How are Golang function parameters passed?

In Go, there are three ways to pass function parameters: pass by value (value or copy), pass by reference (pointer), and pass by immutable reference (structure/array reference, unmodifiable object). When passed by value, the function operates on a copy of the value and does not affect the original value; when passed by reference, the function can modify the original value; when passed by immutable reference, the function can operate on the fields of the object, but does not modify the object itself. These three methods are suitable for different scenarios. For example, passing by value is suitable for read-only operations, passing by reference is suitable for modification operations, and passing by immutable reference is suitable for operating fields of structures/arrays.

Go function parameter passing mechanism

In the Go language, function parameters are passed in three main ways:

• Pass by value (call by value): Pass a value (or a copy of the value) to the function.
• Pass by reference (call by reference): Pass a pointer to the value to the function so that the function can modify the original value.
• Pass by immutable reference (call by value of immutable reference): Pass an immutable reference of a structure or array to the function, so that the function cannot modify the original object, but can modify it Field (if the structure or array has one).

Pass by value

When you pass a parameter by value, a copy of the parameter value is passed. Any modification to this value only affects the copy within the function, not the original value.

func modifyValue(num int) {
    num *= 2
}

func main() {
    x := 10
    modifyValue(x)
    fmt.Println(x) // 输出：10
}

Pass by reference

If you want to allow a function to modify the passed value, you can use pointer pass by reference. This is accomplished by declaring a pointer parameter using the * prefix.

func modifyReference(num *int) {
    *num *= 2
}

func main() {
    x := 10
    modifyReference(&x)
    fmt.Println(x) // 输出：20
}

Passing by immutable reference

Structures and arrays can be passed by passing their immutable reference. This allows the function to access the fields of the structure or array, but not modify the object itself.

type Point struct {
    x int
    y int
}

func modifyPoint(p Point) {
    p.x *= 2
}

func main() {
    point := Point{10, 20}
    modifyPoint(point)
    fmt.Println(point) // 输出：{20 20}
}

Practical case

Suppose we have a function calculateTotalCost, its function is to calculate their total cost given a list of items.

import "fmt"

type Item struct {
    name  string
    price float64
}

func calculateTotalCost(items []Item) float64 {
    total := 0.0
    for _, item := range items {
        total += item.price
    }
    return total
}

func main() {
    items := []Item{
        {"Item 1", 10.00},
        {"Item 2", 20.00},
        {"Item 3", 30.00},
    }

    totalCost := calculateTotalCost(items)
    fmt.Println(totalCost) // 输出：60.00
}

In this case, items is passed as a slice by value. This means that the function will receive a copy of the items variable without modifying the original slice.

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