How can I write data to the beginning of a bytes.Buffer in Go?

Writing to the Beginning of a Buffer in Go

In Go, the bytes.Buffer type provides methods for building a mutable buffer of bytes. By default, data is appended to the buffer using methods like WriteString(). However, it may be desirable to write to the beginning of a buffer.

Is it Possible to Write to the Beginning of a Buffer?

The underlying buffer buf in bytes.Buffer is not exported, making it difficult to manipulate directly. However, there is a workaround that allows writing to the beginning of the buffer.

Solution

To write to the beginning of a buffer, you can follow these steps:

1. Append some data to the buffer to initialize the underlying slice.
2. Read the entire buffer into a string.
3. Reset the buffer.
4. Append your desired data to the beginning.
5. Append the original data after the new data.

Example

The following example demonstrates this approach:

<code class="go">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())
}</code>

Output:

By using this workaround, you can write to the beginning of a buffer in Go, allowing for more flexibility in managing buffer contents.

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