Structure of golang

GoLang Structure

Basic use

First , Give some understanding of the structure .

A structure is a collection of attributes , These attributes can be called fields , It's actually some variables in the program . Through these properties （ Field ） To describe the characteristics of something . actually , Structure is the abstraction of the substance to be described .

1. Defining structure

type struct_name struct{
    
    var_name1 type
    var_name2 type
    var_name3 type
    .
    .
    .
    var_namen type
}

2. Declare and initialize the structure object

Examples of structures :

type Animal struct{
    
    name string
    age int
    sex boolean
}

• Method 1 : Declare struct variables , And then through . Operator access assignment , To initialize

var st1 Animal
st1.name = " cat "
st1.age = 3
st1. sex = false
println(st1.name)
println(st1.age)
println(st1.sex)
// The result is :
// cat 
//3
//false

• Method 2 : Declaration is used in conjunction with {} To initialize

var st1 Animal = Animal{
    
    name: " cat ",
    age: 3,
    sex: false,    // The last comma here cannot be omitted , If you want to omit , Then the right brace should be on this line 
}
// Or abbreviated as the following form 
st2 := Animal{
    
    name: " cat ",
    age: 3,
    sex: false,
}
// Or abbreviated as the following form 
st3 := Animal{
      // When using this form , The assignment order is consistent with the declaration order of variables in the structure .
    " cat ",
    3,
    false,
}

• Method 3 ： Use new() function ： A function designed to create a pointer of a certain type

st4 := new(Animal)
st4.name = " cat "
st4.age = 3
st4.sex = false
fmt.Printf("%p---%T", st4, st4)
// The result is :
//0xc0000593c0---*main.Person

From the return result *main.Person It can be seen that , Use new() Example of structure created by function , The return is a structure pointer , It points to the use new() Function opens up space .

meanwhile , For structure pointers , Can pass . Operator directly accesses the field it points to the structure .

3. Value type and reference data type

Golang In language ：

Value type ：int, float, bool, string, array, struct etc.

Reference type : slice, map ,function, pointer etc.

• Aim at struct, Its example is value type . When assigning a structural variable to another structural variable , A copy will occur . such as ：st1 := st2, here st2 It has been initialized in advance , It will be st2 Copy the value of , Then put st1 In memory space .

Example :

var st2 Animal = Animal{
    
    name: " cat ",
    age: 3,
    sex: false,
}
st1 := st2
st1.name = " Dog "
fmt.Println(st1)
fmt.Println(st2)
// The result is :
//{ Dog  3 false}
//{ cat  3 false}

As can be seen from the above code , modify st1 It doesn't affect st2 Value .

Example :

var st2 *Animal = new(Animal)
st2.name = " cat "
st2.age = 3
st2.sex =false

st1 := st2
st1.name = " Dog "
fmt.Println(st1)
fmt.Println(st2)
// The result is :
//&{ Dog  3 false}
//&{ Dog  3 false}
//0xc0000503c0---0xc0000503c0

so ,st1 and st2 Are all references to structures , All point to the same memory space . So pass st1 or st2 Modify the fields of the structure , Will affect the value of the other party .

Anonymous structure and anonymous field

• Anonymous structure

When we need to use a structure and declare its variables, there are two ways . One is to define the structure first , Then declare its variables ; The other is to use anonymous structures . Use anonymous structures , No need to use type Keyword specifies the name of the structure type .

Use the syntax :

struct{
    
    field1 type
    filed2 type
    .
    .
    .
    filedn type
}{
    
    field1: value1,
    field2: value2,
    field3: valur3,
    .
    .
    .
    fieldn: valuen,
}

analysis ： The content in the first brace , It is the field definition of anonymous structure . The content in the second brace is the initialization of anonymous structure fields

Examples of use :

animal := struct{
    
    name string
    age int
}{
    
    name: " cat ",
    age: 3,
}
fmt.Println(animal)
// The result is :
{
     cat  3}

• Anonymous structure field

For fields in the structure , Sometimes its field name can be omitted . notes ： Variables in the structure can also be called fields .

Example :

type Animal struct{
     
    string
    int
}

Make a statement Animal The structural variable of ：

var a Animal = Animal{
     " cat ", 3}

How to access the fields of a structure without field names ？ In this case , The type of the field is treated as the field name by default . in other words , The type of field plays the role of data type , It also acts as a field name .

Example :

var a Animal = Animal{
     " cat ", 3}
fmt.Println(a.string, a.int)
// The result is :
// cat  3

• According to the above , You can know , The anonymous field of the structure can only be used when the field type of the current structure is not complex . That is, when multiple fields of the same type appear in the structure , Anonymous fields cannot be used . Because the type of anonymous field acts as a field identifier , If anonymous fields of the same type appear , Then it is impossible to determine which field to access .

Example ： The following code snippet , Will make the compiler report errors

type Animal struct{
     
    string
    int
    string   // Here are two string Anonymous field of type , Report errors 
}

Nesting of structures

Simply speaking , There is another definition of structure in the definition of structure , This is the nesting of structures .

When to use it ？ commonly , When one structure has another structure in its attributes, nesting of structures will be used .

as follows :

type Book struct{
      // Definition Book Structure , The name and price of the description book 
    name string
    price float64
}

type Student struct{
      // Definition student Structure , Describe the student's name , Age , And the books I have read 
    name string
    age int
    book Book
}

Reference material

• https://www.bilibili.com/video/BV1jJ411c7s3?p=86&vd_source=008445eb63b17edad78a03ae3d20c08a
• https://www.runoob.com/go/go-structures.html