Go接口用于一组约束和继承

Question

I am still trying to understand Golang interface. Please correct me and help me understand.

Frances Campoy explains, interface is a set of contraints.

So in my case, let's say I have one Store interface that is to be interfaced with contrains, like sort Interface in Go.

type Store interface {
    Earning() int
    Expense() int
}

Then what do I have to do if I want to implement this interface constraint to other packages like StoreA, StoreB? I want to get a message like when I try:

aa := StoreC{}
aa.Add("AAA")
// error saying `StoreC` does not implement method `Add`, constrained by interface `Store`... something

So what do I do if I want to enforce this contraint in other Stores? Do I need to do something like inheritance?

type StoreC interface {
    Store
}

In other words, I wonder how package sort's interface of 3 methods can be enforced to every sort operations in Go. How does it enforce the contraints to any other possible sort uses in Go?

Thanks!

Answer 1

An interface is mostly useful for a user of the interface. By looking at what methods the interface provides, you know what methods you can safely use.

If you have an interface:

type MyInterface interface {
   Frob(int)
   Flanged() bool
}

You know that any function func foo(thing MyInterface) can safely call thing.Frob(..) and thing.Flanged() safely.

Any type that implements both a func (r a_type) Frob(int) and func (r a_type) Flanged() bool will satisfy the interface. The compiler will do this check at compile-time.

Defining a method looks (roughly) like this:

func (reciever a_type) methodname (possibly more args here) possibly_a_return_type {...}

The receiver's type is the type for which the method is defined. The name of the method comes between and then there's the (possibly empty) argument list.