How Can I Display and Track the Pointer Value of a Go Object Throughout Its Lifetime?

How Can I Display the Pointer Value of a Go Object? Unraveling the Significance of Pointer Values

In Go, understanding whether structs are passed by value or reference is crucial. This article dives into this concept and explores a technique for printing the pointer value of an object in Go.

Understanding Value vs. Reference Passing

Go functions pass arguments by value, meaning that the function receives a copy of the original object. This implies that any changes made to the argument within the function do not affect the original object. Conversely, if structures are passed by reference, any changes made within the function are reflected in the original object.

Revealing Pointer Values in Go

To determine whether a struct is passed by value or reference, it's helpful to inspect its pointer value. In Go, & operator returns a pointer to the variable, which can be printed using the %p format specifier. However, this pointer value may change during garbage collection.

To obtain a persistent object ID, using the runtime.SetFinalizer() function is recommended. This involves providing a finalizer, which gets called just before the garbage collector removes the object. By creating a uniqueness map in the finalizer, you can assign a unique ID to each object to identify it throughout its lifetime.

Extending the Example

Consider the following Go code:

package main

import (
    "fmt"
    "runtime"
)

type Something struct {
    number int
    queue  chan int
}

func gotest(s *Something, done chan bool) {
    fmt.Println("from gotest:")
    fmt.Println(&s)
    for num := range s.queue {
        fmt.Println(num)
        s.number = num
    }
    done <- true
}

func main() {
    runtime.GOMAXPROCS(4)
    s := new(Something)
    fmt.Println(&s)
    s.queue = make(chan int)
    done := make(chan bool)
    go gotest(s, done)
    s.queue <- 42
    close(s.queue)
    <-done
    fmt.Println(&s)
    fmt.Println(s.number)
}

Running this code would output:

0x4930d4
from gotest:
0x4974d8
42
0x4930d4
42

This demonstrates that the pointer value within the go routine is different from the original object, but the value of the number field is updated correctly.

Conclusion

While Go functions pass arguments by value, you can still inspect pointer values to understand the behavior of your code. Using techniques like setting finalizers, you can gain insight into the object's identity and reference behavior, which can be especially helpful in concurrent programming scenarios.

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