Introduction to Classes

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Introduction to Classes#

reading

C++ classes from Wikipedia

Classes are a fundamental part of the object-oriented programming paradigm. A class is an object that holds both data and functions that know how to operate on the data.

Tip

In C++, both struct and class create a class, with the difference being whether the data is publicly accessible by default.

We’ll start by making a class that describes a circle:

Listing 111 simple-class.cpp#
#include <iostream>
#include <numbers>
#include <format>

struct Circle {

    // member data
    double radius{};

    // constructor
    Circle(double r) {
        radius = r;
    }

    // member functions
    double circumference() {
        return 2.0 * std::numbers::pi * radius;
    }

    double area() {
        return std::numbers::pi * radius * radius;
    }

};

int main() {

    Circle c(3.0);
    std::cout
        << std::format("circle: r = {:.3f}; C = {:.3f}; A = {:.3f}\n",
                       c.radius, c.circumference(), c.area());

}

Let’s understand the syntax:

  • We use struct just like we did before, but now in addition to data, there are functions inside the { } that define the struct.

  • We start with the member data. In this class, there is only piece of member data, radius—the radius of our circle.

  • We have one special function here, the constructor—it has the same name as the class, Circle. This is what is called when we create an instance of our class, e.g.,

    Circle c(2.0);

    calls our Circle(double r) function in our class Circle.

    Important

    We call the constructor, so we are using () here instead of {}.

  • We have two member functions:

    • circumference() will return the circumference of the circle

    • area() will return the area of the circle

    Notice that we don’t need to pass the radius into this functions. Since they are members of the class, they will use the member data radius directly. We invoke these on our instance of the class, e.g.,

    auto C = c.circumference();

    This is the power of classes—the functions work directly on the data that is a member of the class.

class vs. struct#

try it…

Change the keyword from struct to class —what happens?

Now the data and functions are private by default, so we fail to compile. We need to explicitly make public anything that we want the user to access.

Let’s keep the data (radius) private but make the functions (including the constructor) public. We’ll add a new function r() to access the radius.

Listing 112 simple-class-private.cpp#
#include <iostream>
#include <numbers>
#include <format>

class Circle {

    // member data
    double radius{};

public:

    // constructor
    Circle(double r) {
        radius = r;
    }

    // member functions
    double r() {
        return radius;
    }

    double circumference() {
        return 2.0 * std::numbers::pi * radius;
    }

    double area() {
        return std::numbers::pi * radius * radius;
    }

};

int main() {

    Circle c(3.0);
    std::cout
        << std::format("circle: r = {:.3f}; C = {:.3f}; A = {:.3f}\n",
                       c.r(), c.circumference(), c.area());

}
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