Constructor function overloading in C++

In this tutorial from our Progressive C++ course, we will learn about overloading the constructor functions.

Many constructors functions

You were hired by a university to create a system to help teachers, especially to work with students' grades.

As you took the Progressive C++ course, you will create a fantastic system, with several classes and objects, hardcore.
But let's take it easy.

Let's create a math class, called Math. For simplicity, she'll do just one operation: average two grades, one student, that's all.

The constructor function will tell the student's average right away.

See how our code looks:

#include <iostream>
using namespace std;

class Math
{
    public:
        Math(double, double);
};


Math::Math(double g1, double g2)
{
    cout<<"Average: "<<(g1+g2)/2<<endl;
}


int main()
{
    Math BruceDickinson(10, 8);

    return 0;
}

The result was 9 right, all nice and cool, and student Bruce Dickinson got a great average.

And speaking of market experience, it is SOOOO common for customers to be asking for changes, new features, etc. Get used to it, it is always requests and complaints.

In that case, the university now wants you to average 3 grades.

Time, it's very simple, just add one more parameter to the list of the constructor function, create the object with the 3 notes and send a bullet, that's it, our code looks like this:

#include <iostream>
using namespace std;

class Math
{
    public:
        Math(double, double, double);
};


Math::Math(double g1, double g2, double g3)
{
    cout<<"Average: "<<(g1+g2+g3)/3<<endl;
}


int main()
{
    Math NeilPeart(10, 9, 10);

    return 0;
}

Everything was fine and Neil Peart had an almost perfect average.

But then your university get in contact and says: look, there are students who will make two disciplines and students who will make three, that is, their system has to calculate the average in both cases.

And now? How will the constructor guess? One hour calculates the average by dividing by 2 and in the other example he divided by 3 because he had one more variable.

Create a class for each case? Of course not, as this is a complication.

Programming was made to make the world easier! This is where builder overloading comes in.

Constructors overloading in C++

C++ has a card up its sleeve: it allows you to create as many constructors as you want. That's right, functions with the same name.
But one thing has to be different: the list of parameters.

Example of a list of different parameters:

Math (double, double)
Math (double, int)
Math (int, double)
Math (double, double, double, float, char, int)
...

So, in our example of averages, just create two constructor functions, one that receives two grades and the other that receives three grades:

#include <iostream>
using namespace std;

class Math
{
    public:
        Math(double, double);
        Math(double, double, double);
};

Math::Math(double g1, double g2)
{
    cout<<"Average: "<<(g1+g2)/2<<endl;
}

Math::Math(double g1, double g2, double g3)
{
    cout<<"Average: "<<(g1+g2+g3)/3<<endl;
}


int main()
{
    Math BruceDickinson(10, 8);
    Math NeilPeart(10, 9, 10);

    return 0;
}

See how smart C++ is. When you created the BruceDickinson object, you only passed two arguments, and the object invoked the constructor function that only works with two numbers.

When he created the ana object, he called the other constructor function, the one that works with three values.

That is, C++ calls the correct constructor!
This is the constructor overloading.

Now if you have two constructors with the same parameter list, then there is no way for C++ to guess what function you are trying to invoke, okay?

We will learn a little more about overload when we study a super special class, the string, which does crazy things and wonders with overloading.

Progressive C++