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
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