How Does Go Handle Pass by Reference and Value, and Why Use the Asterisk (*)?

Passing by Reference and Value in Go Functions

The concept of passing by reference and value in Go can be confusing, especially when comparing it to other languages like Java. This article will clarify the differences and explain why Go requires the use of the asterisk (*) in front of certain types.

Passing by Reference vs. Passing by Value

Go technically only passes by value. This means that when you pass a variable to a function, a copy of that variable is created and passed. Any changes made to the copy within the function will not affect the original variable.

However, when passing a pointer to an object, you are effectively passing a pointer by value. The pointer itself is a value that contains the memory address of the object. Any changes made to the object through the pointer will be reflected in the original object outside the function.

The Need for the Asterisk (*)

The asterisk (*) in Go is used to indicate that the type is a pointer to the specified type. For example, the following function expects a pointer to a sql.DB object:

func PutTasks(db *sql.DB) echo.HandlerFunc {
    // ...
}

This syntax is required because Go requires you to explicitly specify whether you are passing an object by value or by reference. If you do not include the asterisk, the function will expect an sql.DB value, which is not a valid type for this function.

Why Pass by Pointer?

Passing by pointer is used when you want to share the value between the caller and the function body. Any changes made to the object within the function will be reflected in the caller's variable. This is necessary when you want to modify the caller's object or when you need to access large objects efficiently without copying them.

Example

Consider the following Go code:

func someFunc(x *int) {
    *x = 2 // Modifies the original variable
    y := 7
    x = &y // Changes the pointer to point to a different variable
}

In this example, the function someFunc takes a pointer to an integer x. Inside the function, the pointer is dereferenced (*) to modify the original variable. Additionally, the pointer is reassigned to point to a different variable y. This change is reflected in the caller's variable because the pointer itself was modified within the function.

Conclusion

Passing by reference and value in Go is a fundamental concept to understand. The asterisk (*) is used to indicate that you are passing a pointer to a type, which allows you to share the value between the caller and the function body. This is especially useful for modifying caller's variables or when dealing with large objects efficiently.

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