Why Does Go Throw 'Invalid Operation: Type *map[key]value Does Not Support Indexing'?

Go: Invalid Operation - Type *map[key]value Does Not Support Indexing

Background:

Go's pointer mechanisms can be confusing, especially when working with maps. In Go, a map is a reference type, meaning that when you pass a map by value, you're passing a copy of the reference, not the actual map itself. However, sometimes we may need to modify the original map that is passed to a function as an argument.

Issue:

When trying to pass a map by pointer to modify its contents, you may encounter the following error:

prog.go:15: invalid operation: b[amount.Currency] (type *Balance does not support indexing)

This error indicates that you're attempting to index a pointer to a map, which is not supported in Go.

Solution:

To resolve this issue, you need to differentiate between accessing the map's value and the pointer to it. Here's the correct way to do it:

func (b *Balance) Add(amount Amount) *Balance {
    // Index the *b (pointer to map) to get the map itself
    current, ok := (*b)[amount.Currency]
    // Continue modifying the indexed map
    // ...
}

By dereferencing the pointer b using *b, you gain access to the map itself and can perform the necessary operations on it.

Alternative Approach:

If the map is the only field in your struct, as is the case in this example, you can avoid passing it by pointer. Instead, pass it by value as shown in the following code:

func Add(b Balance, amount Amount) Balance {
    // ... do your operations on the passed map ...
    return b
}

Go passes maps by reference by default, so even if you pass it by value, it's still treated as a reference internally, and any changes made within the function are reflected in the original map.

Remember, when working with maps, always differentiate between indexing the map itself and the pointer to it. This will help you avoid errors like the one encountered in this case.

The above is the detailed content of Why Does Go Throw 'Invalid Operation: Type *map[key]value Does Not Support Indexing'?. For more information, please follow other related articles on the PHP Chinese website!