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DATE 2010-03-01

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Key: Value:

Key: Value:

MESSAGE
DATE 2010-03-03
FROM Ruben Safir
SUBJECT Subject: [NYLXS - HANGOUT] C++ Workshop _ Syntax Basics - agregate data types

A particularly special array is the charater array, which in C forms the
basis for strings. We already know that a single char is a C and C++
built in data type and we can have an array of chars, and lastly that we
can have string literals, which are constant. For review, lets look at
code examples of each:

Example A:
char car = 'A'; //a single character assigned to a char variable. Note
//the single quote
Example B:
char cararray[] = {'A', 'B', 'C', 'D'}; // The definition and assignment
An array of 4 chars,
//which got it's size with the
//initialization of the array
//and of which each of which
//element can be accessed
//through indexing ie:
//char b = cararray[3] or
//pointers such as
//char b = *(cararray + 3)

Example C:
const char *stringo = "ABCD"; //This is the assignment of a string
//constant literal to a pointer to a
//char constant. This is a real string
//that differs from the above example
//because it creates an array of chars,
//not 4 chars long but 5 chars long
//because it appends a NULL character
//to the end


Example D:
char[] = "My Dog Has Fleas\n";//similar to above with 19 char
//assigned to the array ending with
//a NULL char but not a constant
//literal


There are some important but subtle differences between "true" string
literals and strings formed by manually creating arrays of chars as
shown in the technique of Example B and Example D. We can see an
example of this difference in the following code.

Make a new directory and in GVIM or the editor of your choice create the
following files:

test.cc

------------------------------------------
#include
#include "test.h"
using namespace std;

int main(int argc, char * argv[]){
stringexample();
return 0;
}


----------------------------------------------

test.h

----------------------------------------------

#ifndef TEST_H
#define TEST_H
#endif /* TEST_H */

void stringexample();

--------------------------------------------------

string_ex.cc

--------------------------------------------------

#include
#include "test.h"
using namespace std;

void stingexample(){

char test[] = "My Dog has Fleas\n";
const char * test2 = "My Dog has Fleas\n";
cout << test;
test[3] = 'C';
test[4] = 'a';
test[5] = 't';
cout << test;
cout << test2;
const_cast(test2[3] = 'C');
const_cast(test2[4] = 'a');
const_cast(test2[5] = 't');
cout << test2;

}

--------------------------------------------------

test.cc

--------------------------------------------------

#include
#include "test.h"
using namespace std;

int main(int argc, char * argv[]){
stringexample();
return 0;
}

----------------------------------------------------

and create the following makefile


----------------------------------------------------

test.bin : test.o string_ex.o
g++ -Wall -o test.bin test.o string_ex.o

test.o : test.cc
g++ -Wall -c test.cc

string_ex.o : string_ex.cc test.h
g++ -Wall -c string_ex.cc

----------------------------------------------------

Note that the makefile MUST have those TABS and not spaces

Then run 'make'

gcc gives you the following output

g++ -Wall -c string_ex.cc
string_ex.cc: In function ?void stingexample()?:
string_ex.cc:15: error: assignment of read-only location ?*(test2 + 3u)?
string_ex.cc:15: error: invalid use of const_cast with type ?char?,
which is not a pointer, reference, nor a pointer-to-data-member type
string_ex.cc:16: error: assignment of read-only location ?*(test2 + 4u)?
string_ex.cc:16: error: invalid use of const_cast with type ?char?,
which is not a pointer, reference, nor a pointer-to-data-member type
string_ex.cc:17: error: assignment of read-only location ?*(test2 + 5u)?
string_ex.cc:17: error: invalid use of const_cast with type ?char?,
which is not a pointer, reference, nor a pointer-to-data-member type
make: *** [string_ex.o] Error 1


This is a very useful error message and the GCC compiler is now taking
the programming to school. Lets look at the complaints of the compiler
about our code. The first problem gcc makes is about line 15 in
string_ex.cc which is this line:

const_cast(test2[3] = 'C');

The compiler is telling us that array (or string) and test2 points to is
read only. That variable is defined on line 8:

const char * test2 = "My Dog has Fleas\n";

It is obvious from the code that the data is defined as a "const", less
obvious is that the compiler will complain and refuse to compile if you
do NOT make test2 a "const". Because of the assignment of the string
literal to the char pointer, it must be a const. Therefore, we tried to
cast the const away with const_cast, and that fails as well
because, as the compiler says to us:

invalid use of const_cast with type ?char?, which is not a pointer,
reference, nor a pointer-to-data-member type. We can not just cast away
to constantness of the string literal assigned to test2.

So again we see that arrays and pointers have differences, and arrays
and strings have even great differences. The standard iostream object
"cout" will recognize both as strings for printing to standard output.



Ruben

  1. 2010-03-01 Paul Robert Marino <prmarino1-at-gmail.com> Re: [NYLXS - HANGOUT] this should not get through
  2. 2010-03-01 Ruben Safir <mrbrklyn-at-panix.com> Subject: [NYLXS - HANGOUT] Catching the streams from Flash Video
  3. 2010-03-02 Ruben Safir <mrbrklyn-at-panix.com> Subject: [NYLXS - HANGOUT] [ruben-at-mrbrklyn.com: [jim-at-well.com: [conspire] open source
  4. 2010-03-02 Ruben Safir <mrbrklyn-at-panix.com> Subject: [NYLXS - HANGOUT] Phone Tech Wars
  5. 2010-03-02 Ron Guerin <ron-at-vnetworx.net> Subject: [NYLXS - HANGOUT] NYLUG Workshop / Hacking Society (Smalltalk, C++, Python) Tuesday March 2 6:00PM-8:00PM
  6. 2010-03-02 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] this should not get through
  7. 2010-03-02 Paul Robert Marino <prmarino1-at-gmail.com> Re: [NYLXS - HANGOUT] this should not get through
  8. 2010-03-02 Paul Robert Marino <prmarino1-at-gmail.com> Re: [NYLXS - HANGOUT] this should not get through
  9. 2010-03-03 Ruben Safir <mrbrklyn-at-panix.com> Subject: [NYLXS - HANGOUT] C++ Workshop _ Syntax Basics - agregate data types
  10. 2010-03-03 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] C++ Workshop _ Syntax Basics - agregate data
  11. 2010-03-03 Ruben Safir <mrbrklyn-at-panix.com> Subject: [NYLXS - HANGOUT] C++ Workshop _ Syntax Basics - agregate data
  12. 2010-03-04 Ruben Safir <mrbrklyn-at-panix.com> Subject: [NYLXS - HANGOUT] C++ Workshop _ Syntax Basics
  13. 2010-03-05 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] C++ Workshop _ Syntax Basics
  14. 2010-03-05 Ruben Safir <mrbrklyn-at-panix.com> Subject: [NYLXS - HANGOUT] AlphaOne Labs invited you to the event "Ask-a-Sysadmin
  15. 2010-03-06 Ruben Safir <mrbrklyn-at-panix.com> Subject: [NYLXS - HANGOUT] Professors Open Source Summer Experience -at-RIT
  16. 2010-03-07 Kevin Mark <kevin.mark-at-verizon.net> Re: [NYLXS - HANGOUT] Professors Open Source Summer Experience -at-RIT
  17. 2010-03-09 Ruben Safir <mrbrklyn-at-panix.com> Subject: [NYLXS - HANGOUT] C++ Workshop _ Syntax Basics Structs Unions and
  18. 2010-03-09 From: "Beau Gould" <bg-at-capitalmarketsp.com> Subject: [NYLXS - HANGOUT] [JOB] System Administrator, NYC to 90k
  19. 2010-03-09 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] [JOB] System Administrator, NYC to 90k
  20. 2010-03-10 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] C++ Workshop _ Syntax Basics Structs Unions
  21. 2010-03-13 redpill <redpill-at-rcn.com> Subject: [NYLXS - HANGOUT] NYLXS Library?
  22. 2010-03-13 redpill <redpill-at-rcn.com> Subject: [NYLXS - HANGOUT] NYLXS Library?
  23. 2010-03-14 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  24. 2010-03-14 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  25. 2010-03-15 redpill <redpill-at-rcn.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  26. 2010-03-15 redpill <redpill-at-rcn.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  27. 2010-03-15 Contrarian <adrba-at-nyct.net> Re: [NYLXS - HANGOUT] NYLXS Library?
  28. 2010-03-15 Contrarian <adrba-at-nyct.net> Re: [NYLXS - HANGOUT] NYLXS Library?
  29. 2010-03-15 Contrarian <adrba-at-nyct.net> Re: [NYLXS - HANGOUT] NYLXS Library?
  30. 2010-03-15 Contrarian <adrba-at-nyct.net> Re: [NYLXS - HANGOUT] NYLXS Library?
  31. 2010-03-15 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  32. 2010-03-15 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  33. 2010-03-15 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  34. 2010-03-16 Ruben Safir <mrbrklyn-at-panix.com> Subject: [NYLXS - HANGOUT] C++ Workshop _ Syntax Basic: Flow Control Structures
  35. 2010-03-16 Ron Guerin <ron-at-vnetworx.net> Subject: [NYLXS - HANGOUT] NYLUG Workshop / Hacking Society (Smalltalk, C++, Python) Tuesday March 16 6:00PM-8:00PM
  36. 2010-03-16 redpill <redpill-at-rcn.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  37. 2010-03-16 redpill <redpill-at-rcn.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  38. 2010-03-17 Ruben Safir <mrbrklyn-at-panix.com> Subject: [NYLXS - HANGOUT] C++ Workshop - Switches and Break
  39. 2010-03-17 einker <eminker-at-gmail.com> Subject: [NYLXS - HANGOUT] Revolution OS at Darress Theatre
  40. 2010-03-20 Ron Guerin <ron-at-vnetworx.net> Subject: [NYLXS - HANGOUT] Free Software Round Table TONIGHT at 10PM (live streaming)
  41. 2010-03-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [NYLXS - HANGOUT] C++ Workshop - Syntax Basics -- functions
  42. 2010-03-21 Kevin Mark <kevin.mark-at-verizon.net> Re: [NYLXS - HANGOUT] C++ Workshop - Syntax Basics -- functions
  43. 2010-03-21 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] C++ Workshop - Syntax Basics -- functions
  44. 2010-03-21 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] C++ Workshop - Syntax Basics -- functions
  45. 2010-03-21 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  46. 2010-03-21 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  47. 2010-03-21 Kevin Mark <kevin.mark-at-verizon.net> Re: [NYLXS - HANGOUT] C++ Workshop - Syntax Basics -- functions
  48. 2010-03-21 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] C++ Workshop - Syntax Basics -- functions
  49. 2010-03-21 Ruben Safir <mrbrklyn-at-panix.com> Subject: [NYLXS - HANGOUT] Tina Gasperson commented on Darin Gasperson's status...
  50. 2010-03-21 redpill <redpill-at-rcn.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  51. 2010-03-22 From: "Beau Gould" <bg-at-capitalmarketsp.com> Subject: [NYLXS - HANGOUT] [JOB] Systems Engineer, NYC | 90-100k
  52. 2010-03-23 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  53. 2010-03-24 einker <eminker-at-gmail.com> Subject: [NYLXS - HANGOUT] Free Software Award Winners Announced
  54. 2010-03-24 einker <eminker-at-gmail.com> Subject: [NYLXS - HANGOUT] Free Software Award Winners Announced
  55. 2010-03-25 redpill <redpill-at-rcn.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  56. 2010-03-25 Ruben Safir <mrbrklyn-at-panix.com> Re: [NYLXS - HANGOUT] NYLXS Library?
  57. 2010-03-25 swd <sderrick-at-optonline.net> RE: [NYLXS - HANGOUT] NYLXS Library?
  58. 2010-03-30 Ron Guerin <ron-at-vnetworx.net> Subject: [NYLXS - HANGOUT] NYLUG Workshop / Hacking Society (Smalltalk, C++, Python) Tuesday March 30 6:00PM-8:00PM

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