got it, thank you.
On Saturday, September 17, 2016 at 12:14:34 AM UTC+7, Ian Lance Taylor
wrote:
>
> On Fri, Sep 16, 2016 at 8:24 AM, Giang Tran > 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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