Go: Why Am I Getting 'Invalid Operation: type *map[key]value does not support indexing.'?

Go: Addressing "Invalid Operation" Error in Map Indexing

When working with maps in Go, you may encounter the compiler error "invalid operation: type *map[key]value does not support indexing." This error occurs when you attempt to index a pointer to a map instead of the map itself.

Consider the following code:

func (b *Balance) Add(amount Amount) *Balance {
    current, ok := b[amount.Currency] // Error: indexing pointer to map
    ...
}

To resolve this error, you should modify your code to index the map through the pointer correctly:

func (b *Balance) Add(amount Amount) *Balance {
    current, ok := (*b)[amount.Currency] // Index the map through the pointer
    ...
}

Alternatively, if your struct is solely a map, it's recommended to avoid defining methods that receive map pointers as they offer no benefit. Instead, define methods that receive the map by value, as shown below:

import "fmt"

type Currency string

type Amount struct {
    Currency Currency
    Value float32
}

type Balance map[Currency]float32

func (b Balance) Add(amount Amount) Balance { // Method receives map by value
    current, ok := b[amount.Currency]
    ...
}

func main() {
    b := Balance{Currency("USD"): 100.0}
    b = b.Add(Amount{Currency: Currency("USD"), Value: 5.0})

    fmt.Println("Balance: ", b)
}

This approach avoids the error associated with indexing a map through a pointer while maintaining the reference nature of the map type.

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