Member Access Specifiers

Thus far we have worked with class definition code and class implementation code. There is a third category of code as it relates to a given class. Client code is code that is outside the scope of the class but that uses objects of the class. Generally, client code #includes the header file that contains the class definition.

We now revisit the Fraction class, focusing on its member access specifiers. See Example 2.5.

Example 2.5. src/classes/fraction.h

#ifndef _FRACTION_H_
#define _FRACTION_H_

#include 
using namespace std;

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

#endif

The member access specifiers, public, protected, and private, are used in a class definition to specify where in a program the affected members can be accessed. The following list provides an informal first approximation of the definitions of these three terms. Refinements are contained in footnotes.

  • A public member can be accessed (using an object of the class[1]) anywhere in a program that #includes the class definition file.

    [1] public static members can be accessed without an object. We discuss this in Section 2.10.

  • A protected member can be accessed inside the definition of a member function of its own class, or a member function of a derived class.[2]

    [2] We discuss derived classes in Chapter 6.

  • A private member is only accessible by member functions of its own class.[3]

    [3] Private members are also accessible by friends of the class, which we discuss in Section 2.6.

Accessibility versus Visibility

There is a subtle difference between accessibility and visibility. In order for a named item to be accessible, it must first be visible (in our scope). Not all visible items are accessible. Accessibility depends on member access specifiers: public/private/protected.

Example 2.6 shows client code that demonstrates visibility errors in a variety of ways. This example also focuses on block scope, which extends from an opening brace to a closing brace. A variable declared inside a block is visible and accessible only between its declaration and the closing brace. In the case of a function, the block that contains the function definition also includes the function's parameter list.

Example 2.6. src/classes/fraction-client.cpp

#include "fraction.h"
#include 

int main() {
 const int DASHES = 30;
 using namespace std;

 { <-- 1

 int i;
 for(i = 0; i < DASHES; ++i)
 cout << "=";
 cout << endl;

 }

 // cout << "i = " << i << endl; <-- 2
 Fraction f1, f2;
 f1.set(3, 4);
 f2.set(11,12); <-- 3
 // f2.m_Numerator = 12; <-- 4
 cout << "The first fraction is: " << f1.toString() << endl;
 cout << "
The second fraction, expressed as a double is: "
 << f2.toDouble() << endl;
 return 0;
}
 

(1)nested scopeinner block

(2)errori no longer exists, so it is not visible in this scope.

(3)set through a member function

(4)errorm_Numerator is visible but not accessible.

Now we can describe the difference between struct and class in C++. Stroustrup defines a struct to be a class in which members are by default public, so that

struct T { ...

means precisely

class T {public: ...



Part I: Introduction to C++ and Qt 4

C++ Introduction

Classes

Introduction to Qt

Lists

Functions

Inheritance and Polymorphism

Part II: Higher-Level Programming

Libraries

Introduction to Design Patterns

QObject

Generics and Containers

Qt GUI Widgets

Concurrency

Validation and Regular Expressions

Parsing XML

Meta Objects, Properties, and Reflective Programming

More Design Patterns

Models and Views

Qt SQL Classes

Part III: C++ Language Reference

Types and Expressions

Scope and Storage Class

Statements and Control Structures

Memory Access

Chapter Summary

Inheritance in Detail

Miscellaneous Topics

Part IV: Programming Assignments

MP3 Jukebox Assignments

Part V: Appendices

MP3 Jukebox Assignments

Bibliography

MP3 Jukebox Assignments



An Introduction to Design Patterns in C++ with Qt 4
An Introduction to Design Patterns in C++ with Qt 4
ISBN: 0131879057
EAN: 2147483647
Year: 2004
Pages: 268

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