CMPT 212 Object-Oriented Applications Design in C++ Fall 1997

Read chapters 1 and 2 from C++ Primer Plus (2nd ed.), by Stephen Prata.

Comments

• There are two ways of indicating comments in C++ source code:
  1. Start with /* and end with */.
     • Example:
       /* This is a comment. */
       /*
       Comment.
       */

  2. Start with // and end with the end of the line.
     • Example:
       // This is a comment.
       x = y + z; // comment.

• Suggest use: use // for "normal" comments, and use /* */ when you want to comment out a section of code.
  • Example:
    // r is the radius.
    int r;
    
    /* No longer needed.
    // d is the diamater.
    int d;
    */

  If we used /* */ for "normal" comments, we could have this error:

  • Example:
    /* r is the radius. */
    int r;
    
    /* No longer needed.
    /* d is the diamater. */ (Does the comment end here?)
    int d;
    */

Input/Output Operators

• A stream is a sequence of data. Think of it as a pipe with data flowing through it.
  An input stream has data flowing into the program.
  An output stream has data flowing out of the program.

• The most commonly used input stream is a stream of characters called cin.
  The most commonly used output stream is a stream of characters called cout.

• Normally, cin reads characters from the keyboard and passed them to the program, and cout takes characters from the program and writes them to the screen.

• Another output stream sometimes used is called cerr. This is where a program would output error messages. We use cerr for error messages because we may want normal output to go to one place (such as the screen or a particular file) and abnormal output (such as error messages) to go to another place.

• You do not have to declare cin, cout, or cerr. They are declared in iostream.h. To use these streams, you must include iostream.h in your program:
  #include <iostream.h>

• The input operator >> does two things:
  
  1. reads data from an input stream and puts it into a variable,
  2. converts the data, if necessary, before putting it into the variable.
  • Example: one character is read from the character input stream cin and placed into c.

    char c;
    cin >> c;

  • Example: characters are read from the character input stream cin, converted to an int, and placed into i.

    int i;
    cin >> i;

• Similarly, the output operator << takes the data from the variable, converts it (if necessary), then writes it to the output stream.
  • Example: "W" is displayed on the screen.

    char c = 'W';
    cout << c;

  • Example: "56" is displayed on the screen.

    int i = 56;
    cout << i;

• Like other operatores, >> and << can be concatenated.
  • Example:

    char c;
    int i;
    cin >> c >> i;

  • Example:

    char c = 'W';
    int i = 56;
    cout << c << i;

• It was mentioned that >> and << may convert data from one type to another. By default, they will only do this for built-in types. However, for new types that we create, we can write our own >> and << operators for inputting and outputting values of that type.

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