Go: proposal: spec: replace &T{} composite literals with (*T){}

[alercah]

The use of &T{} for composite literals is strange, and in particular inconsistent with the way you write a type. Consider the following code:

foo := struct{
  m map[int]string
  a [10]int
  sl []string
  st struct{} 
  p *struct{}
}{
  m: map[int]string{},
  a: [10]int{},
  sl: []string{},
  st: struct{},
  p: &struct{},
}

p is the only member for which the type declared does not match the type used in the composite literal. The logical fix is to make a composite literal of type *T mean a composite literal to a pointer to a new value of type T initialized from the literal, in place of &T.

This is probably incompatible with #20446.

[cznic]

foo := struct{
        p *int
}{
        p: &i,
}

[mattn]

&T have two meanings.

• create value T from literal
• and get address of the value

If want to create new int value and get pointer of the value, we can't do it without new builtin. (ex: new(int)) If you mean a := *int or a := *int{} should have meaning to get pointer of new value, compiler will have to look backward literals like C. Then compiler will be slow.

[aarzilli]

But then what's the dereference operator? I think the most logical thing would be to use & in the type signature of pointers, so that if you have a var i int then &i's type is &int and the type signature and literal expression also match. And there is a historical precedent (in C++) for &T being a pointer to T.

But I don't think it's worth it.

[henryas]

If you are to replace &T{} with *T{}, I would be confused. I am used to seeing the _pointer/deref_ (*) and the _address of_ (&) operators. I think Go's usage of them has been consistent with other C-family programming languages. If you are to replace those, you will create unnecessary confusion to many people, considering the popularity of C/C++ and the people's familiarity with them.

I don't agree with this proposal.

[bcmills]

Note that in your example, the lines

  st: struct{},
  p: &struct{},

must today be written as

  st: struct{}{},
  p: &struct{}{},

(two pairs of braces each).

I suspect that, to avoid a parsing ambiguity, the latter case would need to be written as

  p: (*struct{}){},

but today that syntax is rejected by the compiler (https://play.golang.org/p/6GxjWrCfgN).

[bcmills]

Also note that under https://github.com/golang/go/issues/12854 they could be written even more uniformly:

foo := struct{
  m map[int]string
  a [10]int
  sl []string
  st struct{} 
  p *struct{}
}{
  m:  {},
  a:  {},
  sl: {},
  st: {},
  p:  {},
}

[ianlancetaylor]

This idea was discussed during the development of Go 1 (that is, as a change from the earlier versions of Go to the standarized Go 1), but was rejected at that time.

[andlabs]

It is important to understand that & and * aren't interchangeable tokens, but rather opposite and complementary. & means "take the address of value", and * means "get the value at address". From this, it makes no logical sense to say *T{}, because you would wind up reading this as "take the value at the address T{}", and then you realize T{} isn't an address and so the statement makes no sense.

Perhaps memorizing this would help you stop confusing &T{} and *T{} as much (as you said in the proposal for #20446).

This also explains @cznic's comment: what if you want to point to a variable that already exists in a composite literal? (The line that is missing from their comment is var i int.)

[bcmills]

@andlabs The * operator is distinct from the * type-constructor. The & operator is dual to the * operator, but the * type-constructor has no dual.

&T{} is the _expression_ "address of a literal of type T", but *T is the _type_ "pointer to T". The * _operator_ is not involved in this proposal in any visible way.

[andlabs]

@bcmills Yes, but is &T{} in the context of an assignment or composite literal also not an expression? Given what you said, another way to say what I was trying to say is that this proposal confuses and conflates the * type-constructor with the * operator, if that makes more sense.

[bcmills]

The way I read this proposal, it is that &T{} should be read as "the address of a literal of type T", and *T{} should be read as "a literal of type *T". (The latter is currently rejected as a syntax error.)

[andlabs]

Ah, yes, that is different. If we do expand composite literals to all types (a more general version of your reading of the proposal), by current rules it would have to be (*T){...}.

Isn't the current situation due to ambiguity between the binary * operator? A potential solution would be to drop * for indirection, but that seems to be beyond the scope of the current proposal phase for Go 2?

[ianlancetaylor]

There doesn't appear to be strong support for this, and you say yourself it would be incompatible with #20446 / #9097. It was also discussed years ago and rejected. Closing.