go的指针类型,必须初始化后才能赋值

package main

import(
	"fmt"
)

func testPoint5(){
	var a *int = new(int)  // 用new初始化,就可以直接赋值了 如果直接用var a *int 之后直接赋值,就报错

	*a = 100
	fmt.Println("---------------*a = 100-------------------")
	fmt.Printf("*a=%d\n",*a)
	fmt.Printf("地址a=%p\n",a)

	var b *[]int =new([]int)
	fmt.Printf("*b=%v\n",*b)
	// *b[0] =100 // b[0] (type *[]int does not support indexing)  用new方法创建的空数组不能直接赋值

	(*b) = make([]int, 5,100)  
	(*b)[0] =100
	(*b)[1] =200
	fmt.Printf("*b=%v\n",*b)
}

func testPoint6(){
	var a int = 100
	b := a
	fmt.Println("---------------b := a = 100-------------------")
	fmt.Printf("a=%d\n",a)
	fmt.Printf("b=%d\n",b)
	fmt.Printf("&a=%p\n",&a)
	fmt.Printf("&b=%p\n",&b)

	fmt.Println("--------------- a = 120-------------------")
	a = 120
	fmt.Printf("a=%d\n",a)
	fmt.Printf("b=%d\n",b)
	fmt.Printf("&a=%p\n",&a)
	fmt.Printf("&b=%p\n",&b)

	fmt.Println("--------------- a = 130-------------------")
	b= 130
	fmt.Printf("a=%d\n",a)
	fmt.Printf("b=%d\n",b)
	fmt.Printf("&a=%p\n",&a)
	fmt.Printf("&b=%p\n",&b)
}

func testPoint7(){
	// 只有内存地址一样,值怎么变都是一样的
	var a int = 100
	var b *int = &a
	var c *int = b
	fmt.Println("---------------a int = 100-------------------")
	fmt.Printf("地址*a=%d,实际值a=%d\n",&a,a)
	fmt.Printf("地址b=%d, 实际值*b=%d\n",b,*b)
	fmt.Printf("地址c=%d, 实际值*c=%d\n",c,*c)

	fmt.Println("---------------*c = 200-------------------")
	*c = 200
	fmt.Printf("实际值a=%d, 地址*a=%d\n",a,&a)
	fmt.Printf("地址b=%d, 实际值*b=%d\n",b,*b)
	fmt.Printf("地址c=%d, 实际值*c=%d\n",c,*c)

	fmt.Println("---------------a =120-------------------")
	a = 120
	fmt.Printf("a=%d, *a=%d\n",a,&a)
	fmt.Printf("b=%d, *b=%d\n",b,*b)	
	fmt.Printf("c=%d, *c=%d\n",c,*c)

	fmt.Println("---------------*b = 130-------------------")
	*b = 130
	fmt.Printf("a=%d, *a=%d\n",a,a)
	fmt.Printf("b=%d, *b=%d\n",b,*b)	
	fmt.Printf("c=%d, *c=%d\n",c,*c)	
}

func modify(a *int){
	*a =105
}

func testPoint8(){
	var b int = 100
	p := &b
	fmt.Printf("修改之前的b=%d\n",b)
	fmt.Printf("修改之前的p=%d\n",*p)
	modify(p)
	fmt.Printf("修改之后的b=%d\n",b)
}


func modify_arr(a *[3]int){
	(*a)[0] =105
}

func testPoint4(){
	var b [3]int = [3]int{1,2,3}
	fmt.Printf("b=%v\n",b)
	modify_arr(&b)
	fmt.Printf("b=%v\n",b)		
}


func testPoint9(){
	var a *int = new(int)
	*a = 120
	fmt.Printf("a=%v a=%d\n",a,*a)	
	
	var b *[]int = new([]int)
	fmt.Printf("b=%v\n",*b)
	(*b) = make([]int, 5, 100) //初始化切片 也可以简写成 *b = make([]int, 5,100)
	(*b)[0] = 100
	(*b)[2] = 105
	fmt.Printf("b=%v\n",*b)
}

func modifyInt(a int){
	print("00>>",a,"\n")
	a = 162
	print("01>>",a,"\n")
}

func testPoint3()(){
	var a int32
	a = 1000
	fmt.Printf("the addr of a :%p, a:%d a:%v\n",&a,a,a)

	var b *int32
	fmt.Printf("the addr of b:%p  b:%p b:%v\n",&b,b,b)
	// *b =252  // 指针类型必须先初始化才能赋值,否则报错
	if b==nil{
		fmt.Println("b is nil addr")
	}
	b = &a
	fmt.Printf("the addr of b:%p,b:%p b:%v\n",&b,b,b)
	fmt.Printf("the addr of b:%p,b:%d\n",&b,*b)

	fmt.Println("--------------- *b =250-------------------")
	*b =250
	fmt.Printf("the addr of b:%p,b:%d\n",&b,*b)
	fmt.Printf("the addr of a :%p, a:%d a:%d\n",&a,a,a)
}

func main(){
	testPoint3()
	testPoint4()
	testPoint5()
	testPoint6()
	testPoint7()
	testPoint8()
	testPoint9()
}

输出:

the addr of a :0xc00019c058, a:1000 a:1000
the addr of b:0xc0001c6020  b:0x0 b:<nil>
b is nil addr
the addr of b:0xc0001c6020,b:0xc00019c058 b:0xc00019c058
the addr of b:0xc0001c6020,b:1000
--------------- *b =250-------------------
the addr of b:0xc0001c6020,b:250
the addr of a :0xc00019c058, a:250 a:250
b=[1 2 3]
b=[105 2 3]
---------------*a = 100-------------------
*a=100
地址a=0xc00019c078
*b=[]
*b=[100 200 0 0 0]
---------------b := a = 100-------------------
a=100
b=100
&a=0xc00019c0e8
&b=0xc00019c0f0
--------------- a = 120-------------------
a=120
b=100
&a=0xc00019c0e8
&b=0xc00019c0f0
--------------- a = 130-------------------
a=120
b=130
&a=0xc00019c0e8
&b=0xc00019c0f0
---------------a int = 100-------------------
地址*a=824635408632,实际值a=100
地址b=824635408632, 实际值*b=100
地址c=824635408632, 实际值*c=100
---------------*c = 200-------------------
实际值a=200, 地址*a=824635408632
地址b=824635408632, 实际值*b=200
地址c=824635408632, 实际值*c=200
---------------a =120-------------------
a=120, *a=824635408632
b=824635408632, *b=120
c=824635408632, *c=120
---------------*b = 130-------------------
a=130, *a=130
b=824635408632, *b=130
c=824635408632, *c=130
修改之前的b=100
修改之前的p=100
修改之后的b=105
a=0xc00019c160 a=120
b=[]
b=[100 0 105 0 0]