• Structs can group fixed numbers of pieces of data of different types

• Structs can also group related functions

struct Time {
  int hours;
  int minutes;
  int seconds;
};


void read (std::istream& in, Time& time);

void print (std::ostream& out, const Time& time);

bool noLaterThan(const Time& time1, const Time& time2);

struct Time {
  int hours;
  int minutes;
  int seconds;

  void read (std::istream& in);

  void print (std::ostream& out);

  bool noLaterThan(const Time& time2);
};

void Time::print (std::ostream& out)
{
  ⋮
}

• The :: means “within the scope named on the left”

• You’ve seen it used like this: std::string, std::istream

  • In this instance, it means "the function named ‘print’ declared inside the class Time

• distinguishes this function from others elsewhere that might have the same name and parameter types

struct Time {
   ⋮
   void print (std::ostream& out);
   ⋮
};

void print (std::ostream& out)
{
   ⋮
}

void Time::print (std::ostream& out)
{
   ⋮
}

void Time::print (std::ostream& out)
{
    if (hours < 10)
        out << '0';
    out << hours << ':';
    if (minutes < 10)
        out << '0';
    out << minutes << ':';
    if (seconds < 10)
        out << '0';
    out << seconds;
}

• These are data member names

• Notice that there is nothing on the left, e.g., myTime.hours

• Within member function bodies, you can reference data and function members directly

  • implicitly refers to the value named on the left of the call

void Time::print (std::ostream& out)
{
    if (hours < 10)
        out << '0';
    out << hours << ':';
    if (minutes < 10)
        out << '0';
    out << minutes << ':';
    if (seconds < 10)
        out << '0';
    out << seconds;
}

• If we call myTime.printTime(cout);

• Then the highlighted items above refer to myTime.hours, myTime.minutes, and myTime.seconds.

• Header:

  ../fmembers-timeIsMoney/times.h

  #ifndef TIMES_H
  #define TIMES_H
  
  #include <iostream>
  
   * The time of the day, to the nearest second.
  
  
  struct Time {
    int hours;
    int minutes;
    int seconds;
  
  
    /**
     * Read a time (in the format HH:MM:SS) after skipping any
     * prior whitespace.
     */
    void read (std::istream& in);
  
  
    /**
     * Print a time in the format HH:MM:SS (two digits each)
     */
    void print (std::ostream& out);
    
    /**
     *  Compare two times. Return true iff time1 is earlier than or equal to
     *  time2
     *
     *  Pre: Both times are normalized: seconds and minutes are in the range 0..59,
     *         hours are non-negative
     */
    bool noLaterThan(const Time& time2);
  
  };
  
  #endif // TIMES_H
  

• Compilation unit:

  ../fmembers-timeIsMoney/times.cpp

  #include "times.h"
  
  using namespace std;
  
  /**
   * Times in this program are represented by three integers: H, M, & S, representing
   * the hours, minutes, and seconds, respecitvely.
   */
  
  
  /**
   * Read a time from the indicated stream after skipping any 
   * leading whitespace
   */
  void Time::read (istream& in)
  {
    char c;
    in >> hours >> c >> minutes >> c >> seconds;
  }
  
  
  /**
   * Print a time in the format HH:MM:SS (two digits each)
   */
  void Time::print (std::ostream& out)
  {
      if (hours < 10)
          out << '0';
      out << hours << ':';
      if (minutes < 10)
          out << '0';
      out << minutes << ':';
      if (seconds < 10)
          out << '0';
      out << seconds;
  }
  
  /**
   *  Compare two times. Return true iff time1 is earlier than or equal to
   *  time2
   *
   *  Pre: Both times are normalized: sconds and minutes are in the range 0..59,
   *         hours are non-negative
   */
  bool Time::noLaterThan(const Time& time2)
  {
      // First check the hours
      if (hours > time2.hours)
          return false;
      if (hours < time2.hours)
          return true;
      // If hours are the same, compare the minutes
      if (minutes > time2.minutes)
          return false;
      if (minutes < time2.minutes)
          return true;
      // If hours and minutes are the same, compare the seconds
      if (seconds > time2.seconds)
          return false;
      return true;
  }
  

Example: Money with Function Members

• Header:

  ../fmembers-timeIsMoney/money.h

  /*
   * money.h
   *
   *  Created on: Aug 23, 2013
   *      Author: zeil
   */
  
  #ifndef MONEY_H_
  #define MONEY_H_
  
  #include <iostream>
  
  /**
   * An amount of U.S. currency
   */
  struct Money {
  	int dollars;
  	int cents; //< @invariant should be in the range 0..99, inclusive
  
  
    /**
     * Read a money value from the input.  Acceptable formats are
     *
     *        ddd.cc or ddd
     *
     * where ddd is any positive/negative integer of
     * one or more digits denoting dollars, and cc, if
     * supplied, is a two-digit integer.
     *
     * @param in   stream from which to read
     * @param money the value read in. Result is unpredictable if an 
     *              I/O error occurs
     */
    void read (std::istream& in);
    
    /**
     * Print a monetary amount.The output format will always
     * include a decimal point and a two-digit cents amount.
     *
     * @param out the stream to which to print
     * @param money the value to be printed
     */
    void print (std::ostream& out);
    
    /**
     * Compare two Money amounts to see if they are equal
     *
     * @param left  1st value to be compared
     * @param right 2nd value to be compared
     * @return true iff the two amounts are equal
     */
    bool equal (const Money& right);
    
    
    /**
     * Compare two Money amounts to see if the 1st is smaller
     * than the second
     *
     * @param left  1st value to be compared
     * @param right 2nd value to be compared
     * @return true iff left is a smaller amount than right
     */
    bool lessThan (const Money& right);
    
    /**
     * Adds two Money amounts together
     *
     * @param left  1st value to be added
     * @param right 2nd value to be added
     * @return sum of the two amounts
     */
    Money add (const Money& right);
    
    
    /**
     * Subtract one Money amount from another
     *
     * @param left  the minuend
     * @param right the subtrahend
     * @return difference of the two amounts
     */
    Money subtract (const Money& right);
  
  };
  
  
  #endif /* MONEY_H_ */
  

• Compilation unit:

  ../fmembers-timeIsMoney/money.cpp

  /*
   * money.h
   *
   *  Created on: Aug 23, 2013
   *      Author: zeil
   */
  
  #include "money.h"
  #include <iostream>
  
  
  using namespace std;
  
  
  
  /**
   * Read a money value from the input.  Acceptable formats are
   *
   *        ddd.cc or ddd
   *
   * where ddd is any positive/negative integer of
   * one or more digits denoting dollars, and cc, if
   * supplied, is a two-digit integer.
   *
   * @param in   stream from which to read
   * @param money the value read in. Result is unpredictable if an I/O error occurs
   */
  void Money::read (std::istream& in)
  {
    if (!in) return;
    in >> dollars;
    if (!in) return;
    if (in.peek() == '.') // if next input is a '.'
      {
        char decimal;
        in >> decimal;
        in >> cents;
      } else
      cents = 0;
  }
  
  /**
   * Print a monetary amount.The output format will always
   * include a decimal point and a two-digit cents amount.
   *
   * @param out the stream to which to print
   * @param money the value to be printed
   */
  void Money::print (std::ostream& out)
  {
    out << dollars;
    out << '.';
    if (cents < 10)
      out << '0';
    out << cents;
  }
  
  /**
   * Compare two Money amounts to see if they are equal
   *
   * @param left  1st value to be compared
   * @param right 2nd value to be compared
   * @return true iff the two amounts are equal
   */
  bool Money::equal (const Money& right)
  {
    return (dollars == right.dollars)
      && (cents == right.cents);
  }
  
  
  /**
   * Compare two Money amounts to see if the 1st is smaller
   * than the second
   *
   * @param left  1st value to be compared
   * @param right 2nd value to be compared
   * @return true iff left is a smaller amount than right
   */
  bool Money::lessThan (const Money& right)
  {
    if (dollars < right.dollars)
      return true;
    else if (dollars == right.dollars)
      return cents < right.cents;
    return false;
  }
  
  /**
   * Adds two Money amounts together
   *
   * @param left  1st value to be added
   * @param right 2nd value to be added
   * @return sum of the two amounts
   */
  Money Money::add (const Money& right)
  {
    Money result;
    result.dollars = dollars + right.dollars;
    result.cents = cents + right.cents;
    while (result.cents > 99)
      {
        result.cents -= 100;
        ++result.dollars;
      }
    while (result.cents < 0)
      {
        result.cents += 100;
        --result.dollars;
      }
    return result;
  }
  
  
  /**
   * Subtract one Money amount from another
   *
   * @param left  the minuend
   * @param right the subtrahend
   * @return difference of the two amounts
   */
  Money Money::subtract (const Money& right)
  {
    Money result;
    result.dollars = dollars - right.dollars;
    result.cents = cents - right.cents;
    while (result.cents > 99)
      {
        result.cents -= 100;
        ++result.dollars;
      }
    while (result.cents < 0)
      {
        result.cents += 100;
        --result.dollars;
      }
    return result;
  }