中国建设协会官网站,网站开发 程序开发原理,系统让我做神豪,网站开发报价模版了解可赋值规范的重要性当使用type关键字定义类型的时候#xff0c;会遇到一些问题#xff0c;如下#xff1a;func main(){var i int 2pushInt(i) }
type MyInt int //基于int定义MyInt
func pushInt(i MyInt){}结果#xff1a;调用函数pushInt报错
cannot use i (variab…了解可赋值规范的重要性当使用type关键字定义类型的时候会遇到一些问题如下func main(){var i int 2pushInt(i) }
type MyInt int //基于int定义MyInt
func pushInt(i MyInt){}结果调用函数pushInt报错
cannot use i (variable of type int)
as MyInt value in argument to pushIntcompilerIncompatibleAssign而相似的这种调用就不会出错func main(){var i []int []int{2,3,4}pushInt(i)}type MyInt []int //基于[]int 定义MyInt
func pushInt(i MyInt){}结果正常编译运行go语法中的赋值无处不在赋值操作、调用方法时的receiver赋值、调用方法的parameter赋值、方法返回值的接收变量赋值赋值即值拷贝这个大家都懂可是赋值的类型约束是什么赋值原则其实很简单1、类型相同可以进行赋值
2、类型不同的情况至少有一个是unnamed type且底层类型必须兼容。下面会慢慢讲解。go语言规范定义go语言规范中对可赋值的描述比较复杂说到底就是上面的2个原则我们先大概看一下规范内容然后等阐明什么叫类型相同什么叫底层类型相同在回过头来理解该规范。A value x of type V is assignable to a variable of type T (x is assignable to T) if one of the following conditions applies:V and T are identical.V and T have identical underlying types but are not type parameters and at least one of V or T is not a named type.V and T are channel types with identical element types, V is a bidirectional channel, and at least one of V or T is not a named type.T is an interface type, but not a type parameter, and x implements T.x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type, but not a type parameter.x is an untyped constant representable by a value of type T.Additionally, if xs type V or T are type parameters, x is assignable to a variable of type T if one of the following conditions applies:x is the predeclared identifier nil, T is a type parameter, and x is assignable to each type in Ts type set.V is not a named type, T is a type parameter, and x is assignable to each type in Ts type set.V is a type parameter and T is not a named type, and values of each type in Vs type set are assignable to T.什么叫类型相同1、named type有名字的类型只有名称相同才能称为类型相同named的type包括predeclared type即程序预声明的类型如int byte run string等这些都是有名字的。var x int 20 //x的类型是named type --intdefined type即通过type关键字定义的类型定义时根据语法是必须要给定名字的。注意type declaration和type definition的区别type Dog struct{} //类型名字为Dog
type Dog int //类型名字为Dogtype Dog int //类型名字为intDog只是个别名
type Dog struct{} //该类型是unnamedDog只是个别名type parameter类型参数是泛型中的概念其定义了新的类型例如[T ~int]类型名为T底层类型为int底层类型后面讲func name[T ~string](dogName T){} //定义了一个新的类型T
//注意T是一个类型而func name(T string)中T是一个变量。2、literal type字面量类型没有名称只要结构相同类型就相同composite类型都可以用字面量定义新的类型如slice channel等的类型都可以用literal来定义如下列举了几个literal类型定义 var x func(string) int func(s string) int {return 1} //functionvar x struct{ name string } struct{ name string }{name} //structvar x []int []int{1,2,3} //slicevar x [3]int [3]int{1,3,4} //arrayvar x map[int]int make(map[int]int) //mapvar x chan int make(chan int) //channelnum : 23var x *int num //pointervar x interface{String() stringName() string} Inner{name} //interface我们发现其实 指针类型、chan、map、array、slice的类型定义我们平时都是使用unnamed的literal type 形式。因为比较方便。如果我们使用named type反而会比较麻烦type MyMap map[int]int //这样定义类型就比较麻烦3、规范中对类型相同的描述以上两种已经描述何为类型相同规范中是这样描述的A named type is always different from any other type. Otherwise, two types are identical if their underlying type literals are structurally equivalent; that is, they have the same literal structure and corresponding components have identical types. In detail:Two array types are identical if they have identical element types and the same array length.Two slice types are identical if they have identical element types.Two struct types are identical if they have the same sequence of fields, and if corresponding fields have the same names, and identical types, and identical tags. Non-exported field names from different packages are always different.Two pointer types are identical if they have identical base types.Two function types are identical if they have the same number of parameters and result values, corresponding parameter and result types are identical, and either both functions are variadic or neither is. Parameter and result names are not required to match.Two interface types are identical if they define the same type set.Two map types are identical if they have identical key and element types.Two channel types are identical if they have identical element types and the same direction.Two instantiated types are identical if their defined types and all type arguments are identical.4、小试牛刀规范中的小练习以下类型哪些相同type (A0 []stringA1 A0A2 struct{ a, b int }A3 intA4 func(A3, float64) *A0A5 func(x int, _ float64) *[]stringB0 A0B1 []stringB2 struct{ a, b int }B3 struct{ a, c int }B4 func(int, float64) *B0B5 func(x int, y float64) *A1C0 B0D0[P1, P2 any] struct{ x P1; y P2 }E0 D0[int, string]
)相同的类型A0, A1, and []string
A2 and struct{ a, b int }
A3 and int
A4, func(int, float64) *[]string, and A5
B0 and C0
D0[int, string] and E0
[]int and []int
struct{ a, b *B5 } and struct{ a, b *B5 }
func(x int, y float64) *[]string, func(int, float64) (result *[]string), and A5B0 and B1 are different because they are new types created by distinct type definitions; func(int, float64) *B0 and func(x int, y float64) *[]string are different because B0 is different from []string; and P1 and P2 are different because they are different type parameters. D0[int, string] and struct{ x int; y string } are different because the former is an instantiated defined type while the latter is a type literal (but they are still assignable)什么叫底层类型相同什么叫底层类型每种类型都有其底层类型上面提到的predeclared类型和literal类型其底层就是其本身var x int //变量x的类型是predeclared 的intint的底层类型是int
var x []int //类型是literal的[]int其底层类型是[]int指向类型的底层类型是其指向的类型的底层类型。有点拗口上栗子type MyInt int //MyInt指向intint的底层类型是int那么结果是int
type YourInt MyInt //YourInt指向MyInt那么就是MyInt的底层类型那么结果就是int
type HisInt YourInt //HisInt指向YourInt以此类推那么结果就是int类型参数的底层类型是其约束类型。func name[T ~string](n T){}//类型参数定义了新的类型TT的底层类型就是string知道底层类型是什么那么按第一小节“什么叫类型相同”中的规则进行对比即可知道两个类型的底层类型是否相同。回过来看可赋值的规范定义下面会对规范中可赋值定义进行一句句解释A value x of type V is assignable to a variable of type T (x is assignable to T) if one of the following conditions applies:这一部分讲解非type parameter类型参数的情形V and T are identical. 类型相同可以赋值V and T have identical underlying types but are not type parameters and at least one of V or T is not a named type.不是类型参数底层类型相同如果只有一个unnamed type那么底层类型相同两个
都是unnamed type的话是相同类型。V and T are channel types with identical element types, V is a bidirectional channel, and at least one of V or T is not a named type.channel元素相同底层数据类型相同。如果一个unnamed type的话那么底层数据相同。如果两个都是
unnamed type的话那么底层数据相同甚至是类型完全相同。
var c - chan int make(chan int) //类型不同但底层类型相同
var c chan int make(chan int) //类型相同T is an interface type, but not a type parameter, and x implements T.x和T必须有is a的关系。x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type, but not a type parameter.x是nil因为nil是预声明标识符而不是类型。
可以赋值给pointer,function....引用类型和interface类型x is an untyped constant representable by a value of type T.x是untyped int 是unnamed。但是和int32 底层类型相同所以可以赋值
const x 1 20 //x在初始化的时候是untyped intx的取值范围可以超过 164的而x处不会报错
func main() {var y int32 x}这一部分讲解type parameter的情形Additionally, if xs type V or T are type parameters, x is assignable to a variable of type T if one of the following conditions applies:x is the predeclared identifier nil, T is a type parameter, and x is assignable to each type in Ts type set.x是nil是标识符而不是named typeT是type parameter有名字类型不同。
T类型参数的类型集是指针类型可以接受nil
func name3[X *int](age X) {age nil
}V is not a named type, T is a type parameter, and x is assignable to each type in Ts type set.V是unnamed那么就不会和T的类型不同底层类型相同即可
func name[X ~int](age X) {a : 20 agefmt.Println(a)age 30 // age a会报错因为a是named类型而age 30就不会报错
}V is a type parameter and T is not a named type, and values of each type in Vs type set are assignable to T.age是type parameter有unnamed的而x是literal没名字类型不同但底层类型一样
func name2[X IntArr](age X) {var x map[int]int age
}
type IntArr map[int]int完全符合我们的可赋值原则1、类型相同可以进行赋值2、类型不同的情况至少有一个是unnamed type且底层类型必须兼容。描述可能不够准确望网络大佬们指正。
