Pointer addresses with '&'

[jryans10]

Hi,

I'm currently reading a c++ book, and I'm a bit confused about pointers.

Here is some of the code which I'm confused about:

cout << "Addresses of pointers..." << endl;
cout << "aPtr: " << &aPtr << endl;
cout << "bPtr: " << &bPtr << endl << endl;

cout << "Addresses of pointers..." << endl;
cout << "aPtr: " << &aPtr << endl;
cout << "bPtr: " << &bPtr << endl << endl;

To copy to clipboard, switch view to plain text mode 

Now, all this does is show me the addresses of 2 pointers, aPtr and bPtr.

But why do I need the '&'? I tried taking the '&' off and it still works, but the book has the '&' prefix in.

Thanks for your help.

[wysota]

Does it show the same values with and without &?

[jryans10]

Does it show the same values with and without &?

Hi,

I'm not too sure if they're the same, but they are both holding a hex address with or without the & operator.

Thanks,
- John.

[wysota]

If every character in two strings is the same then the value is the same. Otherwise (which is probably what you observe) they are different. In fact what & does is that it takes the address of its argument. So if you pass it an object, it will give you the address of the object (i.e. the pointer). If you pass it a pointer to an object, it will return the address of this pointer (i.e. another pointer that points to this pointer).

See the types below:

Object o;
Object *oPtr = &o;
Object **oPtrPtr = &oPtr;

Object o;
Object *oPtr = &o;
Object **oPtrPtr = &oPtr;

To copy to clipboard, switch view to plain text mode 

[jryans10]

Many thanks for the help.

But surely:

aPointer // a pointer
&aPointer

aPointer // a pointer
&aPointer

To copy to clipboard, switch view to plain text mode 

Is the same?

Thanks.

[wysota]

No. One is e.g. a pointer to object of type X and the other is a pointer to a pointer to an object of type X.

It's not easy to explain it in a short forum post but in general every variable is just an area in computer's memory that contains a hexadecimal value. So if you have a variable "o" of type "Object" then the object is placed in a memory area starting at some specific memory cell (address). Then you have a variable oPtr which is also a value in the memory. This value is the address of the beginning of memory area where "o" resides. But since oPtr is also a variable and is also placed somewhere in the memory, it also has an address. So oPtrPtr is another variable also positioned somewhere in memory space of your program that contains the value that points to the place where oPtr is.

So if "o" is positioned in the cell #1, oPtr is positioned in cell #11 and oPtrPtr is positioned in cell #12 then cell #12 will have the value "11" and cell #11 will have the value "1".

[jryans10]

Aaah I get it now.

Thank you very much for your help!