英文:
Golang: Genric struct implement does not match generic interface
问题
我正在尝试使用Go泛型，并遇到了这个问题。为什么Schedule不能匹配any？
代码
func demo(m Model[any]) Model[any] {
return m
}
type Model[M any] interface {
FindOne(db *gorm.DB, conds ...any) (m M)
}
type DbOperations[M any] struct{}
func (d DbOperations[M]) FindOne(db *gorm.DB, conds ...any) (m M) {
res := db.First(&m, conds)
if res.Error != nil {
return m
}
return m
}
// 由于这个函数，程序无法编译通过
func a() {
m := DbOperations[Schedule]{}
get(m) // <= 错误发生在这里
}
错误信息
../models/operations.go:103:6: 无法将类型为DbOperations[Schedule]的变量m用作get函数的Model[any]类型参数：
DbOperations[Schedule]未实现Model[any]接口（FindOne方法类型不正确）
具有FindOne(db *gorm.DB, conds ...any) (m Schedule)
期望FindOne(db *gorm.DB, conds ...any) any
英文:
I am experimenting with go generics, and I have encountered this issue. Why is Schedule not matching any?
Code
func demo(m Model[any]) Model[any] {
return m
}
type Model[M any] interface {
FindOne(db *gorm.DB, conds ...any) (m M)
}
type DbOperations[M any] struct{}
func (d DbOperations[M]) FindOne(db *gorm.DB, conds ...any) (m M) {
res := db.First(&m, conds)
if res.Error != nil {
return m
}
return m
}
// the program does not compile because of this function
func a() {
m := DbOperations[Schedule]{}
get(m) // <= error occurs here
}
Error message
../models/operations.go:103:6: cannot use m (variable of type DbOperations[Schedule]) as type Model[any] in argument to get:
DbOperations[Schedule] does not implement Model[any] (wrong type for FindOne method)
have FindOne(db *gorm.DB, conds ...any) (m Schedule)
want FindOne(db *gorm.DB, conds ...any) any
答案1
得分: 1
Go语言通常要求你明确指定你使用的类型，并且通常不会自动为你转换变量的类型。所以这段代码不起作用，因为Model[any]（也就是Model[interface{}]）和Model[Schedule]不是相同的类型。（更多细节如下）
你可以尝试将get函数改为泛型函数：
func get[T any](m Model[T]) Model[T] {
return m
}
然后在调用函数时指定类型：
func a() {
m := DbOperations[Schedule]{}
get[Schedule](m)
}
详细说明
Model[any] != Model[SomeType]的原因与你不能将字符串切片传递给接受any切片参数的函数相同：
[]interface{}切片与[]Schedule切片具有不同的内存布局。同样，Model[T]在内存中的布局与Model[any]也不同。Go语言希望你自己完成它们之间的转换工作。（总结自官方解释和这里的Stack Overflow回答）
英文:
The go language usually requires you to be explicit about the types you're using, and generally does not automatically cast variables for you. So this doesn't work because Model[any] (aka Model[interface{}]) and Model[Schedule] are not the same types. (more details below)
What you could do instead, is also make get a generic func:
func get[T any](m Model[T]) Model[T] {
return m
}
and then indicate what the type is when you call the func:
func a() {
m := DbOperations[Schedule]{}
get[Schedule](m)
}
Details
Model[any] != Model[SomeType] for the same reason why you can't pass a string slice to a func that takes an any slice parameter:
A []interface{} slice has a different memory layout than a []Schedule slice. Similarly, a Model[T] is laid out in memory differently than a Model[any]. Go expects you to do the work of converting them. (summarizing the official explanation and SO answers here)
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