Why Can't I Directly Call Pointer Receiver Methods on Go Map Elements?

Why Does Accessing Map Elements Require an Intermediate Variable?

In Go, accessing a map element using dot notation, as in odsMap[segRef].GetValue("@OriginDestinationKey"), raises an error stating that the address of the map element cannot be taken. This is due to the inherent nature of map index expressions.

Unlike slices and arrays, which are addressable, map index expressions are not. This is because map internals can be modified upon adding new entries, rendering them non-addressable by design.

To resolve this issue, map implementations enjoy greater flexibility. However, this flexibility prevents taking the address of map index expressions, which becomes a problem when dealing with pointer receivers.

If non-pointer values are stored in the map and methods with pointer receivers need to be called, it becomes necessary to take the address of the non-pointer value to use as the receiver. However, since map index expressions cannot be addressed, a compile-time error occurs.

To circumvent this limitation, pointer values can be stored in the map, eliminating the need to take the address of an index expression. Alternatively, the value can be assigned to a local variable whose address can be taken and the pointer method called on it.

Caution is advised when choosing this approach, as methods with pointer receivers can modify the pointed object's components and these changes may not be reflected in the value stored in the map.

In summary, for values with pointer-related methods, it's preferable to use them as pointers rather than non-pointer values to avoid addressability issues with map index expressions.

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