Why Can't I Call a Pointer Receiver Method on a Go Map Entry?

Dereferencing a Map Index in Golang

When working with maps in Golang, it's important to understand the limitations of referencing map entries. In a recent programming scenario, a question arose regarding the error: "cannot call pointer method on f[0]".

To clarify the issue, let's examine the following code:

package main

import (
    "fmt"

    "inventory"
)

func main() {
    x := inventory.Cashier{}
    x.AddItem("item1", 13)
    f := x.GetItems()

    fmt.Println(f[0].GetAmount())
}

The problem lies in the type of the map and the method being called. In the Driver.go file, the GetItems method is called on the Cashier struct, which returns a copy of the items map. However, the GetAmount method is a pointer receiver method, which requires the actual struct to be used.

In Go, map entries cannot be addressed directly because their address may change when the map grows or shrinks. As a result, you cannot call pointer receiver methods on map entries.

To resolve this issue, you can modify the GetItems method to return a pointer to the map instead:

func (c *Cashier) GetItems() *map[int]item {
    return &c.items
}

By doing so, you ensure that the GetAmount method can be called directly on the map entry. Here's the corrected main function:

func main() {
    x := inventory.Cashier{}
    x.AddItem("item1", 13)
    f := x.GetItems()
    fmt.Println((*f)[0].GetAmount())
}

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