CODEWITHSUNDEEP
go
Natalie Perret
Natalie Perret

Reputation: 8997

Why `fmt.Println("%T\n", e)` where `e` is an error variable prints out <nil> and not its type?

I was recently playing with the Go language and I bumped into something a bit weird to say the least, let's consider a very simple function:

func main() {
    n, e := fmt.Println(`He said: "Hello"`)
    fmt.Printf("%T\n", n)
}

Which outputs what I was expecting:

He said: "Hello"
int

Now if I want to display the type of e:

func main() {
    n, e := fmt.Println(`He said: "Hello"`)
    fmt.Printf("%T\n", e)
}

and this time prints out:

He said: "Hello"
<nil>

I get the part that there is no error so e is an empty pointer: nil but I would have not expected to be a ~~type~~ on its own.

Why am I not getting the actual type?

If so, is there a workaround? (not saying my use case is a realistic one but curious if there is any possibility)

Upvotes: 2

Views: 127

Answers (2)

cn0047
cn0047

Reputation: 17071

e is an empty pointer

Nope, error itself - interface, so you won't have type here.

Upvotes: 2

peterSO
peterSO

Reputation: 166626

The Go Programming Language Specification

Errors

The predeclared type error is defined as

type error interface {
  Error() string
}

It is the conventional interface for representing an error condition, with the nil value representing no error.

Interface types

An interface type specifies a method set called its interface. A variable of interface type can store a value of any type with a method set that is any superset of the interface. Such a type is said to implement the interface. The value of an uninitialized variable of interface type is nil.

The zero value

When storage is allocated for a variable, either through a declaration or a call of new, or when a new value is created, either through a composite literal or a call of make, and no explicit initialization is provided, the variable or value is given a default value. Each element of such a variable or value is set to the zero value for its type: nil for interfaces.

A zero-value error type, an interface, has no type.Its value is nil.

Upvotes: 5

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