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sepoto

I am having a bit of trouble determining the exact meaning of this line here:

TinyGPS &operator << (char c) {encode(c); return *this;}

It is of course part of the library TinyGPS which I am using to parse an NMEA string from my EM-406A GPS Sensor. I am taking apart the library right now so I can determine the cause of a locking issue with the sensor. I came across this and although I have a good understandings of the basics of C++ I am confused by this. Does anyone know what exactly is happening with this line. I am pasting the context of the whole code below which comes from TinyGPS.h. The rest of it looks pretty straightforward to me so far.

Code: [Select]

/*
TinyGPS - a small GPS library for Arduino providing basic NMEA parsing
Based on work by and "distance_to" and "course_to" courtesy of Maarten Lamers.
Suggestion to add satellites(), course_to(), and cardinal(), by Matt Monson.
Copyright (C) 2008-2012 Mikal Hart
All rights reserved.

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/

#ifndef TinyGPS_h
#define TinyGPS_h

#if defined(ARDUINO) && ARDUINO >= 100
#include "Arduino.h"
#else
#include "WProgram.h"
#endif

#define _GPS_VERSION 12 // software version of this library
#define _GPS_MPH_PER_KNOT 1.15077945
#define _GPS_MPS_PER_KNOT 0.51444444
#define _GPS_KMPH_PER_KNOT 1.852
#define _GPS_MILES_PER_METER 0.00062137112
#define _GPS_KM_PER_METER 0.001
// #define _GPS_NO_STATS

class TinyGPS
{
public:
  enum {
    GPS_INVALID_AGE = 0xFFFFFFFF,      GPS_INVALID_ANGLE = 999999999,
    GPS_INVALID_ALTITUDE = 999999999,  GPS_INVALID_DATE = 0,
    GPS_INVALID_TIME = 0xFFFFFFFF, GPS_INVALID_SPEED = 999999999,
    GPS_INVALID_FIX_TIME = 0xFFFFFFFF, GPS_INVALID_SATELLITES = 0xFF,
    GPS_INVALID_HDOP = 0xFFFFFFFF
  };

  static const float GPS_INVALID_F_ANGLE, GPS_INVALID_F_ALTITUDE, GPS_INVALID_F_SPEED;

  TinyGPS();
  bool encode(char c); // process one character received from GPS
  TinyGPS &operator << (char c) {encode(c); return *this;}

  // lat/long in hundred thousandths of a degree and age of fix in milliseconds
  void get_position(long *latitude, long *longitude, unsigned long *fix_age = 0);

  // date as ddmmyy, time as hhmmsscc, and age in milliseconds
  void get_datetime(unsigned long *date, unsigned long *time, unsigned long *age = 0);

  // signed altitude in centimeters (from GPGGA sentence)
  inline long altitude() { return _altitude; }

  // course in last full GPRMC sentence in 100th of a degree
  inline unsigned long course() { return _course; }

  // speed in last full GPRMC sentence in 100ths of a knot
  inline unsigned long speed() { return _speed; }

  // satellites used in last full GPGGA sentence
  inline unsigned short satellites() { return _numsats; }

  // horizontal dilution of precision in 100ths
  inline unsigned long hdop() { return _hdop; }

  void f_get_position(float *latitude, float *longitude, unsigned long *fix_age = 0);
  void crack_datetime(int *year, byte *month, byte *day,
    byte *hour, byte *minute, byte *second, byte *hundredths = 0, unsigned long *fix_age = 0);
  float f_altitude();
  float f_course();
  float f_speed_knots();
  float f_speed_mph();
  float f_speed_mps();
  float f_speed_kmph();

  static int library_version() { return _GPS_VERSION; }

  static float distance_between (float lat1, float long1, float lat2, float long2);
  static float course_to (float lat1, float long1, float lat2, float long2);
  static const char *cardinal(float course);

#ifndef _GPS_NO_STATS
  void stats(unsigned long *chars, unsigned short *good_sentences, unsigned short *failed_cs);
#endif

private:
  enum {_GPS_SENTENCE_GPGGA, _GPS_SENTENCE_GPRMC, _GPS_SENTENCE_OTHER};

  // properties
  unsigned long _time, _new_time;
  unsigned long _date, _new_date;
  long _latitude, _new_latitude;
  long _longitude, _new_longitude;
  long _altitude, _new_altitude;
  unsigned long  _speed, _new_speed;
  unsigned long  _course, _new_course;
  unsigned long  _hdop, _new_hdop;
  unsigned short _numsats, _new_numsats;

  unsigned long _last_time_fix, _new_time_fix;
  unsigned long _last_position_fix, _new_position_fix;

  // parsing state variables
  byte _parity;
  bool _is_checksum_term;
  char _term[15];
  byte _sentence_type;
  byte _term_number;
  byte _term_offset;
  bool _gps_data_good;

#ifndef _GPS_NO_STATS
  // statistics
  unsigned long _encoded_characters;
  unsigned short _good_sentences;
  unsigned short _failed_checksum;
  unsigned short _passed_checksum;
#endif

  // internal utilities
  int from_hex(char a);
  unsigned long parse_decimal();
  unsigned long parse_degrees();
  bool term_complete();
  bool gpsisdigit(char c) { return c >= '0' && c <= '9'; }
  long gpsatol(const char *str);
  int gpsstrcmp(const char *str1, const char *str2);
};

#if !defined(ARDUINO)
// Arduino 0012 workaround
#undef int
#undef char
#undef long
#undef byte
#undef float
#undef abs
#undef round
#endif

#endif


Thank is advance...

Kind Regards.

Coding Badly


[font=Courier New]TinyGPS &[/font]

...return a reference to a TinyGPS instance...

[font=Courier New]return *this;[/font]

...specifically, a reference to this TinyGPS instance.


Having the operator return a reference allows operators to be chained and prevents a copy of this from being created.  In other words, it makes things like this efficient...

[font=Courier New]TinyGPS MyTinyGps
MyTinyGps << 'a' << 'b' << 'c';
[/font]

sepoto

What does this mean mean exactly? I'm curious about the use of the operator "<<" is this bit shift?

MyTinyGps << 'a' << 'b' << 'c';

Coding Badly


C++ allows operators to be overloaded (redefined).  The TinyGPS library defines the [font=Courier New] << [/font] operator to be "insertion" (instead of bit shift).  Defining[font=Courier New] << [/font] to be "insertion" is very typical for C++ streams (and things that act like a stream).

sepoto

Oh yes I see. I am taking it that the overloaded operator should work with something on the right in the form of (char c) or is that a type casting? I will find this out from some good documentation I am sure. Thank you for the great reply.

tuxduino

Operator overloading works like function / method overloading. So if you define << (char c) that version will be called when there's a char (literal or variable) on its right, like in the example made a couple of posts ago:

Code: [Select]
MyTinyGps << 'a' << 'b' << 'c';

If you want to be able to use string literals (i.e. C-type strings), you have to define another version of << which accepts const char*. Then you can use code like:

Code: [Select]
MyTinyGps << "abc";

johncc


Then you can use code like:
Code: [Select]
MyTinyGps << "abc";


(To OP) Or even

Code: [Select]
MyTinyGps << "abc: " << 13 << ',' <<" value: " << myFloat << endl;

Which would imply similarly overloaded operators taking a parameter of integer, float, and a variable "endl" which presumably contains a newline character...

Cheers,
John

tuxduino

And btw, presumably
Code: [Select]
encode(c); will add c to some internal data structure, like a temporary buffer that the parser will eventually, ehm... parse.

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