Go 语言 import 下划线 _ 使用说明

// main project main.go
package main

import (
	"fmt"
	//_ "main/hello/imp"
	"main/hello/imp"
	"runtime"
)

func main() {
	for skip := 0; ; skip++ {
		pc, file, line, ok := runtime.Caller(skip)
		if !ok {
			break
		}
		fmt.Printf("skip = %v, pc = %v, file = %v, line = %v\n", skip, pc, file, line)
	}

	imp.Print()
}

package imp

import (
	"fmt"
)

func init() {
	fmt.Println("imp-init come here.")
}

func Print() {
	fmt.Println("Hello World! in package imp")
}

输出:

imp-init come here.
skip = 0, pc = 4762118, file = C:/Go/mycode/src/main/main.go, line = 13
skip = 1, pc = 4366150, file = C:/Go/src/runtime/proc.go, line = 185
skip = 2, pc = 4513537, file = C:/Go/src/runtime/asm_amd64.s, line = 2197
Hello World! in package imp

如果修改 main.go 的 import 为一下形式:

// main project main.go
package main

import (
	"fmt"
	_ "main/hello/imp" //这里前面加上了 _
	//"main/hello/imp"
	"runtime"
)

func main() {
	for skip := 0; ; skip++ {
		pc, file, line, ok := runtime.Caller(skip)
		if !ok {
			break
		}
		fmt.Printf("skip = %v, pc = %v, file = %v, line = %v\n", skip, pc, file, line)
	}

	imp.Print() //这已经将会提示编译失败
}

输出:

C:/Go/bin/go.exe build -i [C:/Go/mycode/src/main]
# main
.\main.go:20: undefined: imp in imp.Print
错误: 进程退出代码 2.

前面加上了 _   说明只能调用 init.go 里面的 init() 函数, 当然你不能显示的在 main.go 里面调用

imp.init()

// main project main.go
package main

import (
	"fmt"
	_ "main/hello/imp"
	//"main/hello/imp"
	"runtime"
)

func main() {
	for skip := 0; ; skip++ {
		pc, file, line, ok := runtime.Caller(skip)
		if !ok {
			break
		}
		fmt.Printf("skip = %v, pc = %v, file = %v, line = %v\n", skip, pc, file, line)
	}
	imp.init()
	//imp.Print()
}

输出:

C:/Go/bin/go.exe build -i [C:/Go/mycode/src/main]
# main
.\main.go:19: undefined: imp in imp.init
错误: 进程退出代码 2.

---------------------------------------------------------------------------------------------------

import(. "fmt") 这个点操作的含义就是这个包导入之后在你调用这个包的函数时，
你可以省略前缀的包名，也就是前面你调用的fmt.Println("hello world") 
可以省略的写成Println("hello world")


import(f "fmt") 别名操作调用包函数时前缀变成了重命名的前缀，
即f.Println("hello world")

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