modifiing pointer in other function

hi,

everytime i think i fully understand pointers it doesn't take a day for c++ to show me i'm not ;)

here's what I tried: main function decleares a pointer and the create-function should allocate memory and modify that pointer to point to that location.

Code:

---

#include <iostream>
#include <string>

using namespace std;

void create(string* x)
{
        cout << "adr x before: " << x << endl; // 0xbfb4c2fc
        x=new string("aaa");
        cout << "adr x after: " << x << endl; // 0x804a008
}

int main()
{
        string s;
        cout << "adr s bevore: " << &s << endl; // 0xbfb4c2fc
        create(&s);
        cout << "adr s after: " << &s << endl; // 0xbfb4c2fc
        return 0;
}

---

I think I know why this can't work as wanted (because the given pointer-parameter can't be modified right?) I think it should work using the pointer as return, but let's assume the return of the create function is needed for error checking.
The main question is: that is the way to make it work as wanted? do I have to pass a pointer to the pointer? but how? ...

thank you for comments.

string s - is some variable - you cannot change its address... only its value - you should pass it in the function by pointer or by reference to do it

string *pStr - is pointer to string, you can modify its value...
you can pass it also by pointer or by refeerence to modify it inside some function

Code:

---

void create(string** x)
{
        cout << "adr x before: " << *x << endl; //if it is not initialized - not a good idea to print it
        *x=new string("aaa");
        cout << "adr x after: " << *x << endl; // suppose new was successful
}

...
string* pStr = 0;
create(&pStr);

---

C++ has references for such problems

Code:

---

#include <iostream>
#include <string>

using namespace std;

void create(string *& x)
{
        cout << "adr x before: " << x << endl; // not initialized
        x= new string("aaa");
        cout << "adr x after: " <<  x << endl;
}

int main()
{
        string *s = 0;          // a pointer to a string ( as in your description )
        cout << "adr s bevore: " << s << endl; // was initialized to 0
        create(s);
        cout << "adr s after: " << s << " = " << *s << endl;
    delete s;  // dont forget about this
        return 0;
}

---

Kurt
EDIT: too late, but look at it as a variation of vart's solution

Quote:

---

Originally Posted by ZuK

Code:

---

    delete s;  // dont forget about this
}

---

---

Thank you both! Isn't the destructor called automaticly while the variable *s is going out of scope with the "}" of the main function?

Yup. But the memory allocated will not be freed back to the system.

No. You have to delete anything that was allocated with new.
Kurt
EDIT: The pointer to the string will be destructed ( that's a nop ) but not the object it points to ( the string ).

so for

Code:

---

{
  ...
  string s;
  ...
} //*

---

the destructor is called at //*

and for

Code:

---

{
  ...
  string* s = new string();
  ...
}

---

it is not and delete is required.

right.

all the parameter is passed by value,look at the program below

Code:

---

#include <iostream>
#include <string>

using namespace std;

void create(string* x)
{
        string* _x = x;
        cout << "adr x before: " << x << endl; // 0xbfb4c2fc
        _x = new string("aaa");
        cout << "adr x after: " << x << endl; // 0x804a008
}

int main()
{
        string s;
        cout << "adr s bevore: " << &s << endl; // 0xbfb4c2fc
        create(&s);
        cout << "adr s after: " << &s << endl; // 0xbfb4c2fc
        return 0;
}

---

this program is easy to understand, when the value of x assign to _x, x has no relation with the function create.
your program is similar to it,and the compiler help you to do string* _x = x and _x = new string("aaa")
remember, all the paramter is passed by value
now , let's see another program

Code:

---

#include <iostream>
#include <string>

using namespace std;

void create(string** x)
{
        cout << "adr x before: " << x << endl; // 0xbfb4c2fc
        *x = new string("aaa");
        cout << "adr x after: " << x << endl; // 0x804a008
}

int main()
{
        string* s = NULL;
        cout << "adr s bevore: " << &s << endl; // 0xbfb4c2fc
        create(&s);
        cout << "adr s after: " << &s << endl; // 0xbfb4c2fc
        return 0;
}
this program work perfectly, why?let's modify the function as the compiler always help us
void create(string** x)
{
        string** _x = x;
        *_x = new string("aaa");
}

---

_x and x are point to the same area of the memory, so *_x is *x