如何在Golang中使用此功能？

Hey I'm new to Go syntax. How would I use this function? The part that is throwing me is the pointer at the beginning of the function declaration?

func (p *Pointer) FunctionName(arg string) error {
  dec := json.NewDecoder(strings.NewReader(arg))
  err := dec.Decode(&p)
  return err
}

FunctionName is a method on *Pointer.

If you are asking how to use the code on arbitrary types and not just a *Pointer, then write it as a function:

func FunctionName(v interface{}, arg string) error {
  dec := json.NewDecoder(strings.NewReader(arg))
  err := dec.Decode(v)
  return err
}

Assuming that p is a variable of type *Pointer, then call it like this:

FunctionName(p, "{... JSON text here }")

This is a method definition. The way to use it is

var p Pointer
/* or */
p := new(Pointer)

p.FunctionName(arg)

In Go struct types can contain methods, and that is what you have here. Say I have the following code:

type Foo struct {
    Something string
}

func (f * Foo) PrintSomething() {
    fmt.Println(f.Something)
}

I cannot invoke PrintSomething directly, I must invoke it by using a variable of type Foo. Example:

f := Foo{
    Something: "Something",
}

f.PrintSomething()