英文:
How to access underlying struct field value if via interface field type in Golang?
问题
通过接口直接获取结构字段值是不可能的，但是有一些技巧可以实现。
在文件"./animals/mammal.go"中，你定义了一个接口"Mammal"：
type Mammal interface {
Sound()
}
在同一个文件中，你实现了一个结构体"Dog"：
type Dog struct {
Name string
Age int
}
func (d *Dog) Sound() {
fmt.Println("Say my name...", d.Name)
}
在另一个包"../../police/animals.go"中，你有另一个结构体"Policeman"：
type Policeman struct {
Animal Mammal
Policeman Name
ID int
}
你希望能够像这样实现它：
func (p *Policeman) Arrest() {
fmt.Println("Hands up, police! Bite him, ", p.Animal.Name)
}
但是编译器不允许这样做，因为"p.Animal"是一个接口类型。
然而，有一种应用设计技巧/模式可以实现你的需求。我的观点是，一旦我写了"Policeman"结构体，我希望确保当我将动物从狗改为猫或其他动物时，我不需要重写"Arrest"方法。这就是问题的关键所在！
英文:
Is there some trick (it is impossible directly) on how to get struct field values via interface?
I have an interface in a file ./animals/mammal.go:
type Mammal interface {
Sound()
}
I have implementation in the same file:
type Dog struct {
Name string
Age int
}
func (d *Dog) Sound {
fmt.Println("Say my name...", d.Name)
}
In other package ../../police/animals.go I have another struct:
type Policeman struct {
Animal Mammal
Policeman Name
ID int
}
And I wish to be able to implement it like this:
func (p *Policeman) Arrest {
fmt.Println("Hands up, police! Bite him, ", p.Animal.Name)
}
And compiler won't let me do that, because p.Animal is an interface type, of course...
However, is there an application design trick/pattern, that could allow me to do that. My point is, that once I wrote Policeman struct, I wish to be sure, that WHEN I CHANGE an animal from Dog to Cat or somebody else, I won't rewrite the "Arrest" method, so that's the point of the question!
答案1
得分: 1
如果在你的上下文中，以下陈述是真实的：“所有哺乳动物都有一个名字”，你可以在接口Mammal中添加一个方法Name()，你的示例将像这样工作：
type Mammal interface {
Sound()
Name() string
}
func (p *Policeman) Arrest {
fmt.Println("Hands up, police! Bite him, ", p.Animal.Name())
}
如果这个陈述不是真实的，那么你想要的是不可能的，因为对于没有名字的哺乳动物调用Name()方法的处理方式仍然不清楚。
请注意，你还需要为Dog实现Name()方法。
英文:
If, in your context, the following statement is true "All mammals have a name", you can add a method Name() to the interface Mammal and your example would work like this:
type Mammal interface {
Sound()
Name() string
}
func (p *Policeman) Arrest {
fmt.Println("Hands up, police! Bite him, ", p.Animal.Name())
}
If the statement is not true, then want you want is impossible because it remains unclear what to do with a call to Name() for a mammal that does not have a name.
Note that you then also need to implement the Name() method for Dog.
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