Structs in Go

Struct Types and Struct Type Literals

struct {
	title  string
	author string
	pages  int
}

struct {
	title, author string
	pages         int
}

struct {
	Title  string `json:"title" myfmt:"s1"`
	Author string `json:"author,omitempty" myfmt:"s2"`
	Pages  int    `json:"pages,omitempty" myfmt:"n1"`
	X, Y   bool   `myfmt:"b1"`
}

Struct Value Literals and Struct Value Manipulations

1. S{0, false}. In this variant, no field names are present but all field values must be present by the field declaration orders.
2. S{x: 0, y: false}, S{y: false, x: 0}, S{x: 0}, S{y: false} and S{}. In this variant, each field item is optional and the order of the field items is not important. The values of the absent fields will be set as the zero values of their respective types. But if a field item is present, it must be presented with the FieldName: FieldValue form. The order of the field items in this form doesn't matter. The form S{} is the most used zero value representation of type S.

package main

import (
	"fmt"
)

type Book struct {
	title, author string
	pages         int
}

func main() {
	book := Book{"Go 101", "Tapir", 256}
	fmt.Println(book) // {Go 101 Tapir 256}

	// Create a book value with another form.
	// All of the three fields are specified.
	book = Book{author: "Tapir", pages: 256, title: "Go 101"}

	// None of the fields are specified. The title and
	// author fields are both "", pages field is 0.
	book = Book{}

	// Only specify the author field. The title field
	// is "" and the pages field is 0.
	book = Book{author: "Tapir"}

	// Initialize a struct value by using selectors.
	var book2 Book // <=> book2 := Book{}
	book2.author = "Tapir Liu"
	book2.pages = 300
	fmt.Println(book2.pages) // 300
}

var _ = Book {
	author: "Tapir",
	pages: 256,
	title: "Go 101", // here, the "," must be present
}

// The last "," in the following line is optional.
var _ = Book{author: "Tapir", pages: 256, title: "Go 101",}

About Struct Value Assignments

func f() {
	book1 := Book{pages: 300}
	book2 := Book{"Go 101", "Tapir", 256}

	book2 = book1
	// The above line is equivalent to the
	// following lines.
	book2.title = book1.title
	book2.author = book1.author
	book2.pages = book1.pages
}

Struct Field Addressability

package main

import "fmt"

func main() {
	type Book struct {
		Pages int
	}
	var book = Book{} // book is addressable
	p := &book.Pages
	*p = 123
	fmt.Println(book) // {123}

	// The following two lines fail to compile, for
	// Book{} is unaddressable, so is Book{}.Pages.
	/*
	Book{}.Pages = 123
	p = &(Book{}.Pages) // <=> p = &Book{}.Pages
	*/
}

Composite Literals Are Unaddressable But Can Take Addresses

package main

func main() {
	type Book struct {
		Pages int
	}
	// Book{100} is unaddressable but can
	// be taken address.
	p := &Book{100} // <=> tmp := Book{100}; p := &tmp
	p.Pages = 200
}

In Field Selectors, Dereferences of Receivers Can Be Implicit

package main

func main() {
	type Book struct {
		pages int
	}
	book1 := &Book{100} // book1 is a struct pointer
	book2 := new(Book)  // book2 is another struct pointer
	// Use struct pointers as structs.
	book2.pages = book1.pages
	// This last line is equivalent to the above line.
	// In other words, if the receiver is a pointer,
	// it will be implicitly dereferenced.
	(*book2).pages = (*book1).pages
}

About Struct Value Comparisons

About Struct Value Conversions

• values of type S0 can't be converted to the other four types, and vice versa, because the corresponding field names are different.
• two values of two different types among S1, S2, S3 and S4 can be converted to each other's type.

• values of the type denoted by S2 can be implicitly converted to type S3, and vice versa.
• values of the type denoted by S2 can be implicitly converted to type S4, and vice versa.

• values of the type denoted by S2 must be explicitly converted to type S1, and vice versa.
• values of type S3 must be explicitly converted to type S4, and vice versa.

package main

type S0 struct {
	y int "foo"
	x bool
}

// S1 is an alias of an unnamed type.
type S1 = struct {
	x int "foo"
	y bool
}

// S2 is also an alias of an unnamed type.
type S2 = struct {
	x int "bar"
	y bool
}

// If field tags are ignored, the underlying
// types of S3(S4) and S1 are same. If field
// tags are considered, the underlying types
// of S3(S4) and S1 are different.
type S3 S2 // S3 is a defined (so named) type
type S4 S3 // S4 is a defined (so named) type

var v0, v1, v2, v3, v4 = S0{}, S1{}, S2{}, S3{}, S4{}
func f() {
	v1 = S1(v2); v2 = S2(v1)
	v1 = S1(v3); v3 = S3(v1)
	v1 = S1(v4); v4 = S4(v1)
	v2 = v3; v3 = v2 // the conversions can be implicit
	v2 = v4; v4 = v2 // the conversions can be implicit
	v3 = S3(v4); v4 = S4(v3)
}

Anonymous Struct Types Can Be Used in Field Declarations

var aBook = struct {
	// The type of the author field is
	// an anonymous struct type.
	author struct {
		firstName, lastName string
		gender              bool
	}
	title string
	pages int
}{
	author: struct { // an anonymous struct type
		firstName, lastName string
		gender              bool
	}{
		firstName: "Mark",
		lastName: "Twain",
	},
	title: "The Million Pound Note",
	pages: 96,
}

More About Struct Types

• About Go 101 - why this book is written.
• Acknowledgments

• An Introduction of Go - why Go is worth learning.
• The Go Toolchain - how to compile and run Go programs.

• Become Familiar With Go Code
  • Introduction of Source Code Elements
  • Keywords and Identifiers
  • Basic Types and Their Value Literals
  • Constants and Variables - also introduces untyped values and type deductions.
  • Common Operators - also introduces more type deduction rules.
  • Function Declarations and Calls
  • Code Packages and Package Imports
  • Expressions, Statements and Simple Statements
  • Basic Control Flows
  • Goroutines, Deferred Function Calls and Panic/Recover

• Go Type System
  • Go Type System Overview - a must read to master Go programming.
  • Pointers
  • Structs
  • Value Parts - to gain a deeper understanding into Go values.
  • Arrays, Slices and Maps - first-class citizen container types.
  • Strings
  • Functions - function types and values, including variadic functions.
  • Channels - the Go way to do concurrency synchronizations.
  • Methods
  • Interfaces - value boxes used to do reflection and polymorphism.
  • Type Embedding - type extension in the Go way.
  • Type-Unsafe Pointers
  • Generics - use and read composite types
  • Reflections - the reflect standard package.

• Some Special Topics
  • Line Break Rules
  • More About Deferred Function Calls
  • Some Panic/Recover Use Cases
  • Explain Panic/Recover Mechanism in Detail - also explains exiting phases of function calls.
  • Code Blocks and Identifier Scopes
  • Expression Evaluation Orders
  • Value Copy Costs in Go
  • Bounds Check Elimination

• Concurrent Programming
  • Concurrency Synchronization Overview
  • Channel Use Cases
  • How to Gracefully Close Channels
  • Other Concurrency Synchronization Techniques - the sync standard package.
  • Atomic Operations - the sync/atomic standard package.
  • Memory Order Guarantees in Go
  • Common Concurrent Programming Mistakes

• Memory Related
  • Memory Blocks
  • Memory Layouts
  • Memory Leaking Scenarios

• Some Summaries
  • Some Simple Summaries
  • nil in Go
  • Value Conversion, Assignment and Comparison Rules
  • Syntax/Semantics Exceptions
  • Go FAQ 101

• More Go Related Topics