What does a pair of round brackets syntax expression mean in Go?

Question

(*Book)(nil)

Example:

count, err := db.Model((*Book)(nil)).Count()

What does it mean?

This was just asked 3 hours earlier. https://stackoverflow.com/q/60440743/13860 — Jonathan Hall, Feb 28 '20 at 08:53
The selected answer here is more concise and straightforward tho.. Helps for people who are just getting used to Go like me — taufn, Jul 07 '21 at 04:08

Burak Serdar · Accepted Answer · 2020-02-28T02:38:43.430

7

That is a type conversion. Assuming the db.Model function takes interface{}, it sends an nil interface of type *Book to the function.

To convert a value v to type Book, you'd write:

Book(v)

However, you cannot write Book(nil) because nil is a pointer and Book is not. If you had a type

type BookPtr *Book

Then you could've written BookPtr(nil). Extending that, you want to write *Book(nil), but that means *(Book(nil)) which is invalid, hence:

(*Book)(nil)

edited Feb 28 '20 at 02:38

answered Feb 27 '20 at 23:34

Burak Serdar

So this is needed in this case only? To convert a nil to *Type interface? – Fred Hors Feb 27 '20 at 23:39
If you're talking about typecasting, you'd need it if the value is not nil as well, like (*SomePtr)(someValue). You can use this, for instance, to copy a struct value to another with a different but compatible type. – Burak Serdar Feb 28 '20 at 01:56
Isn't converting value to type Book like this: (Book)v – Stan Nov 09 '22 at 05:26

score 2 · Answer 2 · answered Feb 27 '20 at 23:34

2

'nil' is to Go what NULL/null is to other languages like C#/Java, etc. The *Variable is just getting the pointer value for the Book object of Model.

So in this case, I believe what's happening here is that (*Book)(nil) is setting the pointer value of the Book object of the Model to nil(/null).

Hope this helps in some way.

answered Feb 27 '20 at 23:34

Dan Ferguson

So this is needed in this case only? To convert a nil to *Type interface? – Fred Hors Feb 27 '20 at 23:40
I'm sure there are other times you may want to do this, but commonly I'd say yes. – Dan Ferguson Feb 27 '20 at 23:49

2 Answers2