Copy one struct to another where structs have same members and different types

Question

I have two struct having the same members, I want to copy one struct to another, see the pseudo code below:

type Common struct {
    Gender int
    From   string
    To     string
}

type Foo struct {
    Id    string
    Name  string
    Extra Common
}

type Bar struct {
    Id    string
    Name  string
    Extra Common
}

Then I have foo of struct Foo, and bar of struct Bar, Is there any way to copy bar from foo?

Answer 1

Because the underlying types of Foo and Bar are the same, you can use a conversion to change the type. The following code uses a conversion to copy a value of type Foo to a value of type Bar:

foo := Foo{Id: "123", Name: "Joe"}
bar := Bar(foo)

playground example

The conversion only works when the underlying types are identical except for struct tags.

Answer 2

This is another possible answer

type Common struct { Gender int From string To string }

type Foo struct {
    Id    string
    Name  string
    Extra Common
}

type Bar struct {
    Id    string
    Name  string
    Extra Common
}
foo:=Foo{
    Id:"123",
    Name:"damitha",
    Extra: struct {
        Gender int
        From   string
        To     string
    }{Gender:1 , From:"xx", To:"yy" },
}
bar:=*(*Bar)(unsafe.Pointer(&foo))
fmt.Printf("%+v\n",bar)

Answer 3

If you would like to copy or clone to a different struct, I would suggest using deepcopier

It provides nice features like skipping, custom mapping, and forcing.

You can achieve nested struct copy following way. Install:

go get -u github.com/ulule/deepcopier

Example:

 package main

    import (
        "fmt"
        "strconv"

        "github.com/ulule/deepcopier"
    )

    //FieldStruct -  Field Struct
    type FieldStruct struct {
        Name string `deepcopier:"field:TargetName"`
        Type string `deepcopier:"field:TargetType"`
    }

    //SourceStruct - Source Struct
    type SourceStruct struct {
        Name        string   `deepcopier:"field:TargetName"`
        Age         int      `deepcopier:"field:TargetAge"`
        StringArray []string `deepcopier:"field:TargetStringArray"`
        StringToInt string   `deepcopier:"context"`
        Field       FieldStruct
        Fields      []FieldStruct
    }

    //TargetFieldStruct -  Field Struct
    type TargetFieldStruct struct {
        TargetName string
        TargetType string
    }

    //TargetStruct - Target Struct
    type TargetStruct struct {
        TargetName        string
        TargetAge         int
        TargetStringArray []string
        TargetInt         int
        TargetField       TargetFieldStruct
        TargetFields      []TargetFieldStruct
    }

    //write methods

    //TargetInt - StringToInt
    func (s *SourceStruct) TargetInt() int {
        i, _ := strconv.Atoi(s.StringToInt)
        return i
    }

    func main() {
        s := &SourceStruct{
            Name:        "Name",
            Age:         12,
            StringArray: []string{"1", "2"},
            StringToInt: "123",
            Field: FieldStruct{
                Name: "Field",
                Type: "String",
            },
            Fields: []FieldStruct{
                FieldStruct{
                    Name: "Field1",
                    Type: "String1",
                },
                FieldStruct{
                    Name: "Field2",
                    Type: "String2",
                },
            },
        }

        t := &TargetStruct{}

        //coping data into inner struct
        deepcopier.Copy(&t.TargetField).From(&s.Field)

        // copied array of Struct
        for i := range s.Fields {
            // init a struct
            t.TargetFields = append(t.TargetFields, TargetFieldStruct{})
            // coping the data
            deepcopier.Copy(&t.TargetFields[i]).From(&s.Fields[i])
        }
        //Top level copy
        deepcopier.Copy(t).From(s)

        fmt.Println(t)
    }

Output: &{Name 12 [1 2] 123 {Field String} [{Field1 String1} {Field2 String2}]}

Answer 4

https://github.com/jinzhu/copier (same author of gorm) is also quite a good one, I have nested structs and all I do is:

copier.Copy(&employees, &user)

works great

Answer 5

If you would like to copy or clone to a different struct, I would suggest using deepcopier

It provides nice features like skipping, custom mapping, and forcing. below is an example from github:

Install:

go get -u github.com/ulule/deepcopier

Example:

package main

import (
    "fmt"

    "github.com/ulule/deepcopier"
)

// Model
type User struct {
    // Basic string field
    Name  string
    // Deepcopier supports https://golang.org/pkg/database/sql/driver/#Valuer
    Email sql.NullString
}

func (u *User) MethodThatTakesContext(ctx map[string]interface{}) string {
    // do whatever you want
    return "hello from this method"
}

// Resource
type UserResource struct {
    //copy from field "Name"
    DisplayName            string `deepcopier:"field:Name"`
    //this will be skipped in copy 
    SkipMe                 string `deepcopier:"skip"`
    //this should call method named MethodThatTakesContext 
    MethodThatTakesContext string `deepcopier:"context"`
    Email                  string `deepcopier:"force"`

}

func main() {
    user := &User{
        Name: "gilles",
        Email: sql.NullString{
            Valid: true,
            String: "gilles@example.com",
        },
    }

    resource := &UserResource{}

    deepcopier.Copy(user).To(resource)
    //copied from User's Name field
    fmt.Println(resource.DisplayName)//output: gilles
    fmt.Println(resource.Email) //output: gilles@example.com
    fmt.Println(resource.MethodThatTakesContext) //output: hello from this method
}

Also, some other way you could achieve this is by encoding the source object to JSON and then decode it back to the destination object.