Why Does Changing a String's Value via a Pointer in Go Not Change Its Memory Address?

Immutable String Values and Pointer Address

In Go, the official specification states that strings are immutable, meaning their contents cannot be altered once created. However, a peculiar observation arises from the following code snippet:

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  

Contrary to expectations, after assigning "world" to the dereferenced pointer *ptr, the address of &str remains unchanged.

This phenomenon requires an understanding of what is meant by "immutability" in this context. String values in Go are immutable, not the variables that hold them. In the code sample, str is a variable of type string, and its value can be changed.

The string value "hello" itself is immutable. Assigning "world" to str does not alter "hello" but simply changes the value of the str variable to "world." This change is independent of how the value is set, whether directly or through a pointer.

Immutable string values ensure that shared strings remain consistent throughout the program. For example, if a function receives a string argument, its value cannot be altered within that function. Therefore, subsequent print statements of the same string value will always yield the expected result.

Internally, string values are represented by reflect.StringHeader structs. These structures store a pointer to the UTF-8 encoded value of the text and its byte-length. This data is not accessible, preventing direct modifications. Additionally, string values cannot be indexed and assigned new values.

While the Go spec guarantees the immutability of string values, using the unsafe package can override these guarantees. However, doing so relinquishes the protection and safety provided by the language specification.

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