Prefix Buffer Writes in Golang

In Golang, the bytes.Buffer is a type designed for efficient string concatenation and manipulation. However, some developers may encounter the need to write to the beginning of a buffer, unlike the built-in helper methods (e.g., WriteString) that only append to the buffer.

Write to Beginning of Buffer

While the underlying buf (internal byte buffer) of bytes.Buffer is not exported, it is possible to manipulate its contents indirectly. Here's how you can achieve it:

buffer.WriteString("B")
s := buffer.String()
buffer.Reset()
buffer.WriteString("A"   s)

1. Write to End of Buffer: Initially, write the string "B" to the end of the buffer using WriteString.
2. Retrieve Buffer Contents: Use the String method to retrieve the entire buffer's contents in string format and store it in variable s.
3. Reset Buffer: Reset the buffer to its initial state, which removes all the previous content from the buffer.
4. Write to Beginning of Buffer: Write the string "A" followed by the previously retrieved string s to the buffer using WriteString.

By concatenating "A" and s, we effectively write "A" at the beginning of the buffer, followed by the original contents.

Example

The following code demonstrates the process:

package main

import (
    "bytes"
    "fmt"
)

func main() {
    var buffer bytes.Buffer
    buffer.WriteString("B")
    s := buffer.String()
    buffer.Reset()
    buffer.WriteString("A"   s)
    fmt.Println(buffer.String())
}

Output:

AB

This strategy provides a workaround to write to the beginning of a buffer in Golang despite the limitations of the standard library bytes.Buffer type.