Assignment: shallow or deep copy?

[Kejia]

Hello people,

I don't have a go environment and just have a simple question: Is
assignment to a variable a shallow copy or deep copy? For example:
``
type Pet structure {...}

var huski Pet = new(Pet)
dog := huski
''
Are both `dog' and `huski' pointing from the same address?

[Andrew Gerrand]

A go assignment is a copy of the value itself.

If you assign a pointer value, a copy of that pointer value is made.
The thing the pointer points at is not copied.

> type Pet structure {...}
>
> var huski Pet = new(Pet)
> dog := huski

This code won't compile. new(Pet) returns a value of type *Pet, while
the variable huski is of type Pet.

In this example, huski and dog are both pointers to the same Pet:

var huski *Pet = new(Pet)
dog := huski

In this example, dog is a copy of huski, which is a Pet value:

var huski Pet = Pet{}
dog := huski

You can then modify the huski value without affecting dog.

Andrew

[Ostsol]

In Go, almost everything is passed by value. Maps, slices, and
channels are reference types, so when passing one the receiver points
to the original object. In order to pass a reference to a struct, you
must pass a pointer. A correction to the code you posted would be:

type Pet struct { ... }

var huski *Pet = new(Pet)
dog := huski

'dog' now points the same Pet as 'huski'. 'dog's type is '*Pet', like
'huski'. (Also note that 'new' always returns a pointer.) Consider
the following code, though:

dog := *huski

The asterisk dereferences 'huski', so 'dog' is a copy of 'huski', and
'dog's type is 'Pet'.

-Daniel

[Ostsol]

Addendum: even pointers are simply values, and as such are passed by
value. A map, slice, or channel is a pointer to underlying data, but
it is a pointer that cannot be dereferenced, like other pointers. It
is best to program in Go not thinking in terms of Java or Python, but
C.

-Daniel

[Corey Thomasson]

This is really an implementation detail More than a spec detail. Slices and friends are for specification purposes a reference type. I believe That currently they're actually an underlying structure passed by value and the structure contains a pointer to the referenced data. Iirc

On 9 Jan 2011 20:15, "Ostsol" <ost...@gmail.com> wrote:

Addendum: even pointers are simply values, and as such are passed by
value.  A map, slice, or channel is a pointer to underlying data, but
it is a pointer that cannot be dereferenced, like other pointers.  It
is best to program in Go not thinking in terms of Java or Python, but
C.

-Daniel

On Jan 9, 6:08 pm, Ostsol <ost...@gmail.com> wrote:

> In Go, almost everything is passed by value. ...

[Kejia]

Thank you for the answers.

[Tibi]

In terms of optimization, do we need to think whether a variable
should be passed by value or by pointer value?
I mean if we would like to pass a big struct I guess it is better to
pass it by pointer value although we might not want to might not want
to modify the underlying data.

Tibor

[xophos]

I would very much hope, that the go compiler (or a future Version of
it) can detect cases where a Struct is not written to and use a
pointer internally or even always using a copy on write mechanism.
In this case the Performance should be similar.

[Johann Höchtl]

On Jan 10, 1:06 pm, xophos <xop...@gmail.com> wrote:
> I would very much hope, that the go compiler (or a future Version of
> it) can detect cases where a Struct is not written to and use a
> pointer internally or even always using a copy on write mechanism.
> In this case the Performance should be similar.
>

Such an optimisation would yield different behaviour dependent on
optimisation. Just think about interacting with an FFI. The receiver
has to know if it can expect a literal or a pointer. Only whole
program optimisation would make such optimisations possible.

Johann

[Kejia]

It's tragedy to raise another big fat as Java :-)