Why does printing a `bytes.Buffer` in Go sometimes show the string content and sometimes show its internal representation?

Different Behavior When Printing a bytes.Buffer in Go

When using the bytes.Buffer type, users may encounter different behavior when printing objects of that type. The following code:

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

prints "Hello World", while this code:

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

prints the following:

{[72 101 108 108 111 32 119 111 114 108 100] 0 [72 101 108 108 111 32 119 111 114 108 100 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] 0}

This apparent discrepancy arises because of the existence of a String() method for the bytes.Buffer type. When printing a value of type bytes.Buffer, the String() method is called to produce the string representation of the value. However, when printing a value of type bytes.Buffer, no such method is available, and the default format for a struct is used, which results in the representation seen above.

The differing behavior is further illustrated by the following:

type MyBuffer bytes.Buffer

func (b *MyBuffer) String() string {
    return "MyBuffer with " + b.String()
}

var b MyBuffer
b.WriteString("Hello world")
fmt.Println(b)

In this case, when a MyBuffer value is printed, the custom String() method is called and the "MyBuffer with ..." prefix is added to the output, demonstrating the effect of implementing the String() method.

Understanding this behavior is crucial when working with the bytes.Buffer type in Go, as it affects the formatting of output and can lead to unexpected results if not properly handled.

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