英文:
Creating a map of constructor functions
问题
我想要一个函数名称的映射表，根据环境变量选择接口的实现方式。我在下面的代码中重现了这个问题：
package test
type Fooer interface {
Foo() error
}
type Bar struct{}
func NewBar() (*Bar, error) { return &Bar{}, nil }
func (b *Bar) Foo() error { return nil }
type Baz struct{}
func NewBaz() (*Baz, error) { return &Baz{}, nil }
func (b *Baz) Foo() error { return nil }
var constructors map[string]func() (*Fooer, error)
func init() {
constructors = map[string]func() (*Fooer, error){
"bar": NewBar,
"baz": NewBaz,
}
}
当我运行go build test.go时，会出现以下错误：
./test.go:21: cannot use NewBar (type func() (*Bar, error)) as type func() (*Fooer, error) in map value
./test.go:22: cannot use NewBaz (type func() (*Baz, error)) as type func() (*Fooer, error) in map value
那么我做错了什么？我能够将*Fooer作为构造函数的返回类型吗？实际上，最好的解决方法是什么？（我是Go的新手）
英文:
So I'd like to have a map of names of functions, to choose an implementation of an interface based on an environment variable. I've reproduced it in the following code:
package test
type Fooer interface {
Foo() error
}
type Bar struct{}
func NewBar() (*Bar, error) { return &Bar{}, nil }
func (b *Bar) Foo() error { return nil }
type Baz struct{}
func NewBaz() (*Baz, error) { return &Baz{}, nil }
func (b *Baz) Foo() error { return nil }
var constructors map[string]func() (*Fooer, error)
func init() {
constructors = map[string]func() (*Fooer, error){
"bar": NewBar,
"baz": NewBaz,
}
}
This throws the following errors when I go build test.go:
./test.go:21: cannot use NewBar (type func() (*Bar, error)) as type func() (*Fooer, error) in map value
./test.go:22: cannot use NewBaz (type func() (*Baz, error)) as type func() (*Fooer, error) in map value
So what am I doing wrong? Can I use a *Fooer as the return type of a constructor function? What would be the actual best way to approach this? (I'm new to Go)
答案1
得分: 2
- 不要（几乎不要）传递指向接口的指针。你（几乎）不需要一个
*Fooer，一个Fooer就足够了。 - 是的，你的 Bar 和 Baz 构造函数可以返回一个
Fooer（而不是*Fooer！），但这似乎有些尴尬。 - 将构造函数保存在一个映射中，并查询映射以获取构造函数，似乎有一层过多的间接性。你是否经常创建新的 Bar 和 Baz？
英文:
- Do not (almost never) pass around pointers to interface. You will (almost) never need a
*Fooer, aFooeris enough. - Yes, your Bar and Baz constructors could return a
Fooer(not a*Fooer!) but that seems awkward. - Keeping the constructors in a map and querying the map for a constructor seem like one level of indirection too much. Are you constantly creating new Bar and Baz?
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