Thread: 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.

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?

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