Lesson 14: Accepting command line arguments

In C++ it is possible to accept command line arguments. Command-line arguments are given after the name of a program in command-line operating systems like DOS or Linux, and are passed in to the program from the operating system. To use command line arguments in your program, you must first understand the full declaration of the main function, which previously has accepted no arguments. In fact, main can actually accept two arguments: one argument is number of command line arguments, and the other argument is a full list of all of the command line arguments. 

 int main ( int argc, char *argv[] )

 #include <fstream>#include <iostream>using namespace std;int main ( int argc, char *argv[] ){  if ( argc != 2 ) // argc should be 2 for correct execution    // We print argv[0] assuming it is the program name    cout<<"usage: "<< argv[0] <<" <filename>\n";  else {    // We assume argv[1] is a filename to open    ifstream the_file ( argv[1] );    // Always check to see if file opening succeeded    if ( !the_file.is_open() )      cout<<"Could not open file\n";    else {      char x;      // the_file.get ( x ) returns false if the end of the file      //  is reached or an error occurs      while ( the_file.get ( x ) )        cout<< x;    }    // the_file is closed implicitly here  }}

Lesson 13: More on Functions

In lesson 4 you were given the basic information on functions. However, I left out one item of interest. That item is the inline function. Inline functions are not very important, but it is good to understand them. The basic idea is to save time at a cost in space. Inline functions are a lot like a placeholder. Once you define an inline function, using the 'inline' keyword, whenever you call that function the compiler will replace the function call with the actual code from the function. 

 
#include <iostream>

using namespace std;

inline void hello()
{ 
  cout<<"hello";
}
int main()
{
  hello(); //Call it like a normal function...
  cin.get();
}

Lesson 12: Introduction to Classes 

C++ is a bunch of small additions to C, with a few major additions. One major addition is the object-oriented approach (the other addition is support for *Generic programming, which we'll cover later). As the name object-oriented programming suggests, this approach deals with objects. Of course, these are not real-life objects themselves. Instead, these objects are the essential definitions of real world objects. Classes are collections of data related to a single object type. Classes not only include information regarding the real world object, but also functions to access the data, and classes possess the ability to inherit from other classes. (Inheritance is covered in a later lesson.) 

If a class is a house, then the functions will be the doors and the variables will be the items inside the house. The functions usually will be the only way to modify the variables in this structure, and they are usually the only way even to access the variables in this structure. This might seem silly at first, but the idea to make programs more modular - the principle itself is called "encapsulation". The key idea is that the outside world doesn't need to know exactly what data is stored inside the class--it just needs to know which functions it can use to access that data. This allows the implementation to change more easily because nobody should have to rely on it except the class itself. 

 
#include <iostream>

using namespace std;

class Computer // Standard way of defining the class
{
public:
  // This means that all of the functions below this(and any variables)
  //  are accessible to the rest of the program.
  //  NOTE: That is a colon, NOT a semicolon...
  Computer();
  // Constructor
  ~Computer();
  // Destructor
  void setspeed ( int p );
  int readspeed();
protected:
  // This means that all the variables under this, until a new type of
  //  restriction is placed, will only be accessible to other functions in the
  //  class.  NOTE: That is a colon, NOT a semicolon...
  int processorspeed;
};
// Do Not forget the trailing semi-colon

Computer::Computer()
{
  //Constructors can accept arguments, but this one does not
  processorspeed = 0;
}

Computer::~Computer()
{
  //Destructors do not accept arguments
}

void Computer::setspeed ( int p )
{
  // To define a function outside put the name of the class
  //  after the return type and then two colons, and then the name
  //  of the function.
  processorspeed = p;
}
int Computer::readspeed()  
{
  // The two colons simply tell the compiler that the function is part
  //  of the class
  return processorspeed;
}

int main()
{
  Computer compute;  
  // To create an 'instance' of the class, simply treat it like you would
  //  a structure.  (An instance is simply when you create an actual object
  //  from the class, as opposed to having the definition of the class)
  compute.setspeed ( 100 ); 
  // To call functions in the class, you put the name of the instance,
  //  a period, and then the function name.
  cout<< compute.readspeed();
  // See above note.
}

 
#include <iostream> 

using namespace std;

int main()       
{
  cout<< (char)65 <<"\n"; 
  // The (char) is a typecast, telling the computer to interpret the 65 as a
  //  character, not as a number.  It is going to give the character output of 
  //  the equivalent of the number 65 (It should be the letter A for ASCII).
  cin.get();
}

 
#include <iostream>

using namespace std;

int main()
{
  for ( int x = 0; x < 256; x++ ) {
    cout<< x <<". "<< (char)x <<" "; 
    //Note the use of the int version of x to 
    // output a number and the use of (char) to 
    // typecast the x into a character 	
    // which outputs the ASCII character that 
    // corresponds to the current number
  }
  cin.get();
}

 
int main()       
{
  cout<< char ( 65 ) <<"\n"; 
  cin.get();
}

 
int main()       
{
  cout<< static_cast<char> ( 65 ) <<"\n"; 
  cin.get();
}

Typecasts in practice

 
/* magical function returns the age in years */
int age = getAge();  
/* magical function returns the number of visits */
int pain_visits = getVisits(); 

float visits_per_year = pain_visits / age;

 
float visits_per_year = pain_visits / static_cast<float>(age);
/* or */
float visits_per_year = static_cast<float>(pain_visits) / age;

Lesson 10: C++ File I/O

ifstream a_file;

ifstream a_file ( "filename" );

 
#include <fstream>
#include <iostream>

using namespace std;

int main()
{
  char str[10];

  //Creates an instance of ofstream, and opens example.txt
  ofstream a_file ( "example.txt" );
  // Outputs to example.txt through a_file
  a_file<<"This text will now be inside of example.txt";
  // Close the file stream explicitly
  a_file.close();
  //Opens for reading the file
  ifstream b_file ( "example.txt" );
  //Reads one string from the file
  b_file>> str;
  //Should output 'this'
  cout<< str <<"\n";
  cin.get();    // wait for a keypress
  // b_file is closed implicitly here
}

 
ios::app   -- Append to the file
ios::ate   -- Set the current position to the end
ios::trunc -- Delete everything in the file

 
ofstream a_file ( "test.txt", ios::app );

 
ifstream a_file ( "example.txt" );

if ( !a_file.is_open() ) {
  // The file could not be opened
}
else {
  // Safely use the file stream
}

 
struct Tag {
  Members
};

 
struct Tag name_of_single_structure;

 
name_of_single_structure.name_of_variable;

 
struct example {
  int x;
};
struct example an_example; //Treating it like a normal variable type
an_example.x = 33;  //How to access it's members

 
struct database {
  int id_number;
  int age;
  float salary;
};

int main()
{
  database employee;  //There is now an employee variable that has modifiable 
                      // variables inside it.
  employee.age = 22;
  employee.id_number = 1;
  employee.salary = 12000.21;
}

 
database fn();

 
#include <iostream>

using namespace std;

struct xampl {
  int x;
};

int main()
{  
  xampl structure;
  xampl *ptr;
  
  structure.x = 12;
  ptr = &structure; // Yes, you need the & when dealing with structures
                    //  and using pointers to them
  cout<< ptr->x;    // The -> acts somewhat like the * when used with pointers
                    //  It says, get whatever is at that memory address
                    //  Not "get what that memory address is"
  cin.get();                    
}

Lesson 5: switch case in C and C++

 
switch ( <variable> ) {
case this-value:
  Code to execute if <variable> == this-value
  break;
case that-value:
  Code to execute if <variable> == that-value
  break;
...
default:
  Code to execute if <variable> does not equal the value following any of the cases
  break;
}

 
int a = 10;
int b = 10;
int c = 20;

switch ( a ) {
case b:
  // Code
  break;
case c:
  // Code
  break;
default:
  // Code
  break;
}

 
#include <iostream>

using namespace std;

void playgame()
{
    cout << "Play game called";
}
void loadgame()
{
    cout << "Load game called";
}
void playmultiplayer()
{
    cout << "Play multiplayer game called";
}
	
int main()
{
  int input;
  
  cout<<"1. Play game\n";
  cout<<"2. Load game\n";
  cout<<"3. Play multiplayer\n";
  cout<<"4. Exit\n";
  cout<<"Selection: ";
  cin>> input;
  switch ( input ) {
  case 1:            // Note the colon, not a semicolon
    playgame();
    break;
  case 2:            // Note the colon, not a semicolon
    loadgame();
    break;
  case 3:            // Note the colon, not a semicolon
    playmultiplayer();
    break;
  case 4:            // Note the colon, not a semicolon
    cout<<"Thank you for playing!\n";
    break;
  default:            // Note the colon, not a semicolon
    cout<<"Error, bad input, quitting\n";
    break;
  }
  cin.get();
}