wiki/content/20200826151514-variables.md

date	id	title
20200826	ef9ed7f2-0bee-4405-a6d2-993cdea80029	Golang variables

Basics

The `var` statement declares a list of variables. Type is last. A `var` statement can be at package or function level. Use var when you to set a variable type but don't want to set the variable value yet.

package main

import "fmt"

var c, python, java bool

func main() {
    var i int
    fmt.Println(i, c, python, java)
}

This also works with interfaces.

package main

import "fmt"

type printSomething interface {
    print()
}

type agent struct {
}

func (d *agent) print() {
    fmt.Println("tralala")
}

var (
    a = &agent{}
)

func main() {
    a.print()
}

Initializers

A var declaration can include initializers, one per variable. If an initializer is present, the type can be omitted; the variable will take the type of the initializer.

package main

import "fmt"

var i, j int = 1, 2

func main() {
    var c, python, java = true, false, "no!"
    fmt.Println(i, j, c, python, java)
}

Short variable declarations

Inside a function, the := short assignment statement can be used in place of a var declaration with implicit type. Outside a function, every statement begins with a keyword (var, func, and so on) and so the := construct is not available.

package main

import "fmt"

func main() {
    var i, j int = 1, 2
    k := 3
    c, python, java := true, false, "no!"

    fmt.Println(i, j, k, c, python, java)
}

Basic types

Go's basic types are

bool

string

int int8 int16 int32 int64 uint uint8 uint16 uint32 uint64 uintptr

byte // alias for uint8

rune // alias for int32 // represents a Unicode code point

float32 float64

complex64 complex128

The example shows variables of several types, and also that variable declarations may be "factored" into blocks, as with import statements.

The int, uint, and uintptr types are usually 32 bits wide on 32-bit systems and 64 bits wide on 64-bit systems. When you need an integer value you should use int unless you have a specific reason to use a sized or unsigned integer type.

package main

import (
    "fmt"
    "math/cmplx"
)

var (
    ToBe   bool       = false
    MaxInt uint64     = 1<<64 - 1
    z      complex128 = cmplx.Sqrt(-5 + 12i)
)

func main() {
    fmt.Printf("Type: %T Value: %v\n", ToBe, ToBe)
    fmt.Printf("Type: %T Value: %v\n", MaxInt, MaxInt)
    fmt.Printf("Type: %T Value: %v\n", z, z)
}

Zero values

Variables declared without an explicit initial value are given their zero value.

The zero value is:

• 0 for numeric types
• false for the boolean type
• "" for strings

package main

import "fmt"

func main() {
    var i int
    var f float64
    var b bool
    var s string
    fmt.Printf("%v %v %v %q\n", i, f, b, s)
}

Type conversions

The expression `T(v)` converts the value v to the type T.

package main

 import (
    "fmt"
    "math"
)

func main() {
    var x, y int = 3, 4
    var f float64 = math.Sqrt(float64(x*x + y*y))
    var z uint = uint(f)
    fmt.Println(x, y, z)
}

Type inference

When declaring a variable without specifying an explicit type (either by using the := syntax or var = expression syntax), the variable's type is inferred from the value on the right hand side.

When the right hand side of the declaration is typed, the new variable is of that same type:

var i int
j := i // j is an int

But when the right hand side contains an untyped numeric constant, the new variable may be an int, float64, or complex128 depending on the precision of the constant:

i := 42           // int
f := 3.142        // float64
g := 0.867 + 0.5i // complex128

Constants

Constants are declared like variables, but with the const keyword.

Constants can be character, string, boolean, or numeric values.

Constants cannot be declared using the := syntax.

package main

import "fmt"

const Pi = 3.14

func main() {
    const World = "世界"
    fmt.Println("Hello", World)
    fmt.Println("Happy", Pi, "Day")

    const Truth = true
    fmt.Println("Go rules?", Truth)
}