Denis Koroskin Wrote:

> On Tue, 30 Sep 2008 23:37:50 +0400, Eldar Insafutdinov <e.insafutdinov@gmail.com> wrote:
> 
> > http://www.everfall.com/paste/id.php?m46jrb36o7qu
> >
> > This is a short example. It has all the comments inside. Since "in" keyword tells the compiler to make a copy of an class object - a new object is created.
> 
> No, that's not true. No heap activity is done under the hood, a copy of
> /pointer/ is passed (as opposed to a copy of /instance/).
> In means that changes to the variable won't be visible to the caller:

In my particular case changes are made to the data behind the pointer - and they are not copied because, only pointer itself is copied. In C++ when you call a function like this:
foo(TClass instance);
copy constructor is called to create a proper copy of an object. "in" keyword means exactly the same I guess? it makes a local copy of an object, so that variable is not supposed to be modified. But in my case since I use a binding to a C-library - the real data is behind the pointer. foo() cannot modify the class fields themself, but since pointer both in variable and local copy point to the same data - the semantics of "in" doesn't work.
>>     auto itCopy = it.clone();
Why copy it manually if language tends to do it so by providing appropriate keyword. I should only write a copy constructor for my data and let to do the rest to the compiler.

Eldar Insafutdinov:
> >>     auto itCopy = it.clone();
> Why copy it manually if language tends to do it so by providing appropriate keyword. I should only write a copy constructor for my data and let to do the rest to the compiler.

In D all objects are managed by reference. So when you copy automatically, you are just copying its reference, that's a pointer fully managed by the GC. If you want a copy of the data of an object you ask so to it. This simplifies collections and reduces the number of copies, improving performance.

Instead of "clone()" I suggest "dup", that's the standard in D.

Bye,
bearophile

Reply to bearophile,

> Eldar Insafutdinov:
> 
>>>> auto itCopy = it.clone();
>>>> 
>> Why copy it manually if language tends to do it so by providing
>> appropriate keyword. I should only write a copy constructor for my
>> data and let to do the rest to the compiler.
>> 
> In D all objects are managed by reference.

BTW note that structs ARE pass by value.

On Wed, Oct 1, 2008 at 5:34 AM, bearophile <bearophileHUGS@lycos.com> wrote:
> Eldar Insafutdinov:
>> >>     auto itCopy = it.clone();
>> Why copy it manually if language tends to do it so by providing appropriate keyword. I should only write a copy constructor for my data and let to do the rest to the compiler.
>
> In D all objects are managed by reference. So when you copy automatically, you are just copying its reference, that's a pointer fully managed by the GC. If you want a copy of the data of an object you ask so to it. This simplifies collections and reduces the number of copies, improving performance.
>
> Instead of "clone()" I suggest "dup", that's the standard in D.
>

It has been pointed out though, that dup is a "shallow copy" in D's built-in usage.  If you want a deep copy operation, there is no precedent in the base language, I believe.  Clone is a good one to standardize on for deep copies, I think.

--bb