What are the implications of using `*t` vs. `t` as method receivers in Go, and how can dead loops be prevented when using `fmt.String()`?

Understanding the Nuances of *t vs. t in Go's Method Invocation

In Go, the use of pointers ('*') in method receivers can have significant ramifications on the functioning of the method. This difference is highlighted in 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) // Original implementation
}

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

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

The original implementation of the String() method uses a pointer receiver (*TT), which avoids the pitfall of accessing a nil pointer if t is nil. However, modifying the String() method to use a non-pointer receiver (TT) results in a dead loop.

Reason for Dead Loop:

The key to understanding this behavior lies in how Go's fmt package handles types that implement the fmt.Stringer interface (i.e., types that provide a custom String() method). When printing a value of type *TT, fmt.String() will first check if *TT implements a valid String() method. If it does, it will invoke that method to obtain the string representation of the value. This works well when *TT has a pointer receiver because the method set of *TT includes the String() method.

However, when the receiver of String() is changed to a non-pointer type (i.e., TT), the issue arises. In this case, the method set of TT includes the String() method, which means that when fmt.String() attempts to print the value of t (an instance of TT), it will call t.String(), which in turn will call itself again, leading to an infinite recursion.

Preventing the Dead Loop:

To prevent the dead loop, one can employ a technique called type conversion. By creating a new type using the type keyword and converting the value passed to fmt.String(), one can avoid the infinite recursion:

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

In this case, the new type (TT2) has no methods, so when fmt.String() attempts to print the converted value, it does not invoke a String() method on the converted type.

Conclusion:

Understanding the difference between *t and t in method receivers is crucial to avoid potential pitfalls when printing custom types using the fmt.String() function. By carefully considering the receiver type and employing type conversion when necessary, one can prevent dead loops and ensure the correct functioning of method invocations.

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