Go Interfaces
Internal
Overview
Go has an unique interface system that allows modeling behavior rather than modeling types. An interface expresses the behavior of a type.
There is no need to declare that an interface is implemented, the compiler does the work of determining whether the values of a type satisfy an interface. So, if a type implements the methods of an interface, a value of the type can be stored in a value of that interface type and the compiler ensure the assignments are correct at compile-time.
Interfaces in Go tend to be small, exposing only a few functions, or just one function.
An interface is a type declaration that defines a method set. The method set is a list of method signatures designating methods a type must expose in order to implement the interface. See making a type implement an interface. The method set contains method signatures, so the interface type does not define behavior, but contract. The interfaces allow us to hide the incidental details of the implementation.
TODO Can only structs be interfaces, or there are other things that can be interfaces? "Go in Action" mentions named types.
Interface Name Convention
If the interface type contains only one method, the name of the interface starts with name of the method and ends with the er suffix.
When multiple methods are declared within an interface type, the name of the interface should relate to its general behavior.
Declaration
The interface declaration is introduced by the type keyword, to indicated that this is a user-defined type, followed by the interface name and the keyword interface, followed by the method set declaration between curly braces. Unlike in the struct's case, we don't define fields.
type MyInterface interface {
<function-name-1><function-signature-1>
<function-name-2><function-signature-2>
...
}
Example:
type A interface {
m1(i int) int
m2(s string) string
}
Making a Type Implement an Interface
A type (struct, anything else?) can be made to implement an interface by making the type to expose all methods from the interface's method set. "Exposing" a method means that a method with the same name and signature is declared to use the type in question as value or pointer receiver.
This is called duck typing. There is no "implements" or "extends" keyword in Go.
The example that follows shows how a type B implements interface A:
//
// The interface A declares a method set containing a single method m()
//
type A interface {
m()
}
//
// At this point, the struct B is not yet linked to the interface A in any
// way (it does not implement interface A)
//
type B struct {
i int
}
//
// We make B implement interface A by declaring B as a value receiver for
// m(). Pointer receiver also works. Since A only contains m(), B implements
// A by the virtue of duck typing
//
func (b B) m() {
// simply reports the value of i
fmt.Println(b.i)
}
...
//
// B now implements A, so A methods can be invoked on B
//
b := B{1}
b.m()
...
Interface instances can be used as arguments to functions. See passing interfaces to functions.
Passing Interfaces to Functions
Interfaces can be used as arguments to functions. Passing an interface instance insures that the function body can rely on the fact the interface methods are available on the passed instance. With the example above, we declare a function f() that expects an interface A and we pass a B instance when invoking the function:
...
func f(a A) {
a.m()
}
...
b := B{1}
f(b)
...
Note that the B instance can be passed by value (as in the example above) or by reference (as in the example below). Both cases work:
...
b := B{1}
f(&b)
...
or (same thing)
... bPtr := new(B) f(bPtr) ...
TODO further discussion on the merits of passing the interface by value and by reference.
Interfaces as Fields
Interfaces can be used as fields.