Why Can\'t Go Structs with Value Receivers Fulfill Interfaces with Pointer Receivers?

Why Can't Go Struct Variables Implement Interfaces with Pointer Receivers?

In Go, struct methods can have either value receivers (which operate on the struct value directly) or pointer receivers (which operate on a pointer to the struct). When implementing interfaces, there are subtle restrictions on which types can satisfy the interface depending on the receiver type of the interface methods.

Consider the following code:

type greeter interface {
    hello()
    goodbye()
}

type tourGuide struct {
    name string
}

We can define methods with both value and pointer receivers for tourGuide:

func (t tourGuide) hello() {
    fmt.Println("Hello", t.name)
}

func (t *tourGuide) goodbye() {
    fmt.Println("Goodbye", t.name)
}

Now, here's the key difference:

• Variables or copies of tourGuide can call both value and pointer receiver methods:

  var t1 tourGuide = tourGuide{"James"}
  t1.hello()   // works
  t1.goodbye() // works

• However, interface variables can only be assigned from types that can implement all the interface methods with the same receiver type as the interface itself:

  var g2 greeter = &t2 // works because *tourGuide implements greeter's pointer receiver methods
  
  // illegal: t1 is not assignable to g1
  // var g1 greeter = t1 // error: cannot use tourGuide as type greeter in assignment because tourGuide does not implement greeter (hello method has value receiver)

So, why can't t1 implement the greeter interface?

The reason is that the hello() method in the greeter interface has a pointer receiver, meaning it expects a pointer to a tourGuide value as the receiver. However, t1 is a variable (not a pointer). Even though value variables are addressable, interface values hold copies of the stored values, not references. Therefore, when assigning a value variable to an interface variable, it creates a new copy that cannot be dereferenced and passed as the receiver for pointer receiver methods.

In summary, when implementing interfaces with pointer receiver methods, only pointers to the implementing type can satisfy the interface. Value variables or copies are not assignable to such interfaces because they cannot provide the required pointer receiver type.

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