1.1 Writing a Simple C++ Program

• A function definition has four elements: return type, function name, parameter list, and function body.
• main is required to have return type int, which is a status indicator (0 for success, nonzero values for error types defined by the system, e.g. -1).
  • The value returned from main is accessed in a system-dependent manner: echo %ERRORLEVEL% (Windows) or echo $? (UNIX).
  • Return value -1 will be shown as 255 (unsigned char representation).

1.2 A First Look at Input/Output

• C++ relies on the standard library to provide IO, mostly from iostream.
  • Two fundamental stream types: istream for input and ostream for output.
  • Four IO objects: cin, cout, cerr, and clog. The system associates these objects with the window in which the program is executed.
• #include directive must be written on a single line and outside any function. The name inside the angle brackets refers to a header.
• The << operator takes two operands - ostream and the value to be output - and returns the ostream object. In this way we can chain multiple output operations together. The >> operator is used for input and works similarly to <<.
• std::endl is a special value called a manipulator, which ends a line and flushes the buffer associated with the device.
  • During debugging, it should be ensured that the stream is always flushed to avoid leaving debugging information in the buffer.
• The scope operator :: is used to explicitly specify the namespace. All names defined by the standard library are in the std namespace.

1.3 A Word about Comments

• Warning: an incorrect comment is worse than no comment at all!
• Two kinds of comments in C++: single-line (//, ends with a newline) and multi-line (delimiters /* and */).
  • For multi-line comments, it is a good idea to begin each line with a * to indicate the comment’s extent.
  • One comment pair cannot appear inside another. The compiler error messages may be confusing.
  • The best way to comment a block of code during debugging is to insert single-line comments at the beginning of each line.

#include <iostream> 
/*
 *  Simple main function:
 *  Read two numbers and write their sum 
 */
int main()
{
    // prompt user to enter two numbers
    std::cout << "Enter two numbers:" << std::endl; 
    int v1 = 0, v2 = 0;   // variables to hold the input we read 
    std::cin >> v1 >> v2; // read input
    std::cout << "The sum of " << v1 << " and " << v2
              << " is " << v1 + v2 << std::endl;
    return 0;
}

• The compiler reads from left to right to decide whether a delimiter is part of a string. When a double-quote appears first, the delimiter belongs to the string. When a delimiter appears first, the double-quote belongs to the comment.

std::cout << "/*";                // legal: /*
std::cout << "*/";                // legal: */
// std::cout << /* "*/" */;       // illegal
std::cout << /* "*/" /* "/*" */;  // legal: /*

1.4 Flow of Control

• for (init-statement; condition; expression) statement and while (condition) statement are used for loops with definite and indefinite iteration.
  • A block is a sequence of zero or more statements enclosed by curly braces. It can be used as a statement.
• if is used for conditional execution.
• Read until end-of-file: while (std::cin >> value).
  • When we use an istream as a condition, the state of the stream will be tested. It becomes invalid when the end-of-file is reached or when an input operation fails (incorrect format).
    • Incorrect format: the input will be processed with best effort. For example, if the input is 1.2 for integers, the stream will read 1 and leave . in the input buffer.
  • Entering EOF from keyboard: Ctrl+Z (Windows) / Ctrl+D (UNIX) + Return/Enter.
• Compilation errors: syntax errors, type errors, declaration errors. Correct errors in the sequence they are reported: a single error can have a cascading effect. Recompile after each fix.

1.5 Introducing Classes

• Every class defines a type. The type name is the same as the class name. The author of the class defines all the actions that can be performed on objects of that class.
• Use the dot operator and the call operator to access and call a member function.