Why does using `t` vs `*t` as a receiver in Go\'s `fmt.Stringer` lead to a dead loop?

Understanding the Difference Between t and *t: A Case Study from the fmt Package

In Go, the fmt package provides powerful formatting capabilities, allowing developers to customize the way values are printed. However, a subtle difference between t and *t can lead to unexpected behavior, namely a dead loop when used within a receiver method.

Consider the following code snippet:

package main

import "fmt"

type TT struct {
    a int
    b float32
    c string
}

func (t *TT) String() string {
    return fmt.Sprintf("%+v", *t)
}

func main() {
    tt := &TT{3, 4, "5"}
    fmt.Printf(tt.String())
}

This code will execute successfully, printing the contents of the 'tt' variable as "tt={a:3, b:4, c:5}". However, if we modify the String method as follows:

func (t *TT) String() string {
    return fmt.Sprintf("%+v", t)
}

It will result in a dead loop. The reason lies in the way the fmt package handles values that implement the String() method (implements the fmt.Stringer interface).

When the value of 'tt' is passed to fmt.Println, the fmt package checks if the type TT implements the String() method. Since we defined the String() method with a pointer receiver (*TT), the method set for TT does not include String(). Consequently, the fmt package does not call tt.String().

On the other hand, in the modified code snippet, we define the String() method with a receiver type of *TT. In this case, the method set for TT includes String(). However, since we are passing tt which is of type *TT, calling tt.String() essentially calls the String() method recursively, resulting in an infinite loop.

To prevent this issue, it is advisable to use a different receiver type for the String() method. One approach is to create a new type using the type keyword and perform type conversion on the value being passed. This creates a new type with no methods, avoiding the infinite loop.

For example:

func (t TT) String() string {
    type TT2 TT
    return fmt.Sprintf("%+v", TT2(t))
}

In this case, converting t to TT2 effectively ensures that the String() method will not be called recursively, as TT2 does not have a String() method.

Understanding the distinction between t and *t in the context of the fmt package is crucial to avoid unexpected behavior and ensure efficient code execution.

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