An interview question about interface and implementation in golang

Can the following code be compiled ？ Why? ？

package main

import (
	"fmt"
)

type People interface {
    
	Speak(string) string
}

type Student struct{
    }

func (stu *Student) Speak(think string) (talk string) {
    
	if think == "bitch" {
    
		talk = "You are a good boy"
	} else {
    
		talk = "hi"
	}
	return
}

func main() {
    
	var peo People = Student{
    }
	think := "bitch"
	fmt.Println(peo.Speak(think))
}

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answer

 Compile failed , Value type  `Student{}`  Interface not implemented `People` Methods , It can't be defined as  `People` type .

 stay  golang  In language ,`Student`  and  `*Student`  There are two types , The first is to say  `Student`  In itself , The second is to point to  `Student`  The pointer to .

How to rewrite ？

Change method set to value type

type Student struct{
    }

//  Using value 
func (stu Student) Speak(think string) (talk string) {
    
	if think == "bitch" {
    
		talk = "You are a good boy"
	} else {
    
		talk = "hi"
	}
	return
}

func main() {
    
	var peo People = Student{
    }
	think := "bitch"
	fmt.Println(peo.Speak(think))
}

perhaps , Use the pointer

type Student struct{
    }

func (stu *Student) Speak(think string) (talk string) {
    
	if think == "bitch" {
    
		talk = "You are a good boy"
	} else {
    
		talk = "hi"
	}
	return
}

func main() {
    
	var peo People = &Student{
    } //  Use the pointer 
	think := "bitch"
	fmt.Println(peo.Speak(think))
}

Let's look at a similar problem ：

Please tell me what the following code will output ？ Why? ？

package main

import (
	"fmt"
)

type People interface {
    
	Show()
}

type Student struct{
    }

func (stu *Student) Show() {
    

}

func live() People {
    
	var stu *Student
	if stu == nil {
    
		fmt.Println("C")
	}
	return stu
}


func main() {
    
	p := live()
	if p == nil {
    
		fmt.Println("A")
	} else {
    
		fmt.Println("B")
	}
}

Please think about it. , The answer is below
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answer

C
B

Same as the last question , The difference is *Student There is no initialization value after the definition of , therefore *Student yes nil Of , however *Student Realized People Interface , Interface is not nil.

We compile the code with gdb debug , go build -gcflags "-N -l" solution21.go

$ go build -gcflags "-N -l" solution21.go 
$ gdb solution21
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04.1) 9.2
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from solution21...
Loading Go Runtime support.
(gdb) l
11      type Student struct{}
12
13      func (stu *Student) Show() {
14
15      }
16
17      func live() People {
18              var stu *Student
19              if stu == nil {
20                      fmt.Println("C")
(gdb) 
21              }
22              return stu
23      }
24
25      func main() {
26              p := live()
27              if p == nil {
28                      fmt.Println("A")
29              } else {
30                      fmt.Println("B")
(gdb) 
31              }
32      }
(gdb) 
Line number 33 out of range; /home/yqq/mine/master-go/interview/6-interview-golang/solution21.go has 32 lines.
(gdb) b 27
Breakpoint 1 at 0x47e147: file /home/yqq/mine/master-go/interview/6-interview-golang/solution21.go, line 27.
(gdb) r
Starting program: /home/yqq/mine/master-go/interview/6-interview-golang/solution21 
[New LWP 24400]
[New LWP 24401]
[New LWP 24402]
[New LWP 24403]
[New LWP 24404]
C

Thread 1 "solution21" hit Breakpoint 1, main.main ()
    at /home/yqq/mine/master-go/interview/6-interview-golang/solution21.go:27
27              if p == nil {
(gdb) i locals
p = {tab = 0x4b3188 <Student,main.People>, data = 0x0}
(gdb) 

You can see , p = {tab = 0x4b3188 <Student,main.People>, data = 0x0}, interface By 2 Part of ,tab and data, When comparing p and nil Of course, time is not equal .

We use it gdb Look at the following code

package main

import "fmt"

func main() {
    
	var p interface{
    } = nil
	if p == nil {
    
		fmt.Println("A")
	} else {
    
		fmt.Println("B")
	}

	var q interface{
    } = (*interface{
    })(nil)
	if q == nil {
    
		fmt.Println("C")
	} else {
    
		fmt.Println("D")
	}

}

go build -gcflags "-N -l" solution22.go compile

$ gdb solution22 
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04.1) 9.2
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from solution22...
Loading Go Runtime support.
(gdb) l
1       package main
2
3       import "fmt"
4
5       func main() {
    
6               var p interface{
    } = nil
7               if p == nil {
    
8                       fmt.Println("A")
9               } else {
    
10                      fmt.Println("B")
(gdb) 
11              }
12
13              var q interface{
    } = (*interface{
    })(nil)
14              if q == nil {
    
15                      fmt.Println("C")
16              } else {
    
17                      fmt.Println("D")
18              }
19
20      }
(gdb) b 7
Breakpoint 1 at 0x47e08c: file /home/yqq/mine/master-go/interview/6-interview-golang/solution22.go, line 7.
(gdb) b 14
Breakpoint 2 at 0x47e109: file /home/yqq/mine/master-go/interview/6-interview-golang/solution22.go, line 14.
(gdb) i b
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x000000000047e08c in main.main 
                                                   at /home/yqq/mine/master-go/interview/6-interview-golang/solution22.go:7
2       breakpoint     keep y   0x000000000047e109 in main.main 
                                                   at /home/yqq/mine/master-go/interview/6-interview-golang/solution22.go:14
(gdb) r
Starting program: /home/yqq/mine/master-go/interview/6-interview-golang/solution22 
[New LWP 28172]
[New LWP 28173]
[New LWP 28174]
[New LWP 28175]

Thread 1 "solution22" hit Breakpoint 1, main.main ()
    at /home/yqq/mine/master-go/interview/6-interview-golang/solution22.go:7
7               if p == nil {
    
(gdb) p p
$1 = {
    _type = 0x0, data = 0x0}
(gdb) c
Continuing.
A

Thread 1 "solution22" hit Breakpoint 2, main.main ()
    at /home/yqq/mine/master-go/interview/6-interview-golang/solution22.go:14
14              if q == nil {
    
(gdb) p q
$2 = {
    _type = 0x483360, data = 0x0}
(gdb) 

You can see ,p What's printed out is $1 = {_type = 0x0, data = 0x0}
q What's printed out is {_type = 0x483360, data = 0x0}

So to conclude ：

golang Medium interface The bottom is made up of 2 Part of , namely （type,data）, As long as one of them is not nil, that , When used intefalce And nil The comparison is not equal .