Explanation of the Main.cpp File

In the previous section, there were two files. The gaussian.cpp contained the code that defined the Gaussian class. The main.cpp used the Gaussian class.

The main.cpp file had three parts:

• a header, which is where the #include statements were
• a class declaration
• a main function.

Header

You saw headers in the C++ getting started lessons. In the main.cpp, the header included the iostream library for outputting to the terminal:

#include <iostream>

Class Declaration

Then comes the class declaration. The class declaration is very similar to function declarations, which you learned about previously. In fact, as you'll see later in the lesson, you can put the class declaration into a separate .h file just like you did with function declarations.

The purpose of the class declaration is to give the main function access to the Gaussian class.

// class declaration
class Gaussian
{
    private:
        float mu, sigma2;

    public:

        // constructor functions
        Gaussian ();
        Gaussian (float, float);

        // change value of average and standard deviation 
        void setMu(float);
        void setSigma2(float);

        // output value of average and standard deviation
        float getMu();
        float getSigma2();

        // functions to evaluate 
        float evaluate (float);
        Gaussian mul (Gaussian);
        Gaussian add (Gaussian);
};

Notice that a class declaration looks a lot like the variable declarations and function declarations you've already been using. Declarations tell the program what the variable types will be. The declarations also show the input and output types for functions.

Main Function

And finally comes the main function. The main function instantiates objects of the Gaussian class. So the main function uses the class whereas gaussian.cpp defined the class. You could take the code in gaussian.cpp and put it at the bottom of main.cpp; however, your code files will become quite large and hard to read through.

Here is the code from the main function:

int main ()
{

    Gaussian mygaussian(30.0,20.0);
    Gaussian othergaussian(10.0,30.0);

    std::cout << "average " << mygaussian.getMu() << std::endl;
    std::cout << "evaluation " << mygaussian.evaluate(15.0) << std::endl;

    std::cout << "mul results sigma " << mygaussian.mul(othergaussian).getSigma2() << std::endl;
    std::cout << "mul results average " << mygaussian.mul(othergaussian).getMu() << std::endl;

    std::cout << "add results sigma " << mygaussian.add(othergaussian).getSigma2() << std::endl;
    std::cout << "add results average " << mygaussian.add(othergaussian).getMu() << std::endl;

    return 0;
}