A Very Simple Inline Class in C++

This is the first example of a C++ class definition we have shown you. This not-too-exciting program shows some differences between C++ and Java classes.

/*
 * This program makes use of a trivial C++ class in order to point out
 * some basic syntax and other issues.
 */

#include <iostream>
#include <string>

using namespace std;

/*
 * This class represents an integer value with a label for printing.
 */
class LabeledInt {
public:
        // Create an object.
        LabeledInt(string label, int value = 0) {
                m_label = label;
                m_value = value;
        }

        // Get the labeled value for easy printing
        string to_string() const {
                return m_label + ": " + std::to_string(m_value);
        }
        
        // Change the label.
        void relabel(string label) { m_label = label; }

        // Get or set the value.
        int get() const { return m_value; }
        void set(int value) { m_value = value; }
private:
        string m_label;
        int m_value;
};

void munge(LabeledInt ll)
{
        ll.relabel("Nothing");
        ll.set(0);
}

void set(LabeledInt & ll)
{
        ll.relabel("something");
        ll.set(99);
}

int main()
{
        // Make some.
        LabeledInt li1("Peanuts", 487);
        LabeledInt li2("Acorns", 9871);
        LabeledInt li3("Marbles");

        // Print them.
        cout << "A " << li1.to_string() << ", " << li2.to_string() << ", "
             << li3.to_string() << endl;

        // Some pretty random changes.
        li1.set(li1.get() + 13);
        li3.set(17);
        li2.set(li3.get() * li1.get());

        // Print them.
        cout << "B " << li1.to_string() << ", " << li2.to_string() << ", "
             << li3.to_string() << endl;

        // Compare to Java.
        LabeledInt cpy = li1;
        li1.set(3);
        li1.relabel("Pebbles");
        li2 = li3;
        li2.set(3*li2.get());

        // Print them.
        cout << "C " << li1.to_string() << ", " << li2.to_string() << ", "
             << li3.to_string() << ", " << cpy.to_string() << endl;

        // Note that each function uses a different parameter passing method.
        munge(li1);
        set(li2);

        // Print them.
        cout << "D " << li1.to_string() << ", " << li2.to_string() << ", "
             << li3.to_string() << endl;
}

To note:

• The class is declared in the same program as main and couple other functions. In C++, classes aren’t required to be in a file by themselves, or a file with any particular name. It just needs to be defined before it is used, like anything else. Smaller classes are often placed in code with other things.
• The class definitnion is followed by a the semicolon.
• The public and private controls mean the same as in Java, but the syntax differs. The keywords are followed by a colon, and introduce sections. The classification applies until changed, and may be changed (or changed back) at any time.
• The constructor uses a default parameter. These are allowed in any function or method, but are often particularly convenient in constructors.
• The const after the parameter list declares that the method will not change any object variables. That is, a call to x.f() will not change x.
• There are two top-level functions, munge and set, which are sent an object as a parameter. When sent without marks, objects are passed by value like other parameters. Objects are often passed by reference (with &).
• Note that the variables declared as type Point actually contain the objects, not references. Likewise, assignment copies objects; it does not just copy a reference as in Java.