常量

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Constants 常量

原文：https://go.dev/ref/spec#Constants

There are boolean constants, rune constants, integer constants, floating-point constants, complex constants, and string constants. Rune, integer, floating-point, and complex constants are collectively called numeric constants.

​ 有布尔常量、符文常量、整数常量、浮点常量、复数常量和字符串常量。符文、整数、浮点和复数常量统称为数值常量。

A constant value is represented by a rune, integer, floating-point, imaginary, or string literal, an identifier denoting a constant, a constant expression, a conversion with a result that is a constant, or the result value of some built-in functions such as min or max applied to constant arguments, unsafe.Sizeof applied to certain values, cap or len applied to some expressions, real and imag applied to a complex constant and complex applied to numeric constants. The boolean truth values are represented by the predeclared constants true and false. The predeclared identifier iota denotes an integer constant.

​ 常量值由一个符文字面量、整数字面量、浮点数字面量、虚数字面量或字符串字面量，表示常量的标识符，常量表达式，结果为常量的转换，或一些内置函数的结果值表示，如unsafe.Sizeof应用于某些值，cap或len应用于一些表达式，real和imag应用于复数常量，complex应用于数值常量。布尔真值由预先声明的常数true和false表示。预先声明的标识符iota表示一个整数常量。

个人注释

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package main

import (
	"fmt"
	"unsafe"
)

var arr = [3]int{1, 2, 3}
var sli = []int{1, 2, 3}

const r = 'r'
const i1 = 1
const i2 = 11
const f = 1.2
const x = 1 + 2.3i
const s = "This is a string."
const s1 = s
const (
	iotaNum1 = iota + 1
	iotaNum2 = iota + 2
)

const fi = f + i1
const i2f = float64(i2)

const unsafeSizeOfS = unsafe.Sizeof(s)

const builtIn1 = len(s)

// const builtIn2 = len(sli) // len(sli) (value of type int) is not constant
// const buildIn3 = cap(sli) // cap(sli) (value of type int) is not constant
const buildIn4 = cap(arr)
const ir = real(1 + 2i)
const ii = imag(1 + 2i)
const ic = complex(1, 2)
const isTrue = true
const isFalse = false

func main() {
	fmt.Printf("r =%v,其类型是%T\n", r, r)                                     // r =114,其类型是int32
	fmt.Printf("i1 =%v,其类型是%T\n", i1, i1)                                  // i1 =1,其类型是int
	fmt.Printf("i2 =%v,其类型是%T\n", i2, i2)                                  // i2 =11,其类型是int
	fmt.Printf("f =%v,其类型是%T\n", f, f)                                     // f =1.2,其类型是float64
	fmt.Printf("x =%v,其类型是%T\n", x, x)                                     // x =(1+2.3i),其类型是complex128
	fmt.Printf("s =%v,其类型是%T\n", s, s)                                     // s =This is a string.,其类型是string
	fmt.Printf("s1 =%v,其类型是%T\n", s1, s1)                                  // s1 =This is a string.,其类型是string
	fmt.Printf("iotaNum1 =%v,其类型是%T\n", iotaNum1, iotaNum1)                // iotaNum1 =1,其类型是int
	fmt.Printf("iotaNum2 =%v,其类型是%T\n", iotaNum2, iotaNum2)                // iotaNum2 =3,其类型是int
	fmt.Printf("fi =%v,其类型是%T\n", fi, fi)                                  // fi =2.2,其类型是float64
	fmt.Printf("i2f =%v,其类型是%T\n", i2f, i2f)                               // i2f =11,其类型是float64
	fmt.Printf("unsafeSizeOfS =%v,其类型是%T\n", unsafeSizeOfS, unsafeSizeOfS) // unsafeSizeOfS =16,其类型是uintptr
	fmt.Printf("builtIn1 =%v,其类型是%T\n", builtIn1, builtIn1)                // builtIn1 =17,其类型是int
	//fmt.Printf("builtIn2 =%v,其类型是%T\n", builtIn2, builtIn2)
	//fmt.Printf("buildIn3 =%v,其类型是%T\n", buildIn3, buildIn3)
	fmt.Printf("buildIn4 =%v,其类型是%T\n", buildIn4, buildIn4) // buildIn4 =3,其类型是int
	fmt.Printf("ir =%v,其类型是%T\n", ir, ir)                   // ir =1,其类型是float64
	fmt.Printf("ii =%v,其类型是%T\n", ii, ii)                   // ii =2,其类型是float64
	fmt.Printf("ic =%v,其类型是%T\n", ic, ic)                   // ic =(1+2i),其类型是complex128
	fmt.Printf("isTrue =%v,其类型是%T\n", isTrue, isTrue)       // isTrue =true,其类型是bool
	fmt.Printf("isFalse =%v,其类型是%T\n", isFalse, isFalse)    // isFalse =false,其类型是bool
}

​ 这么说，难道常量值的类型不能是数组、切片、map、channel、结构体？

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package main

import "fmt"

const s1 = [3]int{1, 2, 3}                 // [3]int{…} (value of type [3]int) is not constant
const s2 = []int{1, 2, 3}                  // []int{…} (value of type []int) is not constant
const m1 = map[string]int{"a": 1, "b": 2}  // map[string]int{…} (value of type map[string]int) is not constant
const c <-chan int = 2                     // invalid constant type <-chan int
const s3 = struct{ Name string }{"zlongx"} // struct{Name string}{…} (value of type struct{Name string}) is not constant

func main() {
	fmt.Println(s1)
	fmt.Println(s2)
	fmt.Println(m1)
	fmt.Println(c)
	fmt.Println(s3)
}

​ 确实不能为数组、切片、map、结构体、channel！

​ 那常量的值可以是指针吗？

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package main

import "unsafe"

const s1 = "This is a string"
const s2 string = "This is a string"

var s3 = "This is a string"

const p1 = &s1                 // invalid operation: cannot take address of s1 (untyped string constant "This is a string")
const p2 = uintptr(&s1)        // invalid operation: cannot take address of s1 (untyped string constant "This is a string")
const p3 = unsafe.Pointer(&s1) // nvalid operation: cannot take address of s1 (untyped string constant "This is a string")

const p4 = &s2                 // invalid operation: cannot take address of s2 (constant "This is a string" of type string)
const p5 = uintptr(&s2)        // invalid operation: cannot take address of s2 (constant "This is a string" of type string)
const p6 = unsafe.Pointer(&s2) // invalid operation: cannot take address of s2 (constant "This is a string" of type string)

const p7 = &s3          // &s3 (value of type *string) is not constant
const p8 = uintptr(&s3) // cannot convert &s3 (value of type *string) to type uintptr
const p9 = unsafe.Pointer(&s3) // unsafe.Pointer(&s3) (value of type unsafe.Pointer) is not constant

func main() {
}

​ 看来常量的值也是不能为指针！

​ 常量在编译时就确定了它们的值，并且在程序运行期间是不可修改的。指针类型是动态的，它们包含了变量的内存地址，而这个地址是在运行时确定的。因此，指针类型的值是不适合用作常量的。

​ 常量值可以是变量吗？

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package main

var s = "This is a string"

const ss = s // s (variable of type string) is not constant

func main() {
}

​ 明显，常量的值不能为变量！

In general, complex constants are a form of constant expression and are discussed in that section.

​ 通常，复数常量是常量表达式的一种形式，将在该节中讨论。

Numeric constants represent exact values of arbitrary precision and do not overflow. Consequently, there are no constants denoting the IEEE-754 negative zero, infinity, and not-a-number values.

​ 数值常量表示任意精度的精确值，不会溢出。因此，不存在表示IEEE-754负零、无穷大和非数字值的常量。

个人注释

​ 关于“数值常量表示任意精度的精确值，不会溢出”，还是不能理解！—— 请查看下面的实现限制！！！

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package main

import "fmt"

const f1 = 1.123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890

var f2 = f1

// constant overflow
//const i1 = 1123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890

//var i2 = i1

func main() {
	fmt.Printf("f1=%v，其类型是%T\n", f1, f1) // f1=1.1234567890123457，其类型是float64
	fmt.Printf("f2=%v，其类型是%T\n", f2, f2) // f2=1.1234567890123457，其类型是float64

	fmt.Printf("f1=%.20f，其类型是%T\n", f1, f1)
	fmt.Printf("f1=%.25f，其类型是%T\n", f1, f1)
	fmt.Printf("f1=%.30f，其类型是%T\n", f1, f1)
	fmt.Printf("f1=%.35f，其类型是%T\n", f1, f1)
	fmt.Printf("f1=%.40f，其类型是%T\n", f1, f1)
	fmt.Printf("f1=%.45f，其类型是%T\n", f1, f1)
	fmt.Printf("f1=%.50f，其类型是%T\n", f1, f1)
	fmt.Printf("f1=%.55f，其类型是%T\n", f1, f1)
	fmt.Printf("f1=%.60f，其类型是%T\n", f1, f1)

	fmt.Printf("f2=%.20f，其类型是%T\n", f2, f2)
	fmt.Printf("f2=%.25f，其类型是%T\n", f2, f2)
	fmt.Printf("f2=%.30f，其类型是%T\n", f2, f2)
	fmt.Printf("f2=%.35f，其类型是%T\n", f2, f2)
	fmt.Printf("f2=%.40f，其类型是%T\n", f2, f2)
	fmt.Printf("f2=%.45f，其类型是%T\n", f2, f2)
	fmt.Printf("f2=%.50f，其类型是%T\n", f2, f2)
	fmt.Printf("f2=%.55f，其类型是%T\n", f2, f2)
	fmt.Printf("f2=%.60f，其类型是%T\n", f2, f2)

	//fmt.Printf("i1=%v，其类型是%T\n", i1, i1)
	//fmt.Printf("i2=%v，其类型是%T\n", i2, i2)
}


f1=1.1234567890123457，其类型是float64
f2=1.1234567890123457，其类型是float64
f1=1.12345678901234569125，其类型是float64
f1=1.1234567890123456912476740，其类型是float64
f1=1.123456789012345691247674039914，其类型是float64
f1=1.12345678901234569124767403991427273，其类型是float64
f1=1.1234567890123456912476740399142727255821，其类型是float64
f1=1.123456789012345691247674039914272725582122803，其类型是float64
f1=1.12345678901234569124767403991427272558212280273438，其类型是float64
f1=1.1234567890123456912476740399142727255821228027343750000，其类型是float64
f1=1.123456789012345691247674039914272725582122802734375000000000，其类型是float64
f2=1.12345678901234569125，其类型是float64
f2=1.1234567890123456912476740，其类型是float64
f2=1.123456789012345691247674039914，其类型是float64
f2=1.12345678901234569124767403991427273，其类型是float64
f2=1.1234567890123456912476740399142727255821，其类型是float64
f2=1.123456789012345691247674039914272725582122803，其类型是float64
f2=1.12345678901234569124767403991427272558212280273438，其类型是float64
f2=1.1234567890123456912476740399142727255821228027343750000，其类型是float64
f2=1.123456789012345691247674039914272725582122802734375000000000，其类型是float64

Constants may be typed or untyped. Literal constants, true, false, iota, and certain constant expressions containing only untyped constant operands are untyped.

​ 常量可以是有类型的的或无类型的。字面常量、true、false、iota，以及某些只包含无类型的常量操作数的常量表达式是无类型的。

A constant may be given a type explicitly by a constant declaration or conversion, or implicitly when used in a variable declaration or an assignment statement or as an operand in an expression. It is an error if the constant value cannot be represented as a value of the respective type. If the type is a type parameter, the constant is converted into a non-constant value of the type parameter.

​ 常量可以通过常量声明或转换显式地给出类型，也可以在变量声明、赋值语句 、作为表达式的操作数时，隐式赋予类型。如果常量值不能被表示为相应类型的值，那就是一个错误。如果类型是一个类型参数，常量将被转换为类型参数的一个非常量值。

个人注释

​ 这里提到的隐式赋予类型怎么解释？请看以下给出的示例

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package main

import "fmt"

const A = 1
const B = 2
const C uint8 = 3
const D = 4

func main() {
	var a int8 = 1
	fmt.Printf("A的类型是：%T\n", A)        //A的类型是：int
	fmt.Printf("a的类型是：%T\n", a)        //a的类型是：int8
	fmt.Println("a + A = ", a+A)          //a + A =  2
	fmt.Printf("a + A后的类型是：%T\n", a+A) //a + A后的类型是：int8

	var b uint8 = 2
	fmt.Printf("B的类型是：%T\n", B)        //B的类型是：int
	fmt.Printf("b的类型是：%T\n", b)        //b的类型是：uint8
	fmt.Println("b + B = ", b+B)          //b + B =  4
	fmt.Printf("b + B后的类型是：%T\n", b+B) //b + B后的类型是：uint8

	var c int = 3
	fmt.Printf("C的类型是：%T\n", C)       //C的类型是：uint8
	fmt.Printf("c的类型是：%T\n", c)       //c的类型是：int
	//fmt.Println("c + C = ", c+C)       // invalid operation: c + C (mismatched types int and uint8)
	//fmt.Printf("c + C后的类型是：%T\n", c+C) // invalid operation: c + C (mismatched types int and uint8)

	var d float64 = 1.2
	fmt.Printf("D的类型是：%T\n", D)        //D的类型是：int
	fmt.Printf("d的类型是：%T\n", d)        //d的类型是：float64
	fmt.Println("d + D = ", d+D)          // d + D =  5.2
	fmt.Printf("d + D后的类型是：%T\n", d+D) // d + D后的类型是：float64

}

​ 我们知道算数运算符左右两个操作数的类型必须一致，才能进行运算（可参见 数值型的最后一段话：当不同的数值类型在表达式或赋值中混合使用时，需要进行显示转换。）。

​ 以上示例a + A、b + B 、d + D中，+ 运算符两边的类型分明是不一致的（特别是d + D），但却可以进行运算，可见就是隐式赋予了无类型常量（该常量声明时没有给出明确类型，采用了默认类型）以类型，这应该是编译时就处理好的吧。TODO 待找出出处。

An untyped constant has a default type which is the type to which the constant is implicitly converted in contexts where a typed value is required, for instance, in a short variable declaration such as i := 0 where there is no explicit type. The default type of an untyped constant is bool, rune, int, float64, complex128, or string respectively, depending on whether it is a boolean, rune, integer, floating-point, complex, or string constant.

​ 一个无类型常量有一个默认的类型，该类型是在需要类型化值的上下文中隐式转换为的类型，例如，在一个短变量声明中，如i := 0，没有明确的类型。无类型常量的默认类型分别是bool, rune, int, float64, complex128或string，具体取决于它是一个布尔型常量、rune型常量、整数型常量、浮点型常量、复数型常量还是字符串型常量。

Implementation restriction: Although numeric constants have arbitrary precision in the language, a compiler may implement them using an internal representation with limited precision. That said, every implementation must:

​ 实现限制：尽管数值常量在语言中具有任意的精度，但编译器可以使用有限精度的内部表示法来实现它们。也就是说，每个实现都必须：

  • Represent integer constants with at least 256 bits.
  • 用至少256位来表示整数常量。
  • Represent floating-point constants, including the parts of a complex constant, with a mantissa of at least 256 bits and a signed binary exponent of at least 16 bits.
  • 用至少256位的尾数和至少16位的有符号二进制指数来表示浮点常量，包括复数常量的对应部分。
  • Give an error if unable to represent an integer constant precisely.
  • 如果不能精确表示一个整数常量，则给出一个错误。
  • Give an error if unable to represent a floating-point or complex constant due to overflow.
  • 如果由于溢出而无法表示一个浮点常量或复数常量，则给出一个错误。
  • Round to the nearest representable constant if unable to represent a floating-point or complex constant due to limits on precision.
  • 如果由于精度的限制，无法表示一个浮点常量或复数常量，则四舍五入到最接近的可表示常量。

These requirements apply both to literal constants and to the result of evaluating constant expressions.

​ 这些要求既适用于字面常量，也适用于常量表达式的计算结果。

最后修改 October 10, 2024: 更新 (a4b8f85)