Golang : Fix cannot use buffer (type bytes.Buffer) as type io.Writer(Write method has pointer receiver) error

Tags : golang io-writer bytes-buffer pointer-receiver

Problem :

You created a variable of type bytes.Buffer and encounter this error message when you want to use the variable with bufio.NewWriter() function.

cannot use buffer (type bytes.Buffer) as type io.Writer in argument to bufio.NewWriter: bytes.Buffer does not implement io.Writer (Write method has pointer receiver)

Solution :

From https://golang.org/pkg/bufio/#NewWriter, the NewWriter() will return a pointer *Writer . i.e this explains part the of the error message - Write method has pointer receiver.

To fix this error, you need to provide the memory address of the buffer. In this code example below, you will see that bytes.NewBuffer(nil) is the same as &bytes.Buffer{} and os.Stdout is ok because the underlaying is a pointer of *File type.

 package main

 import (

 func main() {
 // redirect to screen, not an issue because os.Stdout is a pointer
 screenWriter := bufio.NewWriter(os.Stdout)
 fmt.Fprint(screenWriter, "Hello world! 你好世界!\n")
 screenWriter.Flush() // Don't forget to flush!

 // redirect to buffer
 //var buffer bytes.Buffer
 //bufferWriter := bufio.NewWriter(buffer) // <--- not ok, will generate error message 
 //bufferWriter := bufio.NewWriter(&buffer) // <-- ok

 buffer := bytes.NewBuffer(nil) // same as &bytes.Buffer{}, so it is ok as well.
 bufferWriter := bufio.NewWriter(buffer)

 bufferWriter.Flush() // Don't forget to flush!

References :


  See also : Golang : Reset buffer example

Tags : golang io-writer bytes-buffer pointer-receiver

By Adam Ng

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