Golang Struct

Learn to create user-defined types by combining with other types.

Introduction

A struct (short for "structure") is a collection of data fields with declared data types. Golang has the ability to declare and create own data types by combining one or more types, including both built-in and user-defined types. Each data field in a struct is declared with a known type, which could be a built-in type or another user-defined type.

Structs are the only way to create concrete user-defined types in Golang. Struct types are declared by composing a fixed set of unique fields. Structs can improve modularity and allow to create and pass complex data structures around the system. You can also consider Structs as a template for creating a data record, like an employee record or an e-commerce product.

The declaration starts with the keyword type, then a name for the new struct, and finally the keyword struct. Within the curly brackets, a series of data fields are specified with a name and a type.

Example

type identifier struct{
  field1 data_type
  field2 data_type
  field3 data_type
}

Declaration of a Struct Type

A struct type rectangle is declared that has three data fields of different data-types. Here, struct used without instantiate a new instance of that type.

Example

package main
 
import "fmt"
 
type rectangle struct {
	length  float64
	breadth float64
	color   string
}
 
func main() {
	fmt.Println(rectangle{10.5, 25.10, "red"})
}

The rectangle struct and its fields are not exported to other packages because identifiers are started with an lowercase letter. In Golang, identifiers are exported to other packages if the name starts with an uppercase letter, otherwise the accessibility will be limited within the package only.

Creating Instances of Struct Types

The var keyword initializes a variable rect. Using dot notation, values are assigned to the struct fields.

Example

package main
 
import "fmt"
 
type rectangle struct {
	length  int
	breadth int
	color   string
 
	geometry struct {
		area      int
		perimeter int
	}
}
 
func main() {
	var rect rectangle // dot notation
	rect.length = 10
	rect.breadth = 20
	rect.color = "Green"
 
	rect.geometry.area = rect.length * rect.breadth
	rect.geometry.perimeter = 2 * (rect.length + rect.breadth)
 
	fmt.Println(rect)
	fmt.Println("Area:\t", rect.geometry.area)
	fmt.Println("Perimeter:", rect.geometry.perimeter)
}

The struct is printed to the terminal, showing the values have been assigned.

Creating a Struct Instance Using a Struct Literal

Creates an instance of rectangle struct by using a struct literal and assigning values to the fields of the struct.

Example

package main

import "fmt"

type rectangle struct {
	length  int
	breadth int
	color   string
}

func main() {
	var rect1 = rectangle{10, 20, "Green"}
	fmt.Println(rect1)

	var rect2 = rectangle{length: 10, color: "Green"} // breadth value skipped
	fmt.Println(rect2)

	rect3 := rectangle{10, 20, "Green"}
	fmt.Println(rect3)

	rect4 := rectangle{length: 10, breadth: 20, color: "Green"}
	fmt.Println(rect4)

	rect5 := rectangle{breadth: 20, color: "Green"} // length value skipped
	fmt.Println(rect5)
}

Struct Instantiation Using new Keyword

An instance of a struct can also be created with the new keyword. It is then possible to assign data values to the data fields using dot notation.

Example

package main
 
import "fmt"
 
type rectangle struct {
	length  int
	breadth int
	color   string
}
 
func main() {
	rect1 := new(rectangle) // rect1 is a pointer to an instance of rectangle
	rect1.length = 10
	rect1.breadth = 20
	rect1.color = "Green"
	fmt.Println(rect1)
 
	var rect2 = new(rectangle) // rect2 is an instance of rectangle
	rect2.length = 10
	rect2.color = "Red"
	fmt.Println(rect2)
}

Two instances of the rectangle struct are instantiated, rect1 points to the address of the instantiated struct and rect2 is the name of a struct it represents.

Struct Instantiation Using Pointer Address Operator

Creates an instance of rectangle struct by using a pointer address operator is denoted by & symbol.

Example

package main

import "fmt"

type rectangle struct {
	length  int
	breadth int
	color   string
}

func main() {
	var rect1 = &rectangle{10, 20, "Green"} // Can't skip any value
	fmt.Println(rect1)

	var rect2 = &rectangle{}
	rect2.length = 10
	rect2.color = "Red"
	fmt.Println(rect2) // breadth skipped

	var rect3 = &rectangle{}
	(*rect3).breadth = 10
	(*rect3).color = "Blue"
	fmt.Println(rect3) // length skipped
}

Nested Struct Type

Struct can be nested by creating a Struct type using other Struct types as the type for the fields of Struct. Nesting one struct within another can be a useful way to model more complex structures.

Example

package main

import "fmt"

type Salary struct {
	Basic, HRA, TA float64
}

type Employee struct {
	FirstName, LastName, Email string
	Age                        int
	MonthlySalary              []Salary
}

func main() {
	e := Employee{
		FirstName: "Mark",
		LastName:  "Jones",
		Email:     "mark@gmail.com",
		Age:       25,
		MonthlySalary: []Salary{
			Salary{
				Basic: 15000.00,
				HRA:   5000.00,
				TA:    2000.00,
			},
			Salary{
				Basic: 16000.00,
				HRA:   5000.00,
				TA:    2100.00,
			},
			Salary{
				Basic: 17000.00,
				HRA:   5000.00,
				TA:    2200.00,
			},
		},
	}
	fmt.Println(e.FirstName, e.LastName)
	fmt.Println(e.Age)
	fmt.Println(e.Email)
	fmt.Println(e.MonthlySalary[0])
	fmt.Println(e.MonthlySalary[1])
	fmt.Println(e.MonthlySalary[2])
}

Use Field Tags in the Definition of Struct Type

During the definition of a struct type, optional string values may be added to each field declaration.

Example

package main
 
import (
    "fmt"
    "encoding/json"
)
 
type Employee struct {
    FirstName  string `json:"firstname"`
    LastName   string `json:"lastname"`
    City string `json:"city"`
}
 
func main() {
    json_string := `
    {
        "firstname": "Rocky",
        "lastname": "Sting",
        "city": "London"
    }`
 
    emp1 := new(Employee)
    json.Unmarshal([]byte(json_string), emp1)
    fmt.Println(emp1)
 
    emp2 := new(Employee)
    emp2.FirstName = "Ramesh"
    emp2.LastName = "Soni"
    emp2.City = "Mumbai"
    jsonStr, _ := json.Marshal(emp2)
    fmt.Printf("%s\n", jsonStr)
}

The tags are represented as raw string values (wrapped within a pair of ``) and ignored by normal code execution.

Add Method to Struct Type

You can also add methods to struct types using a method receiver. A method EmpInfo is added to the Employee struct.

Example

package main

import "fmt"

type Salary struct {
	Basic, HRA, TA float64
}

type Employee struct {
	FirstName, LastName, Email string
	Age                        int
	MonthlySalary              []Salary
}

func (e Employee) EmpInfo() string {
	fmt.Println(e.FirstName, e.LastName)
	fmt.Println(e.Age)
	fmt.Println(e.Email)
	for _, info := range e.MonthlySalary {
		fmt.Println("===================")
		fmt.Println(info.Basic)
		fmt.Println(info.HRA)
		fmt.Println(info.TA)
	}
	return "----------------------"
}

func main() {

	e := Employee{
		FirstName: "Mark",
		LastName:  "Jones",
		Email:     "mark@gmail.com",
		Age:       25,
		MonthlySalary: []Salary{
			Salary{
				Basic: 15000.00,
				HRA:   5000.00,
				TA:    2000.00,
			},
			Salary{
				Basic: 16000.00,
				HRA:   5000.00,
				TA:    2100.00,
			},
			Salary{
				Basic: 17000.00,
				HRA:   5000.00,
				TA:    2200.00,
			},
		},
	}

	fmt.Println(e.EmpInfo())
}

Assign Default Value for Struct Field

Method of assigning a custom default value can be achieve by using constructor function. Instead of creating a struct directly, the Info function can be used to create an Employee struct with a custom default value for the Name and Age field.

Example

package main

import "fmt"

type Employee struct {
	Name string
	Age  int
}

func (obj *Employee) Info() {
	if obj.Name == "" {
		obj.Name = "John Doe"
	}
	if obj.Age == 0 {
		obj.Age = 25
	}
}

func main() {
	emp1 := Employee{Name: "Mr. Fred"}
	emp1.Info()
	fmt.Println(emp1)

	emp2 := Employee{Age: 26}
	emp2.Info()
	fmt.Println(emp2)
}

This is a technique rather than something that is part of the Golang specification.

Find Type of Struct

The reflect package support to check the underlying type of a struct.

Example

package main

import (
	"fmt"
	"reflect"
)

type rectangle struct {
	length  float64
	breadth float64
	color   string
}

func main() {
	var rect1 = rectangle{10, 20, "Green"}
	fmt.Println(reflect.TypeOf(rect1))         // main.rectangle
	fmt.Println(reflect.ValueOf(rect1).Kind()) // struct

	rect2 := rectangle{length: 10, breadth: 20, color: "Green"}
	fmt.Println(reflect.TypeOf(rect2))         // main.rectangle
	fmt.Println(reflect.ValueOf(rect2).Kind()) // struct

	rect3 := new(rectangle)
	fmt.Println(reflect.TypeOf(rect3))         // *main.rectangle
	fmt.Println(reflect.ValueOf(rect3).Kind()) // ptr

	var rect4 = &rectangle{}
	fmt.Println(reflect.TypeOf(rect4))         // *main.rectangle
	fmt.Println(reflect.ValueOf(rect4).Kind()) // ptr
}

Comparing Structs with the Different Values Assigned to Data Fields

Structs of the same type can be compared using comparison operator.

Example

package main

import "fmt"

type rectangle struct {
	length  float64
	breadth float64
	color   string
}

func main() {
	var rect1 = rectangle{10, 20, "Green"}
	rect2 := rectangle{length: 20, breadth: 10, color: "Red"}

	if rect1 == rect2 {
		fmt.Println("True")
	} else {
		fmt.Println("False")
	}

	rect3 := new(rectangle)
	var rect4 = &rectangle{}

	if rect3 == rect4 {
		fmt.Println("True")
	} else {
		fmt.Println("False")
	}
}

Copy Struct Type Using Value and Pointer Reference

r2 will be the same as r1, it is a copy of r1 rather than a reference to it. Any changes made to r2 will not be applied to r1 and vice versa. When r3 is updated, the underlying memory that is assigned to r1 is updated.

Example

package main

import "fmt"

type rectangle struct {
	length  float64
	breadth float64
	color   string
}

func main() {
	r1 := rectangle{10, 20, "Green"}
	fmt.Println(r1)

	r2 := r1
	r2.color = "Pink"
	fmt.Println(r2)

	r3 := &r1
	r3.color = "Red"
	fmt.Println(r3)

	fmt.Println(r1)
}

Output

{10 20 Green}
{10 20 Pink}
&{10 20 Red}
{10 20 Red}

As both r1 and r3 both reference the same underlying memory, their values are the same. Printing the values of r3 and r1 shows that the values are the same.

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Golang Tutorial

Golang Reference

Beego Framework

Golang Struct

Golang Struct

Introduction

Example

Declaration of a Struct Type

Example

Creating Instances of Struct Types

Example

Creating a Struct Instance Using a Struct Literal

Example

Struct Instantiation Using new Keyword

Example

Struct Instantiation Using Pointer Address Operator

Example

Nested Struct Type

Example

Use Field Tags in the Definition of Struct Type

Example

Add Method to Struct Type

Example

Assign Default Value for Struct Field

Example

Find Type of Struct

Example

Comparing Structs with the Different Values Assigned to Data Fields

Example

Copy Struct Type Using Value and Pointer Reference

Example

Output

Most Helpful This Week