通过fmt.Scan设置结构的值

Question

I want to synchronize state between map[string]string and custom Go structure and came to a conclusion that simplest way to parse it is to use fmt.Sscan for fields.

Unfortunately, direct approach is not working (playground):

var S struct{ I int }

f := reflect.Indirect(reflect.ValueOf(&S)).Field(0)
fmt.Sscan("10", f.Interface())
fmt.Println(S) // {0}

Introducing an intermediate value and using Set(), however, solves the issue:

nv := reflect.New(f.Type())
fmt.Sscan("10", nv.Interface())
f.Set(reflect.Indirect(nv))
fmt.Println(S) // {10}

I wonder why the first approach didn't work. Isn't Interface() returns a kind of reference to a field which can be used to change its value?

Answer 1

The Interface() method returns the value, not a reference to the value. Take the address (by calling Addr()) of the field to scan directly to the field:

var S struct{ I int }
f := reflect.Indirect(reflect.ValueOf(&S)).Field(0)
fmt.Sscan("10", f.Addr().Interface())  
fmt.Println(S) // {0}

https://play.golang.org/p/6_4DjiFZB2