请在GoLang中解释＆和*指针

Question

I recently started learning GoLang. There have been multiple instances where the compiler throws an error when I try to pass variables as arguments inside Go functions. I've been able to debug this sometimes by using a pointer in front of the variable. Both &, and * pointers seem to clear the error. Though, I'd like to understand why. I'm wondering what the difference between &, and * is, and when each should be used. Thank you!

func (ctx *NewContext) SendNotification(rw http.ResponseWriter, req *http.Request, p httprouter.Params) {

    decoder := json.NewDecoder(req.Body)

    var u User

    if err := decoder.Decode(&u); err != nil {
        http.Error(rw, "could not decode request", http.StatusBadRequest)
        return
    }
}

Answer 1

In your example above you defined u as type User, but not a pointer to a User. So you need the &u because the Decode function in the json package is expecting an address or pointer.

If you created the instance of User like this: u := new(User) it would be a pointer since the new function returns a pointer. You could also create a pointer to a user like this: var u *User. If you did either of those, you would have to take out the & in the call to Decode for it to work.

Pointers are basically variables that hold addresses. When you put the & in front of a variable it returns the address. The * could be read as 'redirect of'. So when you create a pointer like this:

var x *int

This can be read as x will redirect to an int. And when you assign a value to x you would give it an address like this: y := 10 x = &y

Where y is some int. So if you were to print out x, you would get the address of y, but if you printed out *x you would redirect to the what x points to which is y's value which is 10. If you were to print out &x, you would get the address of the pointer, x, itself.

If you tried to print out *y, which is just an int, not a pointer, it would throw an error because you would be redirecting with some value that is not an address to redirect to.

Run the below for some pointer fun:

package main

import "fmt"

func main() {
    var y int
    var pointerToY *int
    var pointerToPointerToInt **int

    y = 10
    pointerToY = &y
    pointerToPointerToInt = &pointerToY

    fmt.Println("y: ", y)
    fmt.Println("pointerToY: ", pointerToY)
    fmt.Println("pointerToPointerToInt: ", pointerToPointerToInt)

    fmt.Println("&y: ", &y)     // address of y
    fmt.Println("&pointerToY: ", &pointerToY)// address of pointerToY
    fmt.Println("&pointerToPointerToInt: ", &pointerToPointerToInt) // address of pointerToPointerToInt

    // fmt.Println(*y) throws an error because 
    // you can't redirect without an address.. 
    // y only has int value of 10
    fmt.Println("*pointerToY: ", *pointerToY) // gives the value of y
    fmt.Println("*pointerToPointerToInt: ", *pointerToPointerToInt)     // gives the value of pointerToY which is the address of y

    fmt.Println("**pointerToPointerToInt: ", **pointerToPointerToInt)    // this gives 10, because we are redirecting twice to get y

    if pointerToY == *pointerToPointerToInt {
        fmt.Println("'pointerToY == *pointerToPointerToInt' are the same!")
    }

    if pointerToY == &y {
        fmt.Println("'pointerToY == &y' are the same!")
    }

    if &pointerToY == pointerToPointerToInt {
        fmt.Println("'&pointerToY == pointerToPointerToInt' are the same!")
    }

    if y == **pointerToPointerToInt {
        fmt.Println("'y == **pointerToPointerToInt' are the same!")
    }

    if pointerToY == *pointerToPointerToInt {
        fmt.Println("'pointerToY == *pointerToPointerToInt' are the same!")
    }

}

Hope this helps!