Difference between Golang struct literals & pointers when accessing struct fields

Question

I don't understand the difference between a struct literal and a struct pointer when accessing struct fields. Is there any different internal behavior ?

type Person struct {
    Name string
}

p := &Person{Name: "Alice"}
u := Person{Name: "Bob"}

fmt.Println(p.Name)  // any difference ?
fmt.Println(u.Name)  // any difference ?

I searched for this but posts I found all explain about difference between value & pointer, or "passing a value" vs "passing a pointer" to a method. They are not what I want to know.

Kerrek SB · Accepted Answer · 2015-12-29 01:59:44Z

11

u is a variable of type Person. p is a variable of type "pointer to Person", and it is initialized with the address of an anonymous ("temporary") object. The expression p.Name uses auto-dereferencing of pointers and is equivalent to (*p).Name. The object that p points to lives as long as p points to it and may thereafter be destroyed by Go's non-deterministic memory manager.

Both p.Name and u.Name are expressions of type string, and they're not "passed by pointer" since their address is not taken in the call. In the case of fmt.Println, the value is actually passed "by interface" using Go's structural subtyping form of ad-hoc polymorphism.

edited Dec 29, 2015 at 1:59

answered Dec 29, 2015 at 1:47

Kerrek SB

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2 Comments

kuang Over a year ago

Yes! "auto-dereferencing of pointers" is exactly the keyword that I wanted to search for but did not know. Your answer is very helpful and clear. Thank you.

Jordan Warbelow-Feldstein Over a year ago

For someone who is less familiar with Go, and pointers/refs in general, can this be explained in layman's terms? What are the practical implications?

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