17-methods-2.go

package main

import (  
    "fmt"
)

type rectangle struct {  
    length int
    width  int
}

func area(r rectangle) {  
    fmt.Printf("Area Function result: %d\n", (r.length * r.width))
}

func (r rectangle) area() {  
    fmt.Printf("Area Method result: %d\n", (r.length * r.width))
}

func main() {  
    r := rectangle{
        length: 10,
        width:  5,
    }
    area(r)
    r.area()

    p := &r
    /*
       compilation error, cannot use p (type *rectangle) as type rectangle 
       in argument to area  
    */
    //area(p)

    p.area()//calling value receiver with a pointer
}

/*

Value receivers in methods vs value arguments in functions
This topic trips most go newbies. I will try to make it as clear as possible 😀.

When a function has a value argument, it will accept only a value argument.

When a method has a value receiver, it will accept both pointer and value receivers.

Lets understand this by means of the example above.

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

function func area(r rectangle) in line no.12 accepts a value argument 
and method func (r rectangle) area() accepts a value receiver.

In line no. 25, we call the area function with a value argument area(r) and it will work. 
Similarly In line no. 26 we call the area method r.area() using a value receiver and this will work too.

We create a pointer p to r in line no. 28. 
If we try to pass this pointer to the function area which accepts only a value, 
the compiler will complain. I have commented line no. 33 which does this. 
If you uncomment this line, then the compiler will throw error [compilation error, 
cannot use p (type *rectangle) as type rectangle in argument to area]. This works as expected.

Now comes the tricky part, line no. 35 of the code p.area() calls the method area 
which accepts only a value receiver using the pointer receiver p. 
This is perfectly valid. The reason is that the line p.area(), 
for convenience will be interpreted by Go as (*p).area() since area has a value receiver.

        This program will output,

        Area Function result: 50  
        Area Method result: 50  
        Area Method result: 50 

*/