Why Does Changing a String Pointer Seem to Modify an Immutable String in Go?

Immutable String Values

The Go specification states that strings are immutable, meaning once created, their content cannot be changed. This poses an apparent contradiction to 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  

If strings are immutable, why does changing the value of the pointer assigned to str also change the value of str?

Explanation

The key distinction here is between string values and string variables. str is a variable of type string. When you use the assignment operator (=) on str, you are changing the value of the str variable, not the value of the string that str refers to.

In Go, string values are represented by the reflect.StringHeader struct, which contains a pointer to the data and its length. Once created, this struct cannot be modified.

The & operator returns the address of a variable, not the value of the variable. Therefore, &str returns the address of the str variable, not the address of the string value that str refers to.

When you use the ptr := &str statement, you are creating a pointer to the str variable. Assigning a new value to *ptr changes the value of the str variable, but it does not change the value of the string that str refers to.

Conclusion

In Go, string values are immutable. However, string variables can be modified, which can indirectly change the string value that the variable refers to.

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