This article deeply explores the reason why the function return value address cannot be directly obtained in the Go language, and clarifies the need for the address operator & "home" variable. We provide a canonical method for converting a temporary return value into a desirable address variable and demonstrate it with sample code. At the same time, the article emphasizes the scenarios where the *string type is often unnecessary in Go, explains the characteristics of string as a value type and its transfer efficiency, aiming to help developers avoid unnecessary use of pointers and write code that is more consistent with Go habits.

Understand address operators and temporary values ??in Go language

In Go language programming, trying to directly obtain the address of the return value of a function call, such as &a(), will result in a compilation error cannot take the address of a(). The core reason for this behavior lies in the semantics of the address operator & in the Go language and the characteristics of the objects it operates on.

The & operator is designed to get the memory address of a variable . A variable has a clear "home" in memory, which is a fixed, addressable storage location. However, the return value of function a() is a temporary value. This value is generated immediately after the function call completes, but it is not bound to a named variable, so it does not have a stable and addressable "home" in memory. It is just a calculation result that exists momentarily. The Go compiler does not allow you to directly address this "homeless" temporary value because it does not have a persistent and referable memory location.

Although the compiler of the Go language performs escape analysis and promotes some local variables from the stack to the heap (i.e. "escapes"), this only applies to variables , not the results of temporary expressions. Therefore, even though the return value may theoretically be optimized by the compiler, it cannot be addressed directly.

The correct way to get the function return value address

If you really need to get the address of a function return value in Go, the standard approach that is consistent with Go language habits is to assign it to a temporary variable, and then get the address of the temporary variable. By introducing an intermediate variable, we provide a "home" for this return value, making it an addressable entity.

Here are the canonical steps to achieve this:

1. Assign the return value of function a() to a new local variable, such as tmp.
2. Use the address operator & on this local variable tmp to obtain its memory address.

Sample code:

 package main

import "fmt"

// A function func that returns a string getStringValue() string {
    return "Hello, Go Pointers!"
}

func main() {
    // Error example: directly obtain the address of the function return value // var ptrToString *string = &getStringValue() // Compilation error: cannot take the address of getStringValue()

    // Correct posture: Get the address through a temporary variable tempString := getStringValue() // Assign the function return value to a temporary variable ptrToString := &tempString // Get the address of the temporary variable fmt.Printf("Access string value through pointer: %s\n", *ptrToString)
    fmt.Printf("Memory address of string pointer: %p\n", ptrToString)

    // Further demonstrate the behavior of *string: modify the pointer pointing instead of the string content strA := "String A"
    strB := "String B"

    myPtr := &strA // myPtr points to strA
    fmt.Printf("myPtr initially points to: %s\n", *myPtr) // Output: String A

    myPtr = &strB // myPtr now points to strB
    fmt.Printf("myPtr now points to: %s\n", *myPtr) // Output: String B

    // Note: The string content itself is immutable.
    // Attempting to modify the contents of *myPtr will result in a compilation error or a runtime error,
    // Because strings are immutable, *myPtr = "New Value" is not allowed.
    // Any "modification" of a string creates a new string.
}

Through this two-step operation, we not only solved the compilation error, but also followed the memory model and variable addressing rules of the Go language.

Considerations for using *string in Go language

Although the above method can successfully obtain the address of the string type value, in the actual development of the Go language, the use of the *string type often requires caution. In many cases, using *string may be unnecessary or even a misunderstanding.

Characteristics of string type in Go:

• Value type: string is a value type in Go. This means that when a string is passed as a function parameter or assigned, what is actually passed or copied is its value.
• Efficient transfer: The Go language's string type is internally implemented as a compact structure that contains a pointer to the underlying byte array and a length field. Therefore, even if the string content is very long, passing a string value only copies this small structure (usually 16 bytes), and its efficiency is similar to passing a pointer plus an integer. This makes passing string values ??between functions usually very efficient without worrying about performance issues.
• Immutability: String values ??in Go are immutable. Once a string is created, its contents cannot be modified. Any operation that looks like it modifies a string (such as string concatenation or slicing) actually creates a new string.

* Why ` string` is often unnecessary:**

Since string is a value type and its transmission is efficient, in most cases, the string value can be passed directly. Using *string means you are passing a pointer to a string. Generally, you only need to consider using *string when you need to achieve the following specific behaviors:

1. Modify the string pointed to by a variable: If you need a function that modifies the string pointed to by a variable in the caller's stack frame (rather than modifying the string content itself, since strings are immutable), then you need to pass *string. For example, change a string variable from pointing to "A" to pointing to "B".
2. Represents an optional or nullable string: In some API designs, in order to distinguish an empty string "" from a "non-existent" or "not set" string, *string can be used. If the pointer is nil, it means it is not set; if the pointer is non-nil but points to "", it means an empty string. This may be encountered when interacting with databases or JSON serialization/deserialization, such as declaring *string in a struct field to support null values.

Things to note:

• Avoid unnecessary pointers: The Go language encourages the use of value semantics. Avoiding unnecessary pointers can simplify code logic, reduce the possibility of memory escapes, and may improve performance in certain scenarios.
• Understand the difference between pointers and values: *string changes the address pointed by the pointer (that is, which string variable it points to), not the content of the string. The string content itself is immutable.

Summarize

In the Go language, it is not feasible to directly take the address of the function return value, because the return value is a temporary value and does not have a fixed memory address. The correct approach is to assign it to a temporary variable and then take the address of the variable. However, in most cases, for string, an efficient and immutable value type, passing its value directly is usually a more idiomatic and performant choice in Go. You should consider using *string only when you need to modify the string pointed to by the variable, or when you need to clearly distinguish between "empty string" and "unset string" in specific scenarios. Understanding the semantics of value types and pointers in the Go language is the key to writing code that is efficient, clear, and consistent with the Go language philosophy.

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