英文:
Understanding type assertions with interfaces and pointers in Go
问题
让我们考虑以下程序：
type Fruit interface {
Color() string
}
type Apple struct {
color string
}
func (x *Apple) Color() string {
return x.color
}
func (x *Apple) Compare(y Fruit) bool {
_, ok := y.(*Apple)
if ok {
ok = y.Color() == x.Color()
}
return ok
}
func main() {
a := Apple{"red"}
b := Apple{"green"}
a.Compare(&b)
}
现在，请注意最后一行的a.Compare(&b)。在这里，我传递了一个指向Apple的指针。这样做是正确的，但请注意，我的Compare函数并没有接受指针(y Fruit)。
现在，如果我将最后一行改为a.Compare(b)，那么它会给我以下错误：
cannot use b (type Apple) as type Fruit in argument to a.Compare:
Apple does not implement Fruit (Color method has pointer receiver)
这里发生了什么？
英文:
Let's consider the following program
type Fruit interface {
Color() string
}
type Apple struct {
color string
}
func (x *Apple) Color() string {
return x.color
}
func (x *Apple) Compare(y Fruit) bool {
_, ok := y.(*Apple)
if ok {
ok = y.Color() == x.Color()
}
return ok
}
func main() {
a := Apple{"red"}
b := Apple{"green"}
a.Compare(&b)
}
Now, note the last line that says a.Compare(&b). Here I am passing a pointer to Apple. This works correctly, but note that my Compare
function does NOT accept the pointer (y Fruit).
Now if I change the last line to say a.Compare(b) then it gives me the following error:
cannot use b (type Apple) as type Fruit in argument to a.Compare:
Apple does not implement Fruit (Color method has pointer receiver)
What is ing on here?
答案1
得分: 3
由于这个答案中所述的原因，Apple没有实现Fruit，但*Apple有。如果你在Apple上定义Color（而不是*Apple），代码将会编译通过：
package main
import (
"fmt"
)
type Fruit interface {
Color() string
}
type Apple struct {
color string
}
func (x Apple) Color() string {
return x.color
}
func (x Apple) Compare(y Fruit) bool {
_, ok := y.(Apple)
if ok {
ok = y.Color() == x.Color()
}
return ok
}
func main() {
a := Apple{"red"}
b := Apple{"green"}
fmt.Println(a.Compare(b))
}
英文:
For reasons outlined in this answer, Apple does not implement Fruit, but *Apple does. If you define Color on Apple (instead of *Apple), the code compiles:
package main
import (
"fmt"
)
type Fruit interface {
Color() string
}
type Apple struct {
color string
}
func (x Apple) Color() string {
return x.color
}
func (x Apple) Compare(y Fruit) bool {
_, ok := y.(Apple)
if ok {
ok = y.Color() == x.Color()
}
return ok
}
func main() {
a := Apple{"red"}
b := Apple{"green"}
fmt.Println(a.Compare(b))
}
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