英文:
Difference in string and *string in Gorm model declaration
问题
gorm的文档https://gorm.io/docs/models.html中给出了以下示例。
字段Name和Email的类型分别为string和*string。
这里的主要区别是什么？
另外，如何为存储图像链接列表的Images字段提供数据类型？
应该是[]string还是[]*string？
type User struct {
ID uint
Name string
Email *string
Images []string
Age uint8
Birthday *time.Time
MemberNumber sql.NullString
ActivatedAt sql.NullTime
CreatedAt time.Time
UpdatedAt time.Time
}
英文:
The docs of gorm https://gorm.io/docs/models.html present an example below.
The field Name and Email are described with string and *string.
What is the main difference here?
Also how to provide the datatype for the images field storing a list of images link?
Should it be []string or []*string?
type User struct {
ID uint
Name string
Email *string
Images []string
Age uint8
Birthday *time.Time
MemberNumber sql.NullString
ActivatedAt sql.NullTime
CreatedAt time.Time
UpdatedAt time.Time
}
答案1
得分: 9
Go语言对于每种基本数据类型都有默认值。
int -> 0，string -> ""，bool -> false等等。所以如果你需要添加空值，或者将空值加载到变量中，它应该是一个指针。否则它会被默认为零值。
指针的默认值在Go中是nil。
而复杂的数据类型，比如切片（slices）和映射（maps），会保留引用。所以它们的默认值是nil。因此，在这个例子中，Images []string中的images可以是nil。
下面的代码使用指针类型User1和非指针类型User2展示了默认值的差异。
package main
import (
"fmt"
"time"
)
type User1 struct {
Email *string
Images []string
Birthday *time.Time
}
type User2 struct {
Email string
Images []string
Birthday time.Time
}
func main() {
user1 := User1{}
user2 := User2{}
fmt.Printf("user1 := %+v \n", user1)
//输出：user1 := {Email:<nil> Images:[] Birthday:<nil>}
fmt.Printf("user2 := %+v \n", user2)
//输出：user2 := {Email: Images:[] Birthday:0001-01-01 00:00:00 +0000 UTC}
}
英文:
Go has default values for every primitive data types.
int -> 0, string -> "", bool -> false likewise. So if you need to add null value, or load null value to a variable, it should be a pointer. Otherwise it is defaulted.
Default value of a pointer is nil in Go.
And complex data types such as slices, maps keep references. So their default value is nil. So, Images []string here images can be nil.
Below code with pointer types User1 and without pointer types User2 show the difference in default values.
package main
import (
"fmt"
"time"
)
type User1 struct {
Email *string
Images []string
Birthday *time.Time
}
type User2 struct {
Email string
Images []string
Birthday time.Time
}
func main() {
user1 := User1{}
user2 := User2{}
fmt.Printf("user1 := %+v \n", user1)
//Output : user1 := {Email:<nil> Images:[] Birthday:<nil>}
fmt.Printf("user2 := %+v \n", user2)
//Output : user2 := {Email: Images:[] Birthday:0001-01-01 00:00:00 +0000 UTC}
}
答案2
得分: 5
主要的区别在于，如果你使用指针，你可以将一个空值放入数据库中，否则你必须放入一个字符串。
基本上，如果数据库字段可为空，你应该使用指针。
英文:
The main difference is that if you use pointers, you can put a null value into the DB, else you must put a string.
Esentially if the database field is nullable, you should use pointers.
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