Created
July 19, 2015 02:51
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Surprising behavior for value types with pointer methods.
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| package main | |
| import "fmt" | |
| type T struct { | |
| Thing string | |
| } | |
| func (t *T) Set(thing string) { | |
| t.Thing = thing | |
| } | |
| func SetT(t *T, thing string) { | |
| t.Thing = thing | |
| } | |
| type Setter interface { | |
| Set(thing string) | |
| } | |
| func SetSetter(s Setter, thing string) { | |
| s.Set(thing) | |
| } | |
| func main() { | |
| // Surprising behavior for value types with pointer methods. | |
| // | |
| // Everyone knows that you can trivially call a method defined on a value | |
| // (e.g. 'func (t T) SomeFunc() {}; pointerT := &T{}; t.SomeFunc()'). This | |
| // is simple to explain conceptually: the pointer is copied and used in the | |
| // body of the method as a value type. | |
| // | |
| // But did you know that you can call pointer methods on value types? | |
| var t T = T{Thing: "HI"} // t is of type T (a value, not a pointer). | |
| fmt.Println(t) // Prints "{HI}" | |
| // SetT takes a pointer of T. We must give it t's address (&t) because t | |
| // is a value type, not a pointer type. 'SetT(t, "BRO")' would error. | |
| SetT(&t, "BRO") | |
| fmt.Println(t) // Prints "{BRO}" | |
| // Set is a method defined on the *pointer* T. Even though t is a value, | |
| // not a pointer, THIS IS STILL VALID AND IT MODIFIED t. WTF?? | |
| t.Set("YO") | |
| fmt.Println(t) // Prints "{YO}" | |
| // MORAL OF THE STORY: No matter whether you define a variable as a value | |
| // or a pointer, you can still call pointer methods on it and modify your | |
| // variable. | |
| // | |
| // This basically turns all types into reference types (like Java) when | |
| // methods are called on them. | |
| // BONUS: For interfaces, pointer types have all of the methods defined on | |
| // pointers and values for a type, but value types only have the methods | |
| // defined on values for a type. | |
| // SetSetter(t, "HELLO") // -> error! T does not implement Setter (Set method has pointer receiver) | |
| SetSetter(&t, "HI THERE") // Totally fine. | |
| } |
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