go - 使用go的struct指针作为接口(interface)

Question

我想将结构方法作为函数值传递。如果函数需要返回 interface{} 而它返回 *struct，为什么编译会失败？如果我尝试从声明为返回接口(interface){}(包装函数)的函数中返回 *struct，它会完美地工作。

package main

func main() {
        println("hello")
        testInterface(wrapper)          // works
        instance := MyStruct{}
        testInterface(instance.works)   // works
        testInterface(instance.fails)   // fails: ./main.go:8: cannot use instance.fails (type func(int) *MyStruct) as type func(int) interface {} in argument to testInterface
}

func testInterface(f func() interface{}) {
        f()
        return
}

type MyStruct struct {
}

func (s *MyStruct) works() interface{} {
        return s
}

func (s *MyStruct) fails() *MyStruct {
        return s
}

func wrapper() interface{} {
        s := MyStruct{}
        return s.fails()

Answer 1

那是因为它不适合 assignability标准

A value x is assignable to a variable of type T ("x is assignable to T") in any of these cases:

• x's type is identical to T.
• x's type V and T have identical underlying types and at least one of V or T is not a named type.
• T is an interface type and x implements T.
• x is a bidirectional channel value, T is a channel type, x's type V and T have identical element types, and at least one of V or T is not a named type.
• x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type.
• x is an untyped constant representable by a value of type T.

因此，这解释了为什么 testInterface(instance.fails) 失败:因为 instance.fails 不可分配给 f func() 接口(interface){} 。

现在第二个问题:

It perfectly works if I try to return *struct from function that is declared to return interface{} (wrapper func).

它工作正常，因为 *struct 类型的值可分配给 interface{} 类型，因为:

1. 这条可分配性规则:“T 是一个接口(interface)类型，x 实现了 T。”
2. “一个类型实现任何接口(interface)，包括其方法的任何子集，因此可以实现几个不同的接口(interface)。例如，所有类型都实现空接口(interface):”(粗体是我的)

引用资料:

• Type identity
• Assignability
• Interface types