MESSAGE
| DATE | 2010-03-03 |
| FROM | Ruben Safir
|
| SUBJECT | Re: [NYLXS - HANGOUT] C++ Workshop _ Syntax Basics - agregate data
|
On Wed, Mar 03, 2010 at 11:24:33PM -0500, Ruben Safir wrote:
>
> 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;
>
> }
I'd like to make a correction here as follows
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
>
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