调用接口函数时出错？

package main

import "io"

type MyClass struct{
    writer  *io.Writer
}

func (this *MyClass) WriteIt() {
    this.writer.Write([]byte("Hello World!"))
}

Why is it that when writer, which is an implementation of io.Writer, tries to call the Write() function, displays me this error

this.writer.Write undefined (type *io.Writer has no field or method Write)

Use writer io.Writer without *

As makhov said, it is because writer in your struct definition of MyClass is a pointer to something which implements the Writer interface, not something which implements the writer interface itself. As such, your code should either be:

package main

import "io"

type MyClass struct{
    writer  io.Writer
}

func (this *MyClass) WriteIt() {
    this.writer.Write([]byte("Hello World!"))
}

or

package main

import "io"

type MyClass struct{
    writer  *io.Writer
}

func (this *MyClass) WriteIt() {
    (*(this.writer)).Write([]byte("Hello World!"))
}

Typically it would make sense (and be more idiomatic) to do the first option.