Why would you want to use composition in golang?

Question

In the following code I show what I think is the difference between embedding (where methods get promoted) and composition (where methods are not promoted) in golang.

Why would you ever want to use composition in golang?

type obj1Inherited struct {
    obj2
}

type obj1Composed struct {
    someobj obj2
}

type obj2 struct {
}

func (o obj2) printTest() {
    fmt.Println("obj2")
}

func main() {
    o := obj1Inherited{}
    o.printTest() //fine - printTest is promoted

    obj1Composed := obj1Composed{}
    obj1Composed.someobj.printTest() //fine because I'm using the composed obj
    obj1Composed.printTest() //not fine - printTest is NOT promoted

Answer 1

It is worth going over the section on Embedding in Effective Go.

A common example is having a struct/map with a Mutex.

type SafeStruct struct {
    SomeField string 
    *sync.Mutex
}

It is much easier to type

safe := SafeStruct{SomeField: "init value"}

safe.Lock()
defer safe.Unlock()
safe.SomeField = "new value"

than having to either write appropriate wrapper functions (which are repetitive) or have the stutter of

safe.mutex.Unlock()

when the only thing you would ever do with the mutex field is access the methods (Lock() and Unlock() in this case)

This becomes even more helpful when you are trying to use multiple functions on the embedded field (that implemement an interface like io.ReadWriter).