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?
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}