On Fri, Sep 16, 2016 at 8:24 AM, Giang Tran <[email protected]> wrote:
>
> I have a small test like this
>
> package main
>
> type MError struct {
>
> }
>
> func (m *MError) Error() string {
> return "MError"
> }
>
> func NewMError() *MError {
> return nil
> }
>
> func main() {
> var e error
> e = NewMError()
> println(e.Error())
> }
>
>
>
> I know that interface actually combine like (Type, value), if I change
>
>
> func (m *MError) Error() string {
>
>
> to
>
>
> func (m MError) Error() string {
>
>
> would lead to "panic: value method main.MError.Error called using nil
> *MError pointer",
>
> why we have this different behave?
When you have a pointer, and call a value method, what that means is
that the value is copied out of the pointer and then the method is
invoked. That is, when you write e.Error(), where e is type *MError
and Error is a value method, the actual code is something like
tmp := *e
tmp.Error()
But in this case e is nil, so copying out the value fails, and you get a panic.
Ian
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