Why Does Printing a `bytes.Buffer` in Go Produce Different Output Depending on Whether a Pointer or Value is Used?

Different Behavior in Printing a bytes.Buffer in Go

In Go, when printing a bytes.Buffer using fmt.Println(), the behavior may vary depending on whether you use a pointer to a bytes.Buffer or the value directly. Here's an explanation:

In the first example:

buf := new(bytes.Buffer)
buf.WriteString("Hello world")
fmt.Println(buf)

buf is a pointer to a bytes.Buffer, which means it has a String() method available. When you pass a pointer to fmt.Println(), the String() method is called automatically, which converts the content of the bytes.Buffer to a string. That's why you see "Hello World" printed.

In the second example:

var buf bytes.Buffer
buf.WriteString("Hello world")
fmt.Println(buf)

buf is a value of type bytes.Buffer, not a pointer. As a result, the String() method is not available for this value. Instead, fmt.Println() prints it as a regular struct value, using the default format {field0 field1 ...}. The fields here are the bytes stored in the buffer, represented as a slice of integers.

To always print the content of a bytes.Buffer as a string, regardless of whether you use a pointer or value, you can explicitly call the String() method before printing:

fmt.Println(buf.String())

This will ensure consistent behavior across both cases.

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