Thread: char* behaves strangely

Hello,

I've cast the address of a string to a char* . I expected to be able to use the pointer in the same way as other types, but it behaves quite differently. Could someone explain what is going on?

Why does it print the string and not the address? How does it know where the string ends? Ho *do* I print the address of a char in the string, like for e[2] (I would expect this to be the address of the 3rd letter, s)?

Here is the code. I've compared it to an int*.

Code:

#define DEBUG
#ifdef DEBUG                                    
#define P(A) cout <<  #A << ": " << (A) << endl;// prints variables
#define NL cout << endl;                        // new line
#else
#define P(A)                                    // empty definitions
#define NL
#endif

#include <iostream>
#include <string>
using namespace std;

void main() {

  int a[2];
  a[0] = 5; a[1] = 7;
  string s = "test text";
  
  int* ap = a;
  char* e = (char*)s.c_str();  // string elements

  NL; P(s); P(&s);

  NL; P(e); P(e[0]); P(&e); P(&e[0]); P(&e[2]); P(&(e[0]));

  NL; P(ap); P(ap[0]); P(&ap); P(&ap[0]); P(&ap[1]); P(&(ap[0]));
}

And here is the output

Code:

s: test text
&s: 0012FF7C

e: test text
e[0]: t
&e: 0012FF40
&e[0]: test text
&e[2]: st text
&(e[0]): test text

ap: 0012FF50
ap[0]: 5
&ap: 0012FF44
&ap[0]: 0012FF50
&ap[1]: 0012FF54
&(ap[0]): 0012FF50

char *text=0;

text="This will work"


MyClass *newclass;

newclass=new MyClass();



This doesn't work? Yes, it does.

The pointer returns c_str() is read only. I think alot of c++ string implementations use referance counting so this could cause a problem if you write something like

Code:

#include <string>
#include <iostream>
using namespace std;


int main(void)
{
    string s = "Hello World!\n";
    string r = s;

    *(char*)r.c_str() = 'A';  // modify r should not modify s

    cout << "s = " << s;
    cout << "r = " << r;

    return 0;
}

It works (does not work) on g++
s = Aello World!
r = Aello World!

Thanks for the answers guys.

Bubba wrote:

char *text=0;

text="This will work"

How does the program know not to read any more characters from memory?


zMan wrote:

now if instead I did this this should not work...
char* f;

f = "c:\\my documents\\myfile.txt";

I believe this works in exactly the same way as Bubba's code. Only the pointer is undetermined at start.


Nick, I'm a beginner so I don't know much about string, but if the string variable is a pointer to a char array then that would be the correct result, because r and s are pointing to the same memory space. I got the same from the Borland compiler when I ran your code.

I also would like to know the answers to these questions:

How do I get the address that a char* is pointing to? I've tried to prefix with & but I believe this produces the address of the pointer itself.

How would I get the address of any character that is in a character array?

Seron

>How does the program know not to read any more characters from memory?

because the string literal "This will work" will be null terminated.

>I believe this works in exactly the same way as Bubba's code. Only the pointer is undetermined at start.

Yep, "c:\\my documents\\myfile.txt" is a string literal. You're not storing this string at the memory pointed to by f, but pointing f at the string literal.

>How do I get the address that a char* is pointing to?

You can get the address of it's first element using array notation -

&f[0];

>How would I get the address of any character that is in a character array?

You can get the address the same way as above, but use the appropriate index.

Sorensen wrote:

>How do I get the address that a char* is pointing to?

You can get the address of it's first element using array notation -

&f[0];

>How would I get the address of any character that is in a character array?

You can get the address the same way as above, but use the appropriate index.

Ok, but how would I display those addresses? I've tried the below code and it doesn't work.

Code:

#include <iostream>
using namespace std;

void main() {

  char c[] = "test text";
  			// output
  cout << c << endl;	// test text
  cout << c[2] << endl;	// s
  cout << &c << endl;	// 0012FF80	// is this the address the pointer is pointing to
  					// or the address of the pointer variable ?
  cout << &c[0] << endl;// test text
  cout << &c[2] << endl;// st text
}

Seron

You could use printf, or -

cout << hex << (long)&c[0];