• Ian (13/13) Sep 28 Hi,
• Andy Valencia (17/28) Sep 28 An instance of a class is, of course, an object. The instance's
• H. S. Teoh (12/21) Sep 28 Passing a pointer or ref to a class variable allows the callee to modify

Ian <ian iangarcia.net> writes:

Hi,

I'm coming from C and some C++ so the way D stores class instance 
variables is new to me. If I'm not mistaken the basic unadorned 
instance variable is like a "hidden" pointer. So, when passing 
class instance variables to a function, what would be the point 
of passing a pointer or ref?

I think I answered myself, in that they'd would be pointers or 
references to the variable that holds the... hidden pointer to 
the class instance.

Now I'm unsure. When I pass a class instance to a function by 
value, I'm not creating a copy of the instance, am I?

Thanks,
  Ian

Andy Valencia <dont spam.me> writes:

On Saturday, 28 September 2024 at 18:16:55 UTC, Ian wrote:
 Hi,

 I'm coming from C and some C++ so the way D stores class 
 instance variables is new to me. If I'm not mistaken the basic 
 unadorned instance variable is like a "hidden" pointer. So, 
 when passing class instance variables to a function, what would 
 be the point of passing a pointer or ref?

An instance of a class is, of course, an object.  The instance's 
variables can be int's or float's or struct's.  Values.  The 
instance has storage right there to hold the value.

An instance can also reference another class instance.  Then, 
underneath, that's a pointer.

 I think I answered myself, in that they'd would be pointers or 
 references to the variable that holds the... hidden pointer to 
 the class instance.

I think I see you assuming that how an instance variable treats 
values is different from how a local variable or a struct field 
would treat a value.  My experience is they're all the same.  The 
important difference between struct and class instance is that 
structs want to be values, and you have to go to extra trouble to 
work with pointers thereof. Instances want to be references, and 
you have to go to trouble (shallow or deep copy, presumably) if 
you want to get a value copy.  But all this applies equally to 
instance variables and a struct's fields.

 Now I'm unsure. When I pass a class instance to a function by 
 value, I'm not creating a copy of the instance, am I?

No you aren't.

(Now let the much deeper Dlang minds sweep in and correct me.)

"H. S. Teoh" <hsteoh qfbox.info> writes:

On Sat, Sep 28, 2024 at 06:16:55PM +0000, Ian via Digitalmars-d-learn wrote:
 Hi,
 
 I'm coming from C and some C++ so the way D stores class instance
 variables is new to me. If I'm not mistaken the basic unadorned
 instance variable is like a "hidden" pointer. So, when passing class
 instance variables to a function, what would be the point of passing a
 pointer or ref?

Passing a pointer or ref to a class variable allows the callee to modify
the reference (e.g., make it point to a different instance of the
class).

Passing a class instance by value passes a reference to it, so the
callee can still modify the instance, but they cannot change the
caller's reference to it.


[...]
 Now I'm unsure. When I pass a class instance to a function by value,
 I'm not creating a copy of the instance, am I?

No, because classes are by-reference types.


T

-- 
In theory, software is implemented according to the design that has been
carefully worked out beforehand. In practice, design documents are written
after the fact to describe the sorry mess that has gone on before.