by smallnest

Go的函数参数总是传值

依照Go的FQA, 函数的参数传递总是传值的(pass-by-value)：

As in all languages in the C family, everything in Go is passed by value. That is, a function always gets a copy of the thing being passed, as if there were an assignment statement assigning the value to the parameter. For instance, passing an int value to a function makes a copy of the int, and passing a pointer value makes a copy of the pointer, but not the data it points to. (See a later section for a discussion of how this affects method receivers.)

Map and slice values behave like pointers: they are descriptors that contain pointers to the underlying map or slice data. Copying a map or slice value doesn't copy the data it points to. Copying an interface value makes a copy of the thing stored in the interface value. If the interface value holds a struct, copying the interface value makes a copy of the struct. If the interface value holds a pointer, copying the interface value makes a copy of the pointer, but again not the data it points to.

以下是测试代码，结果贴在下面，不解释了。

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package main

import (
	"fmt"
	"reflect"
	"unsafe"
)

func printInt(i int) {
	fmt.Printf("int i: %p\n", &i)
}

func printInt2(i *int) {
	fmt.Printf("int i: %p\n", i)
}

func printStr(s string) {
	fmt.Printf("string s: %p\n", &s)

	hdr := (*reflect.StringHeader)(unsafe.Pointer(&s))
	data := hdr.Data
	fmt.Printf("string s data: 0x%x\n", data)

}

func printStr2(s *string) {
	fmt.Printf("string s: %p\n", s)
	hdr := (*reflect.StringHeader)(unsafe.Pointer(s))
	data := hdr.Data
	fmt.Printf("string s data: 0x%x\n", data)
}

func printSlice(s []int) {
	fmt.Printf("slice s: %p\n", &s)

	hdr := (*reflect.SliceHeader)(unsafe.Pointer(&s))
	data := hdr.Data
	fmt.Printf("slice s data: 0x%x\n", data)

}

func printSlice2(s *[]int) {
	fmt.Printf("slice s: %p\n", s)
	hdr := (*reflect.SliceHeader)(unsafe.Pointer(s))
	data := hdr.Data
	fmt.Printf("slice s data: 0x%x\n", data)
}

type S struct {
	I int
}

func printStruct(s S) {
	fmt.Printf("struct s: %p, I: %p\n", &s, &(s.I))
}

func printStruct2(s *S) {
	fmt.Printf("struct s: %p, I: %p\n", &s, &(s.I))
}

func printInterface(i interface{}) {
	fmt.Printf("int i: %p\n", &i)
}

func printInterface2(i interface{}) {
	s := i.(S)
	fmt.Printf("struct s: %p, I: %p\n", &s, &(s.I))
}

func printInterface3(i interface{}) {
	s := i.(*S)
	fmt.Printf("struct s: %p, I: %p\n", s, &(s.I))
}

func main() {
	//test int
	i := 10
	fmt.Printf("int i: %p\n", &i)
	printInt(i)
	printInt2(&i)

	//test string
	s := "hello, world"
	fmt.Printf("\n\nstring s: %p\n", &s)
	hdr := (*reflect.StringHeader)(unsafe.Pointer(&s))
	data := hdr.Data
	fmt.Printf("string s data: 0x%x\n", data)
	printStr(s)
	printStr2(&s)

	//slice and map
	sl := []int{1, 2, 3, 4, 5}
	fmt.Printf("\n\nslice s: %p\n", &sl)
	hdr2 := (*reflect.SliceHeader)(unsafe.Pointer(&sl))
	data = hdr2.Data
	fmt.Printf("slice s data: 0x%x\n", data)
	printSlice(sl)
	printSlice2(&sl)

	//struct
	ss := S{I: 10}
	ssp := &ss
	fmt.Printf("\n\nstruct s: %p, I: %p\n", ssp, &(ss.I))
	printStruct(ss)
	printStruct2(ssp)

	//interface to int
	fmt.Printf("\n\nint i: %p\n", &i)
	printInterface(i)
	//interface to struct
	fmt.Printf("\n\nstruct s: %p, I: %p\n", ssp, &(ss.I))
	printInterface2(ss)
	printInterface3(ssp)
}

输出结果：

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int i: 0xc420074188
int i: 0xc4200741b8
int i: 0xc420074188


string s: 0xc4200741c0
string s data: 0xa778d
string s: 0xc4200741e0
string s data: 0xa778d
string s: 0xc4200741c0
string s data: 0xa778d


slice s: 0xc42006e0c0
slice s data: 0xc4200780c0
slice s: 0xc42006e0e0
slice s data: 0xc4200780c0
slice s: 0xc42006e0c0
slice s data: 0xc4200780c0


struct s: 0xc420074210, I: 0xc420074210
struct s: 0xc420074218, I: 0xc420074218
struct s: 0xc420084020, I: 0xc420074210


int i: 0xc420074188
int i: 0xc420074230


struct s: 0xc420074210, I: 0xc420074210
struct s: 0xc420074228, I: 0xc420074228
struct s: 0xc420074210, I: 0xc420074210