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C++ Tutorial, C Tutorial

 

This text enunciates and illustrates features and basic principles of C++. It is aimed at experienced C users who wish to learn C++. It can also be interesting for beginner C++ users who leaved out some possibilities of the language.



1. 

There is a new way to #include libraries (the old method still works yet the compiler roars). The .h extension is no more written and the names of standard C libraries are written beginning with a c. In order for the program to use these libraries correctly using namespace std; has to be added:


using namespace std;
#include <iostream>        // This is a key C++ library
#include <cmath>           // The standard C library math.h

int main ()
{
   double a;

   a = 1.2;
   a = sin (a);

   cout << a << endl;

   return 0;
}


A few hints for beginners:

To compile this program, type it inside (or copy & paste it to) a text editor (gedit, kwrite, kate, kedit, vi, emacs, nano, pico, mcedit, Notepad...),  save it as a file named say test01.cpp (if you are a newbie, best put this file inside your home directory, that is say /home/jones on a Unix-like box).

To compile this source code file, type this command (on most open-source Unix-like boxes) in a console or terminal window:

g++ test01.cpp -o test01

To run the binary executable file test01 that has been produced by the compilation (if there were no errors), type this:

./test01

Each time you modify the test01.cpp source code file, you need to compile it again if you want the modifications to echo in the test01 executable file (type the upward arrow key on your keyboard to recall commands).




2. 

You can use  //  to type a remark:


using namespace std;         // Using the standard library namespace.
#include <iostream>          // The iostream library is often used.

int main ()                  // The program's main routine.
{
   double a;                 // Declaration of variable a.

   a = 456.47;
   a = a + a * 21.5 / 100;   // A calculation.

   cout << a << endl;        // Display the content of a.

   return 0;                 // Program end.
}


(The possibility to use  //  to type remarks has been added to C in C99 and ANSI C 2000.)





3. 

Input from keyboard and output to screen can be performed through cout << and cin >>:


using namespace std;
#include <iostream>

void main()
{
   int a;                    // a is an integer variable
   char s [100];             // s points to a string of max 99 characters

   cout << "This is a sample program." << endl;

   cout << endl;             // Just a line feed (end of line)

   cout << "Type your age : ";
   cin >> a;

   cout << "Type your name: ";
   cin >> s;

   cout << endl;

   cout << "Hello " << s << " you're " << a << " old." << endl;
   cout << endl << endl << "Bye!" << endl;

   return 0;
}





4. 

Variables can be declared everywhere inside the code without using hooks:


using namespace std;
#include <iostream>

int main ()
{
   double a;

   cout << "Hello, this is a test program." << endl;

   cout << "Type parameter a: ";
   cin >> a;
                                                                                                 
   a = (a + 1) / 2;

   double c;

   c = a * 5 + 1;

   cout << "c contains      : " << c << endl;

   int i, j;

   i = 0;
   j = i + 1;

   cout << "j contains      : " << j << endl;

   return 0;
}



Maybe try to use this feature to make your source codes more readable and not to mess them up.
Like in C, variables can be encapsulated between { } hooks. Then they are local to the zone encapsulated between the { and }. Whatever happens with such variables inside the encapsulated zone will have no effect outside the zone:


using namespace std;
#include <iostream>

int main ()
{
   double a;

   cout << "Type a number: ";
   cin >> a;

   {
      int a = 1;
      a = a * 10 + 4;
      cout << "Local number: " << a << endl;
   }

   cout << "You typed: " << a << endl;

   return 0;
}





5. 

A variable can be initialised by a calculation involving other variables:


using namespace std;
#include <iostream>

int main ()
{
   double a = 12 * 3.25;
   double b = a + 1.112;

   cout << "a contains: " << a << endl;
   cout << "b contains: " << b << endl;

   a = a * 2 + b;

   double c = a + b * a;

   cout << "c contains: " << c << endl;

   return 0;
}





6. 

C++ allows to declare a variable to be local to a loop:


using namespace std;
#include <iostream>

int main ()
{   
   int i;                       // Simple declaration of i
   i = 487;

   for (int i = 0; i < 4; i++)  // Local declaration of i 
   {
      cout << i << endl;        // This outputs 0, 1, 2 and 3
   }

   cout << i << endl;           // This outputs 487

   return 0;
}



In case the variable is not declared somewhere above the loop, you may be tempted to use it below the loop. Some early C++ compilers accept this. Then the variable has the value it had when the loop ended. You shouldn't do this. It's a bad practice:


using namespace std;
#include <iostream>

int main ()
{

   for (int i = 0; i < 4; i++)
   {
      cout << i << endl;
   }

   cout << i << endl;           // Bad practice!
   i += 5;                      // Bad practice!
   cout << i << endl;           // Bad practice!

   return 0;
}





7. 

A global variable can be accessed even if another variable with the same name has been declared inside the function:


using namespace std;
#include <iostream>

double a = 128;

int main ()
{
   double a = 256;

   cout << "Local a:  " << a   << endl;
   cout << "Global a: " << ::a << endl;

   return 0;
}





8. 

It is possible to make one variable be another:


using namespace std;
#include <iostream>

int main ()
{
   double a = 3.1415927;

   double &b = a;                            // b is a

   b = 89;

   cout << "a contains: " << a << endl;     // Displays 89.

   return 0;
}



(If you are used at pointers and absolutely want to know what happens, simply think double &b = a is translated to double *b = &a and all subsequent b are replaced by *b.)

The value of REFERENCE b cannot be changed after its declaration. For example you cannot write, a few lines further, &b = c expecting now b is c. It won't work. Everything is said on the declaration line of b. Reference b and variable a are married on that line and nothing will separate them.

References can be used to allow a function to modify a calling variable:


using namespace std;
#include <iostream>

void change (double &r, double s)
{
   r = 100;
   s = 200;
}

int main ()
{
   double k, m;

   k = 3;
   m = 4;

   change (k, m);

   cout << k << ", " << m << endl;        // Displays 100, 4.

   return 0;
}



If you are used at pointers in C and wonder how exactly the program above works, here is how the C++ compiler would translate it to C:


using namespace std;
#include <iostream>

void change (double *r, double s)
{
   *r = 100;
   s = 200;
}

int main ()
{
   double k, m;

   k = 3;
   m = 4;

   change (&k, m);

   cout << k << ", " << m << endl;        // Displays 100, 4.

   return 0;
}



A reference can be used to let a function return a variable:


using namespace std;
#include <iostream>

double &biggest (double &r, double &s)
{
   if (r > s) return r;
   else       return s;
}

int main ()
{
   double k = 3;
   double m = 7;

   cout << "k: " << k << endl;       // Displays  3
   cout << "m: " << m << endl;       // Displays  7
   cout << endl;

   biggest (k, m) = 10;

   cout << "k: " << k << endl;       // Displays  3
   cout << "m: " << m << endl;       // Displays 10
   cout << endl;

   biggest (k, m) ++;

   cout << "k: " << k << endl;       // Displays  3
   cout << "m: " << m << endl;       // Displays 11
   cout << endl;

   return 0;
}



Again, provided you're used at pointer arithmetics and if you wonder how the program above works, just think the compiler translated it into the following standard C program:


using namespace std;
#include <iostream>

double *biggest (double *r, double *r)
{
   if (*r > *s) return r;
   else         return s;
}

int main ()
{
   double k = 3;
   double m = 7;

   cout << "k: " << k << endl;
   cout << "m: " << m << endl;
   cout << endl;

   (*(biggest (&k, &m))) = 10;

   cout << "k: " << k << endl;
   cout << "m: " << m << endl;
   cout << endl;

   (*(biggest (&k, &m))) ++;

   cout << "k: " << k << endl;
   cout << "m: " << m << endl;
   cout << endl;

   return 0;
}



To end with, for people who have to deal with pointers yet do not like it, references are useful to un-pointer variables. Beware this is considered a bad practice. You can go into trouble. See for example http://www.embedded.com/story/OEG20010311S0024.


using namespace std;
#include <iostream>

double *silly_function ()    // This function returns a pointer to a double
{
   static double r = 342;
   return &r;
}

int main ()
{
   double *a;

   a = silly_function();

   double &b = *a;          // Now b is the double towards which a points!

   b += 1;                  // Great!
   b = b * b;               // No need to write *a everywhere!
   b += 4;

   cout << "Content of *a, b and r: " << b << endl;

   return 0;
}





9. 

Namespaces can be declared. The variables declared within a namespace can be used thanks to the :: operator:


using namespace std;
#include <iostream>
#include <cmath>

namespace first
{
   int a;
   int b;
}

namespace second
{
   double a;
   double b;
}

int main ()
{
   first::a = 2;
   first::b = 5;

   second::a = 6.453;
   second::b = 4.1e4;

   cout << first::a + second::a << endl;
   cout << first::b + second::b << endl;

   return 0;
}


CAM simulator Sample with VC++ MFC Source Code


10. 

If they contain just simple lines of code, use no for loops or the like, C++ functions can be declared inline. This means their code will be inserted right everywhere the function is used. That's somehow like a macro. Main advantage is the program will be faster. A little drawback is it will be bigger, because the full code of the function was inserted everywhere it is used:


using namespace std;
#include <iostream>
#include <cmath>

inline double hypothenuse (double a, double b)
{
   return sqrt (a * a + b * b);
}

int main ()
{
   double k = 6, m = 9;

   // Next two lines produce exactly the same code:

   cout << hypothenuse (k, m) << endl;
   cout << sqrt (k * k + m * m) << endl;

   return 0;
}


(The possibility to use inline functions has been added to C in C99 and ANSI C 2000.)






11. 

You know the classical structures of C: for, if, do, while, switch... C++ adds one more structure named EXCEPTION:


using namespace std;
#include <iostream>
#include <cmath>

int main ()
{
   int a, b;

   cout << "Type a number: ";
   cin >> a;
   cout << endl;

   try
   {
      if (a > 100) throw 100;
      if (a < 10)  throw 10;
      throw a / 3;
   }
   catch (int result)
   {
      cout << "Result is: " << result << endl;
      b = result + 1;
   }

   cout << "b contains: " << b << endl;

   cout << endl;

   // another example of exception use:

   char zero []     = "zero";
   char pair []     = "pair";
   char notprime [] = "not prime";
   char prime []    = "prime";

   try
   {
      if (a == 0) throw zero;
      if ((a / 2) * 2 == a) throw pair;
      for (int i = 3; i <= sqrt (a); i++)
      {
         if ((a / i) * i == a) throw notprime;
      }
      throw prime;
   }
   catch (char *conclusion)
   {
      cout << "The number you typed is "<< conclusion << endl;
   }

   cout << endl;

   return 0;
}





12. 

It is possible to define default parameters for functions:


using namespace std;
#include <iostream>

double test (double a, double b = 7)
{
   return a - b;
}

int main ()
{
   cout << test (14, 5) << endl;    // Displays 14 - 5
   cout << test (14) << endl;       // Displays 14 - 7

   return 0;
}





13. 

One important advantage of C++ is the OPERATOR OVERLOAD. Different functions can have the same name provided something allows to distinguish between them: number of parameters, type of parameters...


using namespace std;
#include <iostream>

double test (double a, double b)
{
   return a + b;
}

int test (int a, int b)
{
   return a - b;
}

int main ()
{
   double   m = 7,  n = 4;
   int      k = 5,  p = 3;

   cout << test(m, n) << " , " << test(k, p) << endl;

   return 0;
}





14. 

The OPERATORS OVERLOAD can be used to define the basic symbolic operators for new sorts of parameters:


using namespace std;
#include <iostream>

struct vector
{
   double x;
   double y;
};

vector operator * (double a, vector b)
{
   vector r;

   r.x = a * b.x;
   r.y = a * b.y;

   return r;
}

int main ()
{
   vector k, m;              // No need to type "struct vector"

   k.x =  2;                 // To be able to write
   k.y = -1;                 // k = vector (2, -1)
                             // see chapter 19.

   m = 3.1415927 * k;        // Magic!

   cout << "(" << m.x << ", " << m.y << ")" << endl;

   return 0;
}



Besides multiplication, 43 other basic C++ operators can be overloaded, including +=, ++, the array [], and so on...

The operation cout << is an overload of the binary shift of integers. That way the << operator is used a completely different way. It is possible to overload the << operator for the output of vectors:


using namespace std;
#include <iostream>

struct vector
{
   double x;
   double y;
};

ostream& operator << (ostream& o, vector a)
{
   o << "(" << a.x << ", " << a.y << ")";
   return o;
}

int main ()
{
   vector a;

   a.x = 35;
   a.y = 23;

   cout << a << endl;     // Displays (35, 23)

   return 0;
}





15. 

Tired of defining five times the same function? One definition for int type parameters, one definition for double type parameters, one definition for float type parameters... Didn't you forget one type? What if a new data type is used? No problem: the C++ compiler can generate automatically every version of the function that is necessary! Just tell him how the function looks like by declaring a template function:


using namespace std;
#include <iostream>

template <class ttype>
ttype minimum (ttype a, ttype b)
{
   ttype r;

   r = a;
   if (b < a) r = b;

   return r;
}

int main ()
{
   int i1, i2, i3;
   i1 = 34;
   i2 = 6;
   i3 = minimum (i1, i2);
   cout << "Most little: " << i3 << endl;

   double d1, d2, d3;
   d1 = 7.9;
   d2 = 32.1;
   d3 = minimum (d1, d2);
   cout << "Most little: " << d3 << endl;

   cout << "Most little: " << minimum (d3, 3.5) << endl;

   return 0;
}



The function minimum is used three times in above program yet the C++ compiler generates only two versions of it: int minimum (int a, int b) and double minimum (double a, double b). That does the job for the whole program.

Would you have tried something like calculating minimum (i1, d1) the compiler would have reported that as an error. Indeed the template tells both parameters are of the same type.

You can use a random number of different template data types in a template definition. And not all parameter types must be templates, some of them can be of standard types or user defined (char, int, double...). Here is an example where the minimum function takes parameters of any, possibly different, types and outputs a value that has the type of the first parameter:


using namespace std;
#include <iostream>

template <class type1, class type2>
type1 minimum (type1 a, type2 b)
{
   type1 r, b_converted;
   r = a;
   b_converted = (type1) b;
   if (b_converted < a) r = b_converted;
   return r;
}

int main ()
{
   int i;
   double d;

   i = 45;
   d = 7.41;

   cout << "Most little: " << minimum (i, d) << endl;
   cout << "Most little: " << minimum (d, i) << endl;
   cout << "Most little: " << minimum ('A', i) << endl;

   return 0;
}





16. 

The keywords new and delete can be used to allocate and deallocate memory. They are much sweeter than the functions malloc and free from standard C.

new []
and delete [] are used for arrays.


using namespace std;
#include <iostream>
#include <cstring>

int main ()
{
   double *d;                         // d is a variable whose purpose
                                      // is to contain the address of a
                                      // zone where a double is located


   d = new double;                    // new allocates a zone of memory
                                      // large enough to contain a double
                                      // and returns its address.
                                      // That address is stored in d.

   *d = 45.3;                         // The number 45.3 is stored
                                      // inside the memory zone
                                      // whose address is given by d.

   cout << "Type a number: ";
   cin >> *d;

   *d = *d + 5;

   cout << "Result: " << *d << endl;

   delete d;                          // delete deallocates the
                                      // zone of memory whose address
                                      // is given by pointer d.
                                      // Now we can no more use that zone.


   d = new double[15];                // allocates a zone for an array
                                      // of 15 doubles. Note each 15
                                      // double will be constructed.
                                      // This is pointless here but it
                                      // is vital when using a data type
                                      // that needs its constructor be
                                      // used for each instance.

   d[0] = 4456;
   d[1] = d[0] + 567;

   cout << "Content of d[1]: " << d[1] << endl;

   delete [] d;                       // delete [] will deallocate the
                                      // memory zone. Note each 15
                                      // double will be destructed.
                                      // This is pointless here but it
                                      // is vital when using a data type
                                      // that needs its destructor be
                                      // used for each instance (the ~
                                      // method). Using delete without
                                      // the [] would deallocate the
                                      // memory zone without destructing
                                      // each of the 15 instances. That
                                      // would cause memory leakage.

   int n = 30;

   d = new double[n];                 // new can be used to allocate an
                                      // array of random size.
   for (int i = 0; i < n; i++)
   {
      d[i] = i;
   }

   delete [] d;


   char *s;

   s = new char[100];

   strcpy (s, "Hello!");

   cout << s << endl;

   delete [] s;

   return 0;
}





17.

In standard C a struct just contains data. In C++ a struct definition can also include functions. Those functions are own to the struct and are meant to operate on the data of the struct. Those functions are called METHODS. Example below defines the method surface() on the struct vector:


using namespace std;
#include <iostream>

struct vector
{
   double x;
   double y;

   double surface ()
   {
      double s;
      s = x * y;
      if (s < 0) s = -s;
      return s;
   }
};

int main ()
{
   vector a;

   a.x = 3;
   a.y = 4;

   cout << "The surface of a: " << a.surface() << endl;

   return 0;
}


In the example above, a is an INSTANCE of struct "vector". (Note that the keyword "struct" was not necessary when declaring vector a.)

Just like a function, a method can be an overload of any C++ operator, have any number of parameters (yet one parameter is always implicit: the instance it acts upon), return any type of parameter, or return no parameter at all.


What is a class? It's a struct yet that tends to keep its data hidden. Only the methods of the class can access the data. You can't access the data directly, unless authorized by the public: directive. Here is an example of a class definition. It behaves exactly the same way as the struct example above because the class data x and y are kept public:


using namespace std;
#include <iostream>

class vector
{
public:

   double x;
   double y;

   double surface ()
   {
      double s;
      s = x * y;
      if (s < 0) s = -s;
      return s;
   }
};

int main ()
{
   vector a;

   a.x = 3;
   a.y = 4;

   cout << "The surface of a: " << a.surface() << endl;

   return 0;
}


In the example above, the main() function changes the data of instance a directly, using a.x = 3 and a.y = 4. This is made possible by the public: directive in the class definition. This is a bad practice. See chapter 30.


A method is allowed to change the variables of the instance it is acting upon:


using namespace std;
#include <iostream>

class vector
{
public:

   double x;
   double y;

   vector its_oposite()
   {
      vector r;

      r.x = -x;
      r.y = -y;

      return r;
   }

   void be_oposited()
   {
      x = -x;
      y = -y;
   }

   void be_calculated (double a, double b, double c, double d)
   {
      x = a - c;
      y = b - d;
   }

   vector operator * (double a)
   {
      vector r;

      r.x = x * a;
      r.y = y * a;

      return r;
   }
};

int main ()
{
   vector a, b;

   a.x = 3;
   a.y = 5;

   b = a.its_oposite();

   cout << "Vector a: " << a.x << ", " << a.y << endl;
   cout << "Vector b: " << b.x << ", " << b.y << endl;

   b.be_oposited();
   cout << "Vector b: " << b.x << ", " << b.y << endl;

   a.be_calculated (7, 8, 3, 2);
   cout << "Vector a: " << a.x << ", " << a.y << endl;

   a = b * 2;
   cout << "Vector a: " << a.x << ", " << a.y << endl;

   a = b.its_oposite() * 2;
   cout << "Vector a: " << a.x << ", " << a.y << endl;

   cout << "x of oposite of a: " << a.its_oposite().x << endl;

   return 0;
}





18. 

Very special and essential methods are the CONSTRUCTOR and DESTRUCTOR. They are automatically called whenever an instance of a class is created or destroyed (variable declaration, end of program, new, delete...).

The constructor will initialize the variables of the instance, do some calculation, allocate some memory for the instance, output some text... whatever is needed.

Here is an example of a class definition with two overloaded constructors:


using namespace std;
#include <iostream>

class vector
{
public:

   double x;
   double y;

   vector ()                     // same name as class
   {
      x = 0;
      y = 0;
   }

   vector (double a, double b)
   {
      x = a;
      y = b;
   }

};

int main ()
{
   vector k;                     // vector () is called

   cout << "vector k: " << k.x << ", " << k.y << endl << endl;

   vector m (45, 2);             // vector (double, double) is called

   cout << "vector m: " << m.x << ", " << m.y << endl << endl;

   k = vector (23, 2);           // vector created, copied to k, then erased

   cout << "vector k: " << k.x << ", " << k.y << endl << endl;

   return 0;
}



It is a good practice to try not to overload the constructors. Best is to declare only one constructor and give it default parameters wherever possible:


using namespace std;
#include <iostream>

class vector
{
public:

   double x;
   double y;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
   }
};

int main ()
{
   vector k;
   cout << "vector k: " << k.x << ", " << k.y << endl << endl;

   vector m (45, 2);
   cout << "vector m: " << m.x << ", " << m.y << endl << endl;

   vector p (3);
   cout << "vector p: " << p.x << ", " << p.y << endl << endl;

   return 0;
}



The destructor is often not necessary. You can use it to do some calculation whenever an instance is destroyed or output some text for debugging... But if variables of the instance point towards some allocated memory then the role of the destructor is essential: it must free that memory! Here is an example of such an application:


using namespace std;
#include <iostream>
#include <cstring>

class person
{
public:

   char *name;
   int age;

   person (char *n = "no name", int a = 0)
   {
      name = new char [100];                 // better than malloc!
      strcpy (name, n);
      age = a;
      cout << "Instance initialized, 100 bytes allocated" << endl;
   }

   ~person ()                               // The destructor
   {
      delete name;                          // instead of free!

                                            // delete [] name would be more
                                            // academic but it is not vital
                                            // here since the array contains
                                            // no C++ sub-objects that need
                                            // to be deleted.

      cout << "Instance going to be deleted, 100 bytes freed" << endl;
   }
};

int main ()
{
    cout << "Hello!" << endl << endl;

    person a;
    cout << a.name << ", age " << a.age << endl << endl;

    person b ("John");
    cout << b.name << ", age " << b.age << endl << endl;

    b.age = 21;
    cout << b.name << ", age " << b.age << endl << endl;

    person c ("Miki", 45);
    cout << c.name << ", age " << c.age << endl << endl;

    cout << "Bye!" << endl << endl;

   return 0;
}



Here is a short example of an array class definition. A method that is an overload of the [] operator and that outputs a reference (&) is used in order to generate an error if it is tried to access outside the limits of an array:


using namespace std;
#include <iostream>
#include <cstdlib>

class array
{
public:
   int size;
   double *data;

   array (int s)
   {
      size = s;
      data = new double [s];
   }

   ~array ()
   {
      delete [] data;
   }

   double &operator [] (int i)
   {
      if (i < 0 || i >= size)
      {
         cerr << endl << "Out of bounds" << endl;
         exit (EXIT_FAILURE);
      }
      else return data [i];
   }
};

int main ()
{
   array t (5);

   t[0] = 45;                       // OK
   t[4] = t[0] + 6;                 // OK
   cout << t[4] << endl;            // OK

   t[10] = 7;                       // error!

   return 0;
}





19. 

If you cast an object like a vector, everything will happen all right. For example if vector k contains (4, 7), after the cast m = k the vector m will contain (4, 7) too. The values of k.x and k.y have simply been copied to m.x and m.y. Now suppose you're playing with objects like the person class above. Those objects contain a pointer to a character string. If you cast such person object by writing p = r it is necesary that some function does the work to make p be a correct copy of r. Indeed otherwise p.name will point to the physical same character string as r.name. What's more the former character string pointed towards by p.name is lost and becomes a memory zombie. The result will be catastrophic: a mess of pointers and lost data. The methods that will do the job are the COPY CONSTRUCTOR and an overload of the = operator:


using namespace std;
#include <iostream>
#include <cstring>

class person
{
public:

   char *name;
   int age;

   person (char *n = "no name", int a = 0)
   {
      name = new char[100];
      strcpy (name, n);
      age = a;
   }

   person (const person &s)      // The COPY CONSTRUCTOR
   {
      name = new char[100];
      strcpy (name, s.name);
      age = s.age;
   }

   person& operator= (const person &s)  // overload of =
   {
      strcpy (name, s.name);
      age = s.age;
      return *this;
   }

   ~person ()
   {
      delete [] name;
   }
};

int main ()
{
   person p;
   cout << p.name << ", age " << p.age << endl << endl;

   person k ("John", 56);
   cout << k.name << ", age " << k.age << endl << endl;

   p = k;
   cout << p.name << ", age " << p.age << endl << endl;

   p = person ("Bob", 10);
   cout << p.name << ", age " << p.age << endl << endl;

   return 0;
}



The copy constructor allows your program to make copies of instances when doing calculations. It is a key method. During calculations, instances are created to hold intermediate results. They are modified, casted and destroyed without you being aware. This is why those methods can be useful even for simple objects (see chapter 14.).

In all the examples above the methods are defined inside the class definition. That makes them automatically be inline methods.






20. 

If a method cannot be inline, if you do not want it to be inline, if you want the class definition contain the minimum of information (or simply if you want the usual separated .h header file and .cpp source code file), then you must just put the prototype of the method inside the class and define the method below the class (or in a separated .cpp source file):


using namespace std;
#include <iostream>

class vector
{
public:

   double x;
   double y;

   double surface();         // The ; and no {} show it is a prototype
};

double vector::surface()
{
   double s = 0;

   for (double i = 0; i < x; i++)
   {
      s = s + y;
   }

   return s;
}

int main ()
{
   vector k;

   k.x = 4;
   k.y = 5;

   cout << "Surface: " << k.surface() << endl;

   return 0;
}



For beginners :

If you intent to develop a serious C or C++ software, you need to separate the source code in .h header files and .cpp source files. This is a short example of how it is done. The program above is split in three files :

A header file vector.h:


class vector
{
public:

   double x;
   double y;

   double surface();
};



A source code file vector.cpp:


 
using namespace std;
#include "vector.h"

double vector::surface()
{
double s = 0;

for (double i = 0; i < x; i++)
{
s = s + y;
}

return s;
}



And a source code file main.cpp:


using namespace std;
#include <iostream>
#include "vector.h"

int main ()
{
   vector k;

   k.x = 4;
   k.y = 5;

   cout << "Surface: " << k.surface() << endl;

   return 0;
}


Assuming vector.cpp is perfect, you compile it once and for all into a .o "object file". The command below produces that object code file, that will bear the name vector.o:

g++ -c vector.cpp

Each time you modify the main.cpp source code file you compile it into say a test20 executable file. You tell the compiler explicitely it has to link the vector.o object file into the final test20 executable:

g++ main.cpp vector.o -o test20

Run the executable this way:

./test20

This has several advantages:
  • The source code of vector.cpp need to be compiled only once. This spares a lot of time on big softwares. (Linking the vector.o file into the test20 executable is very fast.)
  • You can give somebody the .h file and the .o file(s). That way he can use your software but not change it because he doesn't have the .cpp file(s) (don't rely too much on this, wait till you master these questions).
Note you can compile main.cpp too into an object file and then link it with vector.o:

g++ -c main.cpp

g++ main.o vector.o test20

If you want to look like a real C or C++ programmer you need to condense all this in a Makefile and compile using the make command. The file content beneath is an oversimplified version of such a Makefile. Copy it in a file named Makefile. Please note, and this is very important, that you need to replace the spaces before the g++ commands by a Tab character.



test20: main.o vector.o
        g++ main.o vector.o -o test20

main.o: main.cpp vector.h
        g++ -c main.cpp

vector.o: vector.cpp vector.h
        g++ -c vector.cpp




In order to compile, making use of that Makefile, type this command:

make test20

The make command will parse through the file Makefile and infere what it has to do. To start with it will understand that test20 depends on main.o and vector.o. So it will automatically launch "make main.o" and "make vector.o". Then it will check if test20 allready exists and check for the date stamps of test20, main.o and vector.o. If test20 allready exists and main.o and vector.o have a date stamp earlier than test20, the make command understands current version of test20 is up to date so it has nothing to do. It will just report it did nothing. Otherwise, if test20 does not exist, or main.o or vector.o are more recent than test20, the command that creates an up to date version of test20 is executed, that is g++ main.o vector.o -o test20.

This next version of Makefile is closer to a standard Makefile:



all: test20

test20: main.o vector.o
    g++ main.o vector.o -o test20

main.o: main.cpp vector.h
    g++ -c main.cpp

vector.o: vector.cpp vector.h
    g++ -c vector.cpp

clean:
    rm -f *.o test20 *~ #*




You trigger the compilation by just typing the make command. The first line in the Makefile implies that if you just type make you intent "make test20":

make

This command erases all the files produced during compilation and all text editors backup files:

make clean






21. 

When a method is applied to an instance, that method may use the instance's variables, modify them... But sometimes it is necessary to know the address of the instance. No problem, the keyword this is intended therefore:


using namespace std;
#include <iostream>
#include <cmath>

class vector
{
public:

   double x;
   double y;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
   }

   double module()
   {
      return sqrt (x * x + y * y);
   }

   void set_length (double a = 1)
   {
      double length;

      length = this->module();

      x = x / length * a;
      y = y / length * a;
   }
};

int main ()
{
   vector c (3, 5);

   cout << "The module of vector c: " << c.module() << endl;

   c.set_length(2);            // Transforms c in a vector of size 2.

   cout << "The module of vector c: " << c.module() << endl;

   c.set_length();             // Transforms b in an unitary vector.

   cout << "The module of vector c: " << c.module() << endl;

   return 0;
}





22. 

Of course it is possible to declare arrays of objects:


using namespace std;
#include <iostream>
#include <cmath>

class vector
{
public:

   double x;
   double y;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
   }

   double module ()
   {
      return sqrt (x * x + y * y);
   }
};

int main ()
{
   vector s [1000];

   vector t[3] = {vector(4, 5), vector(5, 5), vector(2, 4)};

   s[23] = t[2];

   cout << t[0].module() << endl;

   return 0;
}





23. 

Here is an example of a full class declaration:


using namespace std;
#include <iostream>
#include <cmath>

class vector
{
public:

   double x;
   double y;

   vector (double = 0, double = 0);

   vector operator + (vector);
   vector operator - (vector);
   vector operator - ();
   vector operator * (double a);
   double module();
   void set_length (double = 1);
};

vector::vector (double a, double b)
{
   x = a;
   y = b;
}

vector vector::operator + (vector a)
{
   return vector (x + a.x, y + a.y);
}

vector vector::operator - (vector a)
{
   return vector (x - a.x, y - a.y);
}

vector vector::operator - ()
{
   return vector (-x, -y);
}

vector vector::operator * (double a)
{
   return vector (x * a, y * a);
}

double vector::module()
{
   return sqrt (x * x + y * y);
}

void vector::set_length (double a)
{
   double length = this->module();

   x = x / length * a;
   y = y / length * a;
}

ostream& operator << (ostream& o, vector a)
{
   o << "(" << a.x << ", " << a.y << ")";
   return o;
}

int main ()
{
   vector a;
   vector b;
   vector c (3, 5);

   a = c * 3;
   a = b + c;
   c = b - c + a + (b - a) * 7;
   c = -c;

   cout << "The module of vector c: " << c.module() << endl;

   cout << "The content of vector a: " << a << endl;
   cout << "The oposite of vector a: " << -a << endl;

   c.set_length(2);            // Transforms c in a vector of size 2.

   a = vector (56, -3);
   b = vector (7, c.y);

   b.set_length();             // Transforms b in an unitary vector.

   cout << "The content of vector b: " << b << endl;

   double k;
   k = vector(1, 1).module();  // k will contain 1.4142.
   cout << "k contains: " << k << endl;

   return 0;
}



It is also possible to define the sum of vectors without mentioning it inside the vector class definition. Then it will not be a method of the class vector. Just a function that uses vectors:


vector operator + (vector a, vector b)
{
   return vector (a.x + b.x, a.y + b.y);
}



In the example above of a full class definition, the multiplication of a vector by a double is defined. Suppose we want the multiplication of a double by a vector be defined too. Then we must write an isolated function outside the class:


vector operator * (double a, vector b)
{
   return vector (a * b.x, a * b.y);
}



Of course the keywords new and delete work for class instances too. What's more, new automatically calls the constructor in order to initialize the objects, and delete automatically calls the destructor before deallocating the zone of memory the instance variables take:


using namespace std;
#include <iostream>
#include <cmath>

class vector
{
public:

   double x;
   double y;

   vector (double = 0, double = 0);

   vector operator + (vector);
   vector operator - (vector);
   vector operator - ();
   vector operator * (double);
   double module();
   void set_length (double = 1);
};

vector::vector (double a, double b)
{
   x = a;
   y = b;
}

vector vector::operator + (vector a)
{
   return vector (x + a.x, y + a.y);
}

vector vector::operator - (vector a)
{
   return vector (x - a.x, y - a.y);
}

vector vector::operator - ()
{
    return vector (-x, -y);

}

vector vector::operator * (double a)
{
    return vector (a * x, a * y);
}

double vector::module()
{
   return sqrt (x * x + y * y);
}

void vector::set_length (double a)
{
   vector &the_vector = *this;

   double length = the_vector.module();

   x = x / length * a;
   y = y / length * a;
}

ostream& operator << (ostream& o, vector a)
{
   o << "(" << a.x << ", " << a.y << ")";
   return o;
}

int main ()
{
   vector c (3, 5);

   vector *r;                  // r is a pointer to a vector.

   r = new vector;             // new allocates the memory necessary
   cout << *r << endl;         // to hold a vectors' variable,
                               // calls the constructor who will
                               // initialize it to 0, 0. Then finally
                               // new returns the address of the vector.

   r->x = 94;
   r->y = 345;
   cout << *r << endl;

   *r = vector (94, 343);
   cout << *r << endl;

   *r = *r - c;
   r->set_length(3);
   cout << *r << endl;

   *r = (-c * 3  +  -*r * 4) * 5;
   cout << *r << endl;

   delete r;                   // Calls the vector destructor then
                               // frees the memory.

   r = &c;                     // r points towards vector c
   cout << *r << endl;

   r = new vector (78, 345);   // Creates a new vector.
   cout << *r << endl;         // The constructor will initialise
                               // the vector's x and y at 78 and 345

   cout << "x component of r: " << r->x << endl;
   cout << "x component of r: " << (*r).x << endl;

   delete r;

   r = new vector[4];          // creates an array of 4 vectors

   r[3] = vector (4, 5);
   cout << r[3].module() << endl;

   delete [] r;                // deletes the array

   int n = 5;
   r = new vector[n];          // Cute!

   r[1] = vector (432, 3);
   cout << r[1] << endl;

   delete [] r;

   return 0;
}





24. 

A class' variable can be declared static. Then only one instance of that variable exists, shared by all instances of the class. It must be initialised outside the class declaration :


using namespace std;
#include <iostream>

class vector
{
public:

   double x;
   double y;
   static int count;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
      count++;
   }

   ~vector()
   {
      count--;
   }
};

int vector::count = 0;

int main ()
{
   cout << "Number of vectors:" << endl;

   vector a;
   cout << vector::count << endl;

   vector b;
   cout << vector::count  << endl;

   vector *r, *u;

   r = new vector;
   cout << vector::count << endl;

   u = new vector;
   cout << a.count << endl;

   delete r;
   cout << vector::count << endl;

   delete u;
   cout << b.count << endl;

   return 0;
}





25. 

A class variable can also be constant. That's just like static, except it is alocated a value inside the class declaration and that value may not be modified:


using namespace std;
#include <iostream>

class vector
{
public:

   double x;
   double y;
   const static double pi = 3.1415927;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
   }

   double cilinder_volume ()
   {
      return x * x / 4 * pi * y;
   }
};

int main()
{
   cout << "The value of pi: " << vector::pi << endl << endl;

   vector k (3, 4);

   cout << "Result: " << k.cilinder_volume() << endl;

   return 0;
}





26. 

A class can be DERIVED from another class. The new class INHERITS the variables and methods of the BASE CLASS. Additional variables and/or methods can be added:


using namespace std;
#include <iostream>
#include <cmath>

class vector
{
public:

   double x;
   double y;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
   }

   double module()
   {
      return sqrt (x*x + y*y);
   }

   double surface()
   {
       return x * y;
   }
};

class trivector: public vector   // trivector is derived from vector
{
public:
   double z;                      // added to x and y from vector

   trivector (double m=0, double n=0, double p=0): vector (m, n)
   {
      z = p;                      // Vector constructor will
   }                              // be called before trivector
                                  // constructor, with parameters
                                  // m and n.

   trivector (vector a)           // What to do if a vector is
   {                              // cast to a trivector
      x = a.x;
      y = a.y;
      z = 0;
   }

   double module ()               // define module() for trivector
   {
      return sqrt (x*x + y*y + z*z);
   }

   double volume ()
   {
       return this->surface() * z;         // or x * y * z
   }
};

int main ()
{
   vector a (4, 5);
   trivector b (1, 2, 3);

   cout << "a (4, 5)    b (1, 2, 3)    *r = b" << endl << endl;

   cout << "Surface of a: " << a.surface() << endl;
   cout << "Volume of b: " << b.volume() << endl;
   cout << "Surface of base of b: " << b.surface() << endl;

   cout << "Module of a: " << a.module() << endl;
   cout << "Module of b: " << b.module() << endl;
   cout << "Module of base of b: " << b.vector::module() << endl;

   trivector k;
   k = a;               // thanks to trivector(vector) definition
                        // copy of x and y,       k.z = 0
   vector j;
   j = b;               // copy of x and y.       b.z leaved out

   vector *r;
   r = &b;

   cout << "Surface of r: " << r->surface() << endl;
   cout << "Module of r: " << r->module() << endl;

   return 0;
}





27. 

In the program above, r->module() calculates the vector module, using x and y, because r has been declared a vector pointer. The fact r actually points towards a trivector is not taken into account. If you want the program to check the type of the pointed object and choose the appropriate method, then you must declare that method virtual inside the base class.

(If at least one of the methods of the base class is virtual then a "header" of 4 bytes is added to every instance of the classes. This allows the program to determine towards what a vector actually points.)


using namespace std;
#include <iostream>
#include <cmath>

class vector
{
public:

   double x;
   double y;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
   }

   virtual double module()
   {
      return sqrt (x*x + y*y);
   }
};

class trivector: public vector
{
public:
   double z;

   trivector (double m = 0, double n = 0, double p = 0)
   {
      x = m;                  // Just for the game,
      y = n;                  // here I do not call the vector
      z = p;                  // constructor and I make the
   }                          // trivector constructor do the
                              // whole job. Same result.

   double module ()
   {
      return sqrt (x*x + y*y + z*z);
   }
};

void test (vector &k)
{
    cout << "Test result:          " << k.module() << endl;
}

int main ()
{
   vector a (4, 5);
   trivector b (1, 2, 3);

   cout << "a (4, 5)    b (1, 2, 3)" << endl << endl;

   vector *r;

   r = &a;
   cout << "module of vector a: " << r->module() << endl;

   r = &b;
   cout << "module of trivector b: " << r->module() << endl;

   test (a);

   test (b);

   vector &s = b;

   cout << "module of trivector b: " << s.module() << endl;

   return 0;
}





28. 

Maybe you wonder if a class can be derived from more than one base class. Answer is yes:


using namespace std;
#include <iostream>
#include <cmath>

class vector
{
public:

   double x;
   double y;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
   }

   double surface()
   {
      return fabs (x * y);
   }
};

class number
{
public:

   double z;

   number (double a)
   {
      z = a;
   }

   int is_negative ()
   {
      if (z < 0) return 1;
      else       return 0;
   }
};

class trivector: public vector, public number
{
public:

   trivector(double a=0, double b=0, double c=0): vector(a,b), number(c)
   {
   }           // The trivector constructor calls the vector
               // constructor, then the number constructor,
               // and in this example does nothing more.

   double volume()
   {
      return fabs (x * y * z);
   }
};

int main ()
{
   trivector a(2, 3, -4);

   cout << a.volume() << endl;
   cout << a.surface() << endl;
   cout << a.is_negative() << endl;

   return 0;
}





29. 

Class derivation allows to construct "more complicated" classes build above base classes. There is another application of class derivation: allow the programmer to write generic functions.

Suppose you define a base class with no variables. It makes no sense to use instances of that class inside your program. But you write a function whose purpose is to sort instances of that class. That function will be able to sort any types of objects provided they belong to a class derived from that base class! The only condition is that inside every derived class definition, all methods the sort function needs are correctly defined:


using namespace std;
#include <iostream>
#include <cmath>

class octopus
{
public:

   virtual double module() = 0;  // = 0 implies function is not
                                 // defined. This makes instances
                                 // of this class cannot be declared.
};

double biggest_module (octopus &a, octopus &b, octopus &c)
{
    double r = a.module();
    if (b.module() > r) r = b.module();
    if (c.module() > r) r = c.module();
    return r;
}

class vector: public octopus
{
public:

   double x;
   double y;

   vector (double a = 0, double b = 0)
   {
      x = a;
      y = b;
   }

   double module()
   {
      return sqrt (x * x + y * y);
   }
};

class number: public octopus
{
public:

   double n;

   number (double a = 0)
   {
      n = a;
   }

   double module()
   {
      if (n >= 0) return n;
      else        return -n;
   }
};

int main ()
{
    vector k (1,2), m (6,7), n (100, 0);
    number p (5),   q (-3),  r (-150);

    cout << biggest_module (k, m, n) << endl;
    cout << biggest_module (p, q, r) << endl;

    cout << biggest_module (p, q, n) << endl;

   return 0;
}



Perhaps you think "okay, that's a good idea to derive classes from the class octopus because that way I can apply to instances of my classes methods and function that were designed a generic way for the octopus class. But what if there exists another base class, named cuttlefish, which has very interesting methods and functions too? Do I have to make my choice between octopus and cuttlefish when I want to derive a class?" No, of course. A derived class can be at the same time derived from octopus and from cuttlefish. That's POLYMORPHISM. The derived class simply has to define the methods necessary for octopus together with the methods necessary for cuttlefish:


class octopus
{
   virtual double module() = 0;
};

class cuttlefish
{
   virtual int test() = 0;
};

class vector: public octopus, public cuttlefish
{
   double x;
   double y;

   double module ()
   {
      return sqrt (x * x + y * y);
   }

   int test ()
   {
      if (x > y) return 1;
      else       return 0;
   }
}





30. 

  The public: directive means the variables or the methods below can be accessed and used everywhere in the program.

If you want the variables and methods to be accessible only to methods of the class AND to methods of derived classes then you must put the keyword protected: above them.

If you want variables or methods be accessible ONLY to methods of the class then you must put the keyword private: above them.

The fact variables or methods are declared private or protected means nothing external to the class may access or use them. That's ENCAPSULATION. (If you want to give to a specific function the right to access those variables and methods then you must include that function's prototype inside the class definition, preceded by the keyword friend.)

The good practice is to encapsulate all the variables of a class. This can sound strange if you're common to structs in C. Indeed a struct only makes sense if you can access its data... In C++ you have to create methods to acces the data inside a class. Example below uses the basic example of chapter 17 yet declares the class data to be protected:


using namespace std;
#include <iostream>

class vector
{
protected:

   double x;
   double y;

public:

   void set_x (int n)
   {
      x = n;
   }
   void set_y (int n)
   {
      y = n;
   }
double surface ()
{
double s;
s = x * y;
if (s < 0) s = -s;
return s;
}
};

int main ()
{
vector a;

a.set_x (3);
a.set_y (4);

cout << "The surface of a: " << a.surface() << endl;

return 0;
}


The example above is a bit odd since the class data x and y can be set yet they cannot be read back. Any attempt in function main () to read a.x or a.y will result in a compilation error. In next example x and y can be read back:


using namespace std;
#include <iostream>

class vector
{
protected:

   double x;
   double y;

public:

   void set_x (int n)
   {
      x = n;
   }
   void set_y (int n)
   {
      y = n;
   }

   double get_x ()
   {
      return x;
   }
 
   double get_y ()
   {
      return y;
   }
double surface ()
{
double s;
s = x * y;
if (s < 0) s = -s;
return s;
}
};

int main ()
{
vector a;

a.set_x (3);
a.set_y (4);

cout << "The surface of a: " << a.surface() << endl;
cout << "The width of a: " << a.get_x() << endl;
cout << "The height of a: " << a.get_y() << endl;

return 0;
}


In C++ one is not supposed to access the data of a class directly. Methods have to be declared. Why this? Many reasons exist. One is this allows tho change the way the data is memorized inside the class. Another reason is this allows data inside the class to be cross-dependent. Suppose x and y must always be of the same sign, otherwize ugly things can happen... If one is allowed to access the class data directly, it would be easy to impose say a positive x and a negative y. In example below this is severely controlled:


using namespace std;
#include <iostream>

int sign (double n)
{
   if (n >= 0) return 1;
   return -1;
}

class vector
{
protected:

   double x;
   double y;

public:

   void set_x (int n)
   {
      x = n;
      if (sign (x) != sign(y)) y = -y;
   }
   void set_y (int n)
   {
      y = n;
      if (sign (y) != sign(x)) x = -x;
   }

   double get_x ()
   {
      return x;
   }
 
   double get_y ()
   {
      return y;
   }
double surface ()
{
double s;
s = x * y;
if (s < 0) s = -s;
return s;
}
};

int main ()
{
vector a;

a.set_x (-3);
a.set_y (4);

cout << "The surface of a: " << a.surface() << endl;
cout << "The width of a: " << a.get_x() << endl;
cout << "The height of a: " << a.get_y() << endl;

return 0;
}





31. 

Let's talk about input/output. In C++ that's a very broad subject.

Here is a program that writes to a file:


using namespace std;
#include <iostream>
#include <fstream>

int main ()
{
   fstream f;

   f.open("c:\\test.txt", ios::out);

   f << "This is a text output to a file." << endl;

   double a = 345;

   f  << "A number: " << a << endl;

   f.close();

   return 0;
}



Here is a program that reads from a file:


using namespace std;
#include <iostream>
#include <fstream>

int main ()
{
   fstream f;
   char c;

   cout << "What's inside the test.txt file from";
   cout << "the C: hard disk root " << endl;
   cout << endl;

   f.open("c:\\test.txt", ios::in);

   while (! f.eof() )
   {
      f.get(c);                          // Or c = f.get()
      cout << c;
   }

   f.close();

   return 0;
}





32. 

Roughly said, it is possible to do on character arrays the same operations as on files. This is very useful to convert data or manage memory arrays.

Here is a program that writes inside a character array:


using namespace std;
#include <iostream>
#include <strstream>
#include <cstring>
#include <cmath>

int main ()
{
   char a[1024];
   ostrstream b(a, 1024);

   b.seekp(0);                           // Start from first char.
   b << "2 + 2 = " << 2 + 2 << ends;     // ( ends, not endl )
                                         // ends is simply the
                                         // null character   '\0'
   cout << a << endl;

   double v = 2;

   strcpy (a, "A sinus: ");

   b.seekp(strlen (a));
   b << "sin (" << v << ") = " << sin(v) << ends;

   cout << a << endl;

   return 0;
}



A program that reads from a character string:


using namespace std;
#include <iostream>
#include <strstream>
#include <cstring>

int main ()
{
   char a[1024];
   istrstream b(a, 1024);

   strcpy (a, "45.656");

   double k, p;

   b.seekg(0);                       // Start from first character.
   b >> k;

   k = k + 1;

   cout << k << endl;

   strcpy (a, "444.23 56.89");

   b.seekg(0);
   b >> k >> p;

   cout << k << ", " << p + 1 << endl;

   return 0;
}





33. 

This program performs formated output two different ways. Please note the width() and setw() MODIFIERS are only effective on the next item output to the stream. The second next item will not be influenced.


using namespace std;
#include <iostream>
#include <iomanip>

int main ()
{
   int i;

   cout << "A list of numbers:" << endl;
   for (i = 1; i <= 1024; i *= 2)
   {
      cout.width (7);
      cout << i << endl;
   }

   cout << "A table of numbers:" << endl;
   for (i = 0; i <= 4; i++)
   {
      cout << setw(3) << i << setw(5) << i * i * i << endl;
   }

   return 0;
}

Full Screen with CWnd and Diagram Like Microsoft Visio 2007

Using GDI+ with MFC or native C/VC++

VC++ Example: splitter control in dialog UpdateWindow GetWindowRect GetDlgItem

VC++ development Sample: Build and Setting Multiple workspaces and project

You now have a basic knowledge about C++. Inside good books you will learn many more things. The file management system is very powerful, it has many other possibilities than those illustrated here. There is also a lot more to say about classes: template classes, virtual classes...

In order to work correctly with C++ you will need a good reference book, just like you need one for C. You will also need information on how C++ is used in your particular domain of activity. The standards, the global approach, the tricks, the typical problems encountered and their solutions... The best reference is of course the books written by Bjarn Stroustrup himself (I don't remind which one of them I read). Following book contains almost every detail about C and C++ and is constructed a way similar to this text:

 

 

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