Explain Go struct instantiation and assignment with examples

                                                                                                               

Golang The various instantiation and assignment methods of struct, sometimes by value transfer, sometimes by pointer, are confusing, so I decided to sort it out and understand the whole thing. .

Define a structure first and explain it below with the code.

package mainimport "fmt"type Person struct {
    Name string
    Age int
    Descprtion string}

Example 1

p instantiates a struct in the most conventional way, and the variable p gets a Person structure.

p := Person{}
p.Name = "小明"

fmt.Printf("p:%+v 变量地址：%p\n", p, &p)
fmt.Println("===========")

// result:
// p:{Name:小明 Age:0 Descprtion:} 变量地址：0xc000078480
// ===========

Example 2

The variable p1 is assigned by p. Since the Golang language is value transfer, after the assignment, the modification to p1 will not It will not affect p;

It can also be seen from the first output that Golang’s assignment does not have a copy on write mechanism like PHP variable assignment. .

p1 := p
fmt.Printf("p1:%+v 变量地址：%p\n", p1, &p1) // 不存在写时复制
p1.Name = "小明p1"
fmt.Printf("p:%+v 变量地址：%p\n", p, &p)
fmt.Printf("p1:%+v 变量地址：%p\n", p1, &p1)
fmt.Println("===========")

// result:
// p1:{Name:小明 Age:0 Descprtion:} 变量地址：0xc0000784e0
// p:{Name:小明 Age:0 Descprtion:} 变量地址：0xc000078480
// p1:{Name:小明p1 Age:0 Descprtion:} 变量地址：0xc0000784e0
// ===========

Example 3

Use the address operator to assign the address of p to p2. The variable p2 is a pointer that stores the address pointing to p. When p2 modifies the element Name in the structure, the value corresponding to the structure accessed through p also changes accordingly.

p2 := &p // 等同于 var p2 *Person = &p
fmt.Printf("p2:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p2, p2, &p2)
p2.Name = "小明p2"
fmt.Printf("p1:%+v 变量地址：%p\n", p, &p)
fmt.Printf("p2:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p2, p2, &p2)
fmt.Println("===========")

// result:
// p2:&{Name:小明 Age:0 Descprtion:} 指针变量指向地址（变量值）：0xc000078480 变量地址：0xc000006030
// p1:{Name:小明p2 Age:0 Descprtion:} 变量地址：0xc000078480
// p2:&{Name:小明p2 Age:0 Descprtion:} 指针变量指向地址（变量值）：0xc000078480 变量地址：0xc000006030
// ===========

Example 4

Variable p3 is obtained from new(Person). new will open up a piece of memory and return the memory address to p3, that is, p3 is a pointer to this memory.

p3 is a pointer to the structure. It has two ways to operate the structure,

p3.Age = 3 and *p3 = Person{Name: "Xiao Ming p3" }, If the operation is performed after the second method, the modification of the structure in the first method will be overwritten.

Since p3 is a pointer, when p3 is assigned to p5, p5 will also point to this memory address.

p3 := new(Person)
fmt.Printf("p3:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p3, p3, &p3)
p3.Age = 3 // 等同于 (*p3).Age = 3
fmt.Println("================ 操作 Age ================")
fmt.Printf("p3:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p3, p3, &p3)
*p3 = Person{
    Name: "小明p3",
}
fmt.Println("================ 操作 Name ================")
fmt.Printf("p3:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p3, p3, &p3)
p5 := p3
fmt.Println("================ p5 := p3 ================")
fmt.Printf("p3:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p3, p3, &p3)
fmt.Printf("p5:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p5, p5, &p5)
p3.Name = "小明p3修改"
fmt.Println("================ p3 修改 ================")
fmt.Printf("p3:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p3, p3, &p3)
fmt.Printf("p5:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p5, p5, &p5)
fmt.Println("===========")

// result:
// p3:&{Name: Age:0 Descprtion:} 指针变量指向地址（变量值）：0xc000078630 变量地址：0xc000006038
// ================ 操作 Age ================
// p3:&{Name: Age:3 Descprtion:} 指针变量指向地址（变量值）：0xc000078630 变量地址：0xc000006038
// ================ 操作 Name ================
// p3:&{Name:小明p3 Age:0 Descprtion:} 指针变量指向地址（变量值）：0xc000078630 变量地址：0xc000006038
// ================ p5 := p3 ================
// p5:&{Name:小明p3 Age:0 Descprtion:} 指针变量指向地址（变量值）：0xc000078630 变量地址：0xc000006040
// ================ p3 修改 ================
// p3:&{Name:小明p3修改 Age:0 Descprtion:} 指针变量指向地址（变量值）：0xc000078630 变量地址：0xc000006038
// p5:&{Name:小明p3修改 Age:0 Descprtion:} 指针变量指向地址（变量值）：0xc000078630 变量地址：0xc000006040
// ===========

Example 5

The instantiation method of p4 will also get a pointer. This instantiation method is the same as the instantiation of p3, but The p4 format is more commonly used.

p4 := &Person{
    Name: "小明p4",}fmt.Printf("%+v %p\n", p4, &p4)// result:// &{Name:小明p4 Age:0 Descprtion:} 0xc000006048

Attached is the complete code:

package mainimport "fmt"type Person struct {
    Name string
    Age int
    Descprtion string}func main() {
    p := Person{}
    p.Name = "小明"

    fmt.Printf("p:%+v 变量地址：%p\n", p, &p)
    fmt.Println("===========")

    p1 := p
    fmt.Printf("p1:%+v 变量地址：%p\n", p1, &p1) // 不存在写时复制
    p1.Name = "小明p1"
    fmt.Printf("p:%+v 变量地址：%p\n", p, &p)
    fmt.Printf("p1:%+v 变量地址：%p\n", p1, &p1)
    fmt.Println("===========")

    p2 := &p
    fmt.Printf("p2:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p2, p2, &p2)
    p2.Name = "小明p2"
    fmt.Printf("p1:%+v 变量地址：%p\n", p, &p)
    fmt.Printf("p2:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p2, p2, &p2)
    fmt.Println("===========")

    p3 := new(Person)
    fmt.Printf("p3:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p3, p3, &p3)
    p3.Age = 3 // 等同于 (*p3).Age = 3
    fmt.Println("================ 操作 Age ================")
    fmt.Printf("p3:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p3, p3, &p3)
    *p3 = Person{
        Name: "小明p3",
    }
    fmt.Println("================ 操作 Name ================")
    fmt.Printf("p3:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p3, p3, &p3)
    p5 := p3
    fmt.Println("================ p5 := p3 ================")
    fmt.Printf("p3:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p3, p3, &p3)
    fmt.Printf("p5:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p5, p5, &p5)
    p3.Name = "小明p3修改"
    fmt.Println("================ p3 修改 ================")
    fmt.Printf("p3:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p3, p3, &p3)
    fmt.Printf("p5:%+v 指针变量指向地址（变量值）：%p 变量地址：%p\n", p5, p5, &p5)
    fmt.Println("===========")

    p4 := &Person{
        Name: "小明p4",
    }
    fmt.Printf("%+v %p\n", p4, &p4)}

End!

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