Array of objects in C++

In this tutorial from our C++ e-book, we will learn how to create an array of objects, as well as how to manipulate them.

Creating an array with Objects in C++

In the past tutorial, we created a class that averaged two or three grades for students. We will do something similar in this tutorial.

Let's create the Student class, and initially it gets two grades. The constructor function correctly sets variables, and has a function called getAverage() that returns the average of the two grades:

#include <iostream>
using namespace std;
class Student
{
    private:
        double grade1, grade2;

    public:
        Student();
        Student(double, double);
        double getAverage();
};

Student::Student()
{
    grade1 = 0;
    grade2 = 0;
}
double Student::getAverage()
{
    return (grade1+grade2)/2;
}

int main()
{
    Student NeilPeart(10, 8);
    cout<<NeilPeart.getAverage()<<endl;

    return 0;
}

Note that we created only one object, the student Neil Peart. But which school has only one student? There are thousands, but let's take it easy. We will now work with 5 students. We could do:

Student alu1, alu2, alu3, alu4, alu5;

But, what if it was a real project, with a thousand students? Would you do it up to the alu1000?
Of course not, right, buddy!

You studied by Progressive C++, you are a smart person and you will soon remember our beloved and beautiful arrays, to work with many variables at the same time.

To declare 5 integers, we do:

int num[5];

To declare 5 chars, we do:

char letter[5];

So guess what we do to create 5 objects of the Student class? That's right:

Student alu[5];

Run the code to the code below:

#include <iostream>
using namespace std;

class Student
{
    private:
        double grade1, grade2;

    public:
        Student();
        Student(double, double);
        double getAverage();
};

Student::Student()
{
    grade1 = 0;
    grade2 = 0;
}

Student::Student(double g1, double g2)
{
    grade1 = g1;
    grade2 = g2;
}

double Student::getAverage()
{
    return (grade1+grade2)/2;
}


int main()
{
    Student alu[5];
    cout<<alu[0].getAverage()<<endl;

    return 0;
}

The result will be 0, as we don't initialize the array objects.

How to Initialize Objects from an Array

We could initialize the objects one by one, after declaring:

    Student alu[5];
    alu[0] = Student(10,9);
    alu[1] = Student(10,8);
    alu[2] = Student(10,7);
    alu[3] = Student(10,6);
    alu[4] = Student(10,5);

Or we could just initialize everything when instantiating objects:

    Student alu[5] = { Student(10,9),
                       Student(10,8),
                       Student(10,7),
                       Student(10,6),
                       Student(10,5) };

Or even directly:

    Student alu[5] = { {10,9},
                       {10,8},
                       {10,7},
                       {10,6},
                       {10,5} };

If you have other constructor functions (that receive one grade or 3, for example), you can also place them directly in the code, C++ will know how to call the correct constructor. And if you don't provide any information about any element of the array, C++ will invoke the default constructor (it's always good to have one in the class).

Accessing members of the Object Array

Ok, we have already declared and initialized several objects at once, now you need to learn how to access their members.

When we had an object named 'name', we accessed its members using a dot:

name.var
name.func()

Here's the same thing, but the variable is an array element:

arr[0].grade
arr[1].getAverage();

That is, the same thing, just use the cute array and the dot, followed by the member of the class / object.

The following program creates 5 objects, initializes with the students' grade and displays their averages with the getAverage() function:

#include <iostream>
using namespace std;

class Student
{
    private:
        double grade1, grade2;

    public:
        Student();
        Student(double, double);
        double getAverage();
};

Student::Student()
{
    grade1 = 0;
    grade2 = 0;
}

Student::Student(double g1, double g2)
{
    grade1 = g1;
    grade2 = g2;
}

double Student::getAverage()
{
    return (grade1+grade2)/2;
}


int main()
{
    Student alu[5] = { {10,9},
                       {10,8},
                       {10,6},
                       {10,5} };


    cout<<alu[0].getAverage()<<endl;
    cout<<alu[1].getAverage()<<endl;
    cout<<alu[2].getAverage()<<endl;
    cout<<alu[3].getAverage()<<endl;
    cout<<alu[4].getAverage()<<endl;

    return 0;
}

Note that we only initialized the first four students, so the fifth one was initialized by the default constructor. What was the average of the fifth student (alu[4])?

Now do another test: create a class, which only has the default constructor, displaying a message on the screen. Create an array of size 10, of these objects. What happens?

Progressive C++