2.2. Class Definitions

2.2. Class Definitions
	Chapter 2. Top of the `class`

2.2. Class Definitions

[ fromfile: classes.xml id: definingclasses ]

C++ has another datatype called class that is similar to struct.
A class definition looks like this:

class ClassName {
    public:
         publicMembers 
    private:
         privateMembers 
};

The first line of the class definition is called the classHead.

The features of a class include data members, member functions, and access specifiers (public, private, protected).
Member functions are used to initialize, manipulate, or otherwise manage the data members.

After you have defined a class, you can use the class name as a type for variables, parameters, and returns from functions.
Variables of a class type are called objects, or instances, of the class.

Member functions for class ClassName specify the behavior of all objects of type ClassName.
Each member function can access all other members of the class.
Nonmember functions can only manipulate objects indirectly by calling member functions.

The set of values of the data members of an object is called the state of the object.

Header Files

To define a class (or any other type) you should place its definition in a header file, preferably with the same name as the class, and with the .h extension.

Example 2.3. src/classes/fraction/fraction.h

#ifndef _FRACTION_H_ 
#define _FRACTION_H_  

#include <QString>


class Fraction {
public:
    void set(int numerator, int denominator);
    double toDouble() const;
    QString toString() const;
private:
    int m_Numerator;
    int m_Denominator;
};

#endif

Notice the #ifndef-#define ... #endif wrapper.

Member function definitions are in a separate implemention file.

Example 2.4 is an implementation file that contains definitions of the functions declared in Example 2.3.

Example 2.4. src/classes/fraction/fraction.cpp

#include <QString>
#include "fraction.h"


void Fraction::set(int nn, int nd) {
    m_Numerator = nn;
    m_Denominator = nd;
}

double Fraction::toDouble() const {
    return 1.0 * m_Numerator / m_Denominator;
}

QString Fraction::toString() const {
  return QString("%1 / %2").arg(m_Numerator).arg(m_Denominator);
}

Every identifier has a scope (Section 20.2), a region of code in which a name is "known" (or visible) and accessible.

Class member names have class scope.
Class scope is quite different from block scope.
Note the use of the scope resolution operator Fraction::.

The toString() member function typically returns a string containing a "snapshot" of the current state of an object.
To increase flexibility, you can give the toString() function one or more parameters that permit the string to have a variety of formats.


2.1. First, There Was `struct`		2.3. Member Access Specifiers