How Does Go's String Immutability Work, Considering That String Variables Can Be Reassigned?

Immutable String Values: Decoding the Go Spec

Introduction

The Go specification emphasizes the immutability of strings, stating that the contents of a string cannot be altered once created. This can be puzzling when dealing with string variables and pointers, as demonstrated by the following code:

str := "hello"
fmt.Printf("%p\n",&str) // 0x1040c128
fmt.Printf("%v\n",str) // hello
ptr := &str
*ptr = "world"
fmt.Printf("%p\n",&str) // 0x1040c128
fmt.Printf("%v\n",str) // world  

Here, we reassign the value of the string variable str through a pointer, leading us to question the concept of string immutability. Let's explore the underlying mechanics to unravel this apparent paradox.

The Truth behind Immutability

String immutability refers specifically to the values of strings, not their variables. str is a string variable, not a string value. Thus, the value of str can be modified, which is what we observe when reassigning through a pointer.

The string value, such as "hello", is what remains immutable. When we assign "world" to str, we're simply replacing one immutable value with another. The address of str remains the same because it's a variable, not a value.

Immutability in Practice

String values are immutable under the hood. They are represented by a struct containing a pointer to the UTF-8 encoded byte array and its length:

type StringHeader struct {
    Data uintptr
    Len  int
}

These fields are inaccessible to the programmer, ensuring the immutability of the string value.

Limitations of Immutability

While string values are generally immutable, there are exceptions when using the unsafe package. This package allows bypassing type safety checks and, therefore, potentially modifying string values. However, such operations fall outside the guarantees provided by the Go specification and may lead to unexpected behaviors.

Conclusion

String immutability in Go exists at the value level, ensuring that the contents of strings cannot be altered indirectly. This guarantees the integrity and consistency of string data throughout a program.

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