the interface IComb and will add a public member function named Comb to the CComb class. Simply add the needed C++ code to this function to give the method the functionality you desire. The code for the Comb function can be seen below:


											STDMETHODIMP CComb::Comb(int n, int k, int *NchooseK)
{
  float fAnswer = 1.0,
        fN = (float) n,
        fK = (float) k,
        fTemp = 1.0;

  if (k < 0)
    *NchooseK = 0;
  else {
    for(; fTemp <= fN - fK ;)
      fAnswer *= (fTemp + fK) / fTemp++;

    *NchooseK = (int) fAnswer;
  }

  return S_OK;
}

The code for the Comb function is pretty sweet, in my opinion. A full explanation of why it works can be found in the extended readings. Once you enter this code into the Comb function you are ready to compile your COM component. Go to the Build menu and select Build Math.dll. Once the build process is completed, the component should be compiled, the DLL created and registered! You can now use this COM component in your ASP pages! A quick example can be seen below:


											<% @LANGUAGE="VBScript" %>
<% Option Explicit %>
<%
  Dim objComb
  Set objComb = Server.CreateObject("Math.Comb")
%>
  6 choose 4 = <%=objComb.Comb(6,4)%><BR>
  7 choose 3 = <%=objComb.Comb(7,3)%><BR>
  52 choose 5 = <%=objComb.Comb(52,5)%><BR>
  10 choose -4 = <%=objComb.Comb(10,-4)%>
<%
  Set objComb = Nothing   'Clean up...
%>

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