TweetFollow Us on Twitter

C++ Overview
Volume Number:5
Issue Number:9
Column Tag:Jörg's Folder

C++ Overview

By Jörg Langowski, MacTutor Editorial Staff

“An Overview Of C++”

MacHack ’89 brought me not only a colorful set of screwdrivers from APDA, but also a new assignment: yours truly is supposed to run a tutorial column on C++. C++ is a very interesting programming language. Supposedly, the new Finder was written in it. Also, it exists on a couple of Unix systems. In fact, shopping for a Unix system, we recently met a representative who assured us that C++ would be delivered with the system. Jordan Matthews, and other people from Apple, spoke very highly about C++ at the MacHack, and assured us that we would get our fingers on a pre-release of Apple’s C++ for MPW, supposedly to be delivered by the end of this year.

As you might have guessed, Apple hasn’t sent us the pre-release yet, and I have yet to use a working C++ compiler. So far my only ‘hands-on’ experience is Bjarne Stroustrup’s book, The C++ Programming Language (Addison-Wesley 1987), which I highly recommend.

The style of the book is rather terse, and you have to work your way through. A good example is that after the introduction, not much is said about ‘object-oriented’ programming, until you hit page 213:

“A list specified in terms of pointers to a class can hold objects of any class derived from that class. That is, it may be heterogeneous. This is probably the single most important and useful aspect of derived classes, and it is essential in the style of programming presented in the following example. That style of programming is often called object based or object oriented; it relies on operations applied in a uniform manner to objects on heterogeneous lists.”

So now you know what you’ve really done when you used MacApp The fact that Stroustrup refers to object-oriented programming in this rather abstract way made me dig out an old introduction to Simula 67, which was the first language to introduce object-oriented concepts. There, too (the book dates from 1973) no reference is made to OOP as we know it today. All the important constructs - classes, instances, methods, overriding - are already there, and one could have implemented today’s programming style in Simula; only computers were much smaller, and most programs did not demand OOP concepts.

The C++ Design

Stroustrup’s team designed C++ for dealing with simulation problems not unlike those that Simula was developed for. However, C++ is a much broader concept than simply a set of ‘object-oriented’ extensions to C; it is a redesign of the C language. To use another quotation from Stroustrup’s book, “C++ was designed to enable larger programs to be structured in a rational way so that it would not be unreasonable for a single person to deal with 25,000 lines of code”. To achieve such an ambitious goal, the most important point is to allow the user to extend the language to accommodate new ‘shorthand notations’ for things that have to be done over and over again. For instance, in a program that uses matrix algebra, given the 25-row by 35-column matrix C and 25-row by 15-column matrix B,

A = ^B*C

is much easier to read than

matmul (A,transpose(B),C,15,25,35).

To be able to use such a shorthand for matrix multiplication, we would need two features built into the language: a. data structures that carry additional information, such as row and column size for a matrix, but which is normally hidden to the user; and b. the capability to redefine operators - like ‘*’ or ‘^’ - depending on the context in which they are used. The latter feature would then cause a ‘*’ to behave differently depending on whether it is used to multiply two integers, reals, vectors or matrices. Some of this behavior is already built into most compilers: integer and real multiply generate different code. But this behavior cannot be modified. C++ allows you to modify your operators in any odd way.

Classes

Let’s assume we wanted to define an array structure, matrix, whose size is not defined at compile time and for which space will be dynamically allocated at run time. In C, one might write

typedef struct matrix
{
    int rows, cols;
    int *m;  /* pointer to matrix  data */
}
matrix a

and then write an indexing function elem(i,j) which refers to the (i,j)th element of the matrix a by

*(a.m + 2*(i*a.cols + j)).

Of course, it would be much easier to simply define a two-dimensional array and write a[i][j], but let’s stay with this definition for a while; unlike the usual array definition, this matrix is resizeable and space is allocated dynamically at run time. We would have to find a block of memory to hold the matrix data and put a pointer to it in m.

When we access an array, we are often not interested in its actual dimensions, as long as the indices are not out of range. In C++ we can define the matrix type in such a way that only certain functions have access to information ‘private’ to the array (such as its dimensions), and all access to the array’s data is done through these access functions, called methods. Data structures that may carry private information are called classes in C++. (According to the manual, classes are ‘user-defined types’ - the most general definition that one might imagine!). A class is just like a struct in which some of the fields cannot be ‘seen from the outside’, and in which the interface to these private fields is defined through method declarations. The C++ class definition for the matrix type would look very similar to a struct declaration, with some additions. The syntax of the class declaration is:

/* 1 */

class matrix
{
     int rows, cols;
     int *m;  /* pointer to matrix  data */
public:
     int rowsize() { return rows}
     int colsize() { return cols }
     void set_size(int,int);
     int& elem(int,int);
     matrix(int,int); 
     ~matrix;
     }
matrix a

Those of you who have had some experience with NEON [let’s make the point again that it is a shame that NEON has disappeared ] might recall that its class definition looked similar:

:class matrix <super object
     2 <indexed
     int rows
     int cols
   :M rowsize    ;M
  
etc 
;class

NEON, however, did not have the label public: for separating the private and public parts of the class declaration. In NEON, all variables were private and all the methods were public.

The C++ class declaration is similar to a C struct declaration, with the possibility to include functions and to hide parts of the declaration from the outside. The public functions in a class that constitute the interface to the outside world are called methods.

There are two principal ways to define a method. One can write the method code inside the class declaration (as for rowsize and colsize in the example above), or one can just declare the method and write the method code later, as for set_size or elem. elem returns the reference to an integer that is the (i,j)th element of the matrix and might be defined as follows:

/* 3 */

int& matrix::elem(int i, int j)  { return m[i*cols + j] };

There is a fundamental difference between methods defined inside and outside of a class declaration. The methods defined outside will be called through a subroutine call, while inside-defined methods are inline-expanded by the compiler. Writing a.rowsize will not generate a JSR to the function code, but code that will directly reference the hidden field a.row. However, any method that is defined outside a class declaration may also be defined as an inline method by prefixing it with the keyword inline:

/* 4 */

inline int& matrix::elem(int i, int j)  { return m[i*cols + j] };

There are two more special methods in the class declaration which carry the name of the class, or respectively the class name prefixed with a tilde (~). These are the so-called constructor and destructor methods; they are called when a new object is declared (as in matrix a;) or deleted (when one leaves the block that the object was declared in). Constructors and destructors are important when heap space has to be allocated for an object (our matrix will need it) and deallocated when the object is no longer defined.

Operators

Our dynamically sized matrix might be defined in a slightly different way which allows to access the elements in the usual way, writing a[i][j] instead of a.elem(i,j). One first defines a one-dimensional array class (as in Stroustrup’s book):

/* 5 */

class vector 
{
     int* v;
     int sz;
public:
     vector(int); ~vector();
     int size () { return sz; }
     void set_size(int);
     int& operator[](int);
     int& elem(int i) { return v[i] };
};

and then builds the two-dimensional class on top of it:

/* 6 */

 class matrix : vector
{
     vector*& mv;
     int rows, cols;
public:
     matrix(int,int); ~matrix;
     int rowsize () { return rows; }
     int colsize () { return cols; }
     void set_size(int,int);
     vector*& operator[](int);
     int& elem(int i, int j) { return mv[i][j] };
};

(I hope this is approximately correct while I’m waiting for the C++ system to try this out and get ready for your embarrassing remarks). In the program, one would declare matrix a(10,20) and access the (i,j)th element by writing a[i][j]. The array indexing operator [] has been re-declared in the class declaration, and will now support checking of index bounds, if we wish so.

The actual implementation of the operators has of course to be done separately. We would write

/* 6 */

 int& vector::operator[](int i) { /* body of code */ }

and

/* 7 */

 vector*& matrix::operator[](int i) { /* body of code */ }

to implement the new definitions.

The matrix multiplication operator may now be defined easily. We write

/* 8 */

matrix operator*(matrix& a, matrix& b)
{
     matrix c(a.colsize,b.rowsize);
     if (a.rowsize != b.colsize) error “index mismatch”;
     for (int i=1 ; i<a.colsize ; i++)
          for (int j=1 ; j<b.rowsize ; j++)
          {
               int sum = 0;
               for (int k=1 ; k<a.rowsize ; k++)  
                    sum = sum + a[i][k]*b[k][j];
               c[i][j] = sum;
          };
     return c;
};

Again, I hope this would work in an actual example. It is not the most efficient way to program the matrix multiplication; the good way to do it would be using friend definitions. This concept is explained in Stroustrup’s book, and I’m going to come back to it in the next column, where I can supply some examples.

The expression a*b, where a and b are of type matrix, would return a pointer to another object of class matrix, which contains the product of a and b. To make sense of the expression c = ^a*b, we would also have to define the transpose operator, ‘^’, and the assignment operator, ‘=’. I won’t write these definitions down here; you might try to work them out, or better, test them if you have a C++ system available.

Operator redefinition is one of the most important concepts of C++, since it makes the code much more readable. The redefinition of an existing operator (like +, *, etc.) is called operator overloading; when such a redefined operator is used, the compiler will automatically search the existing definitions to find one that works on the data types provided. Thus, even if one redefined * for matrices, integer and real multiplications would still work as before. I have not found out yet whether dynamic binding is possible for operators by declaring them virtual (see below), but I’m sure I’ll soon be able to test that.

Class Hierarchies

We have seen the syntax of a class definition which was derived from another class, class matrix : vector { }. If we define a derived class this way, none of the methods in the superclass will be accessible through an object of the subclass; all subclass methods have to be explicitly defined in the subclass declaration. If we write, on the other hand, class matrix : public vector { }, any method from class vector that is not redefined in class matrix is usable on objects of class matrix as well. This is the way we very often wish objects to behave; methods that are redefined in a subclass should override the superclass definition, but if an object does not ‘know’ about a method it should look for a definition higher up in the hierarchy.

In a class hierarchy we should therefore be able to apply a method to an arbitrary object whose exact type is not known at compile time. If the object’s type is known at compile time, the compiler will simply generate a JSR to the appropriate method code, passing arguments as required. This is known as early binding in object-oriented jargon. If the type is not known, we must check at run time what type of object is given the method call, and see whether the method is defined in the object’s class declaration or somewhere higher up in the hierarchy. This is called late binding; a run time error message will be generated if the method can’t be found for a particular object.

Late binding is important if we have a list of objects to which the same method should be applied, for instance a list of shapes - rectangles, circles, polygons - to be drawn on a screen. If the list is kept in an array shapelist[i], we could then simply write

/* 9 */

for (i=1;i<=N;i++) shapelist[i].draw;

to draw all the objects. This is very similar to Object Pascal, where we would write analogously

{10}

for i :=1 to N do shapelist[i].draw;

However, in Object Pascal late binding is always used when early binding can’t be applied. In C++, we have to tell the compiler that a method could be used for late binding by declaring it virtual:

/* 11 */

class TShape {
    TShape* Next, Prev;
    Rect boundRect;
    RgnHandle ShapeRgn;
public:
    virtual void Create(rect *theRect); 
    virtual void Track(rect *oldRect,*newRect);
    virtual void Draw();
    virtual void Erase();
    virtual void Free();
}

This is the generic definition of a shape for which methods for drawing, erasing, etc. exist, but may or may not be defined in the top class; they may be overridden in the descendant classes, and the actual binding may be known only at run time. The figure illustrates the definition of a class hierarchy of shapes in C++ and in Object Pascal.

This more or less concludes my quick overview of the main characteristics of C++ (of course, all the features of C are still present in the language). Don’t laugh at the mistakes that are probably still in the examples; this happens when one writes programs without a compiler. There are many details I haven’t gone into here; we’ll get to know them in the following columns, with corresponding examples. Forth friends, don’t despair; you’ll get your share soon again, too.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Together 3.7 - Store and organize all of...
Together helps you organize your Mac, giving you the ability to store, edit and preview your files in a single clean, uncluttered interface. Features Smart storage. With simple drag-and-drop... Read more
EtreCheck 3.1.4 - For troubleshooting yo...
EtreCheck is an app that displays the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to... Read more
Postbox 5.0.9 - Powerful and flexible em...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more
DiskCatalogMaker 6.5.16 - Catalog your d...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast. Finder-like intuitive look and feel. Super-fast search algorithm. Can compress catalog data... Read more
TrailRunner 3.8.827 - Route planning for...
TrailRunner is the perfect companion for runners, bikers, hikers, and all people wandering under the sky. Plan routes on a geographical map. Import GPS or workout recordings and journalize your... Read more
VueScan 9.5.61 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Postbox 5.0.9 - Powerful and flexible em...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more
VueScan 9.5.61 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
DiskCatalogMaker 6.5.16 - Catalog your d...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast. Finder-like intuitive look and feel. Super-fast search algorithm. Can compress catalog data... Read more
TrailRunner 3.8.827 - Route planning for...
TrailRunner is the perfect companion for runners, bikers, hikers, and all people wandering under the sky. Plan routes on a geographical map. Import GPS or workout recordings and journalize your... Read more

Latest Forum Discussions

See All

Amateur Surgeon 4 Guide: Become the worl...
It's time to wield your trusty pizza cutter again, as Amateur Surgeon has returned with a whole fresh set of challenges (and some old, familiar ones, too). Starting anew isn't easy, especially when all you have at your disposal is a lighter, the... | Read more »
Le Parker: Sous Chef Extraordinaire (Ga...
Le Parker: Sous Chef Extraordinaire 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »
Telltale Games really is working on a Gu...
Telltale Games' next episodic adventure is indeed Guardians of the Galaxy. A document tied to the voice actors strike suggested that the project was in the work, but now we have direct confirmation following an announcement at the Game Awards that... | Read more »
Amateur Surgeon returns to iOS and Andro...
Amateur Surgeon and its two sequels disappeared from the App Store some time and it was sad days for all. But now, just in time for the holidays, the Adult Swim favorite makes its joyous return in the shape of Amateur Surgeon 4, a remake with... | Read more »
The best board games on mobile
Sometimes you need to ditch all of the high speed, high action games in favor of something a little more traditional. If you don't feel like parting ways from your mobile device, though, there are still plenty of ways to get that old-school fix.... | Read more »
The best Facebook Messenger Instant Game...
Facebook's new Instant Games is now here, meaning you can play games with your friends directly via Facebook. It's a fun new way to connect with friends, of course, but it's also proving to be a solid gaming experience in its own right, with a... | Read more »
You can now play game's on Facebook...
Facebook launched its new Instant Games platform in an exciting new attempt to engage its user base. As a result, you can now play a number of different games directly through Facebook Messenger. All of these games run with HTML5, meaning you play... | Read more »
Apollo Justice Ace Attorney (Games)
Apollo Justice Ace Attorney 1.00.00 Device: iOS Universal Category: Games Price: $.99, Version: 1.00.00 (iTunes) Description: Court Is Back In Session Star as rookie defense attorney, Apollo Justice, as he visits crime scenes,... | Read more »
KORG iWAVESTATION (Music)
KORG iWAVESTATION 1.0 Device: iOS Universal Category: Music Price: $19.99, Version: 1.0 (iTunes) Description: A revolutionary new world of sound.The Wave Sequence Synthesizer for iPad - KORG iWAVESTATION | Read more »
Don't Grind Guide: Tips for becomin...
Don’t Grind is a surprising, derpy little one touch game with fun hand-drawn graphics. The goal is simple -- get the high score without being chopped to bits. That can be tough when you’re not used to the game, and that’s compounded by the fact... | Read more »

Price Scanner via MacPrices.net

Monday roundup of Holiday Mac sales: Up to $3...
Take up to $300 off MSRP on the price of a new Apple Mac at B&H Photo today as part of their Holiday sale. Shipping is free, and B&H charges NY sales tax only. Touch Bar MacBook Pros are in... Read more
12-inch WiFi Apple iPad Pros on sale for up t...
B&H Photo has 12″ WiFi Apple iPad Pros on sale for up to $50 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 32GB WiFi iPad Pro: $749 $50 off MSRP... Read more
9-inch Apple WiFi iPad Pros on sale for $20-$...
B&H Photo has 9.7″ Apple WiFi iPad Pros on sale for $20-$50 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 9″ Space Gray 256GB WiFi iPad Pro: $779.95 $20 off MSRP... Read more
Holiday sale: Apple MacBook Airs available fo...
B&H Photo has 13″ MacBook Airs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/128GB MacBook Air (MMGF2LL/A): $899 $100 off MSRP - 13″ 1.6GHz/... Read more
13-inch Silver Touch Bar MacBook Pro in stock...
Amazon has the new 2016 13″ 2.9GHz/256GB Silver Touch Bar MacBook Pro (MLVP2LL/A) in stock today and on sale for $1749 including free shipping. That’s $50 off MSRP, and it’s the lowest price... Read more
Parallels Toolbox 1.3 for Mac Offers 25 Singl...
Parallels has launched Parallels Toolbox 1.3 for Mac, an upgrade that adds five new utilities to the stand-alone application which was released in August and is available exclusively online at http... Read more
OWC Mercury Elite Pro Dual mini Ultra-Portabl...
OWC has introduced the new OWC Mercury Elite Pro Dual mini, a powerful yet ultra-portable dual-drive RAID solution. The new Mercury Elite Pro Dual mini packs phenomenal performance into a small... Read more
Clearance 13-inch Retina MacBook Pros availab...
B&H Photo has clearance 2015 13″ Retina Apple MacBook Pros available for up to $200 off original MSRP. Shipping is free, and B&H charges NY tax only: - 13″ 2.7GHz/128GB Retina MacBook Pro: $... Read more
Roundup of 2016 13-inch 2.0GHz MacBook Pro sa...
B&H has the non-Touch Bar 13″ MacBook Pros in stock today for $50-$100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz MacBook Pro Space Gray (MLL42LL/A): $1449 $... Read more
New 13-inch 2.0GHz Space Gray MacBook Pro in...
Adorama has the new 13″ 2.0GHz Space Gray MacBook Pro (non-Touch Bar, MLL42LL/A) in stock for $1499 including a free 3-year AppleCare Protection Plan. Shipping is free, and Adorama charges sales tax... Read more

Jobs Board

Lead *Apple* Solutions Consultant - Apple (...
# Lead Apple Solutions Consultant Job Number: 53586123 Pittsburgh, Pennsylvania, United States Posted: Nov. 28, 2016 Weekly Hours: 40.00 **Job Summary** The Lead ASC Read more
*Apple* Retail - Multiple Positions- Plano,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions- Kansas...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions- Chicago...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Hardware Design Validation Engineer - *Apple...
The Apple Watch team is looking for a Hardware Design Validation Engineer. This person will be part of the Apple Watch hardware team with responsibilities for Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.