Why Can\'t Go Methods Defined on *T Be Used by T?

Why Go Disallows Method Sets on *T to Be Used by T

In Go, methods defined on a value type T can be used by both T and *T, while methods defined on a pointer type *T cannot be used by T. This seemingly arbitrary distinction stems from the complexities of memory in computer systems.

Inability to Obtain Pointers on Demand

Consider the case where a method on *T is to be called using a T variable. To achieve this, a pointer to the T variable must be obtained. However, obtaining a pointer is not always guaranteed to be possible.

Go's specification explicitly states the conditions under which an address-of operation (&) can be performed. These include accessing variables, pointer indirections, and array indexing operations. However, it excludes accessing a variable stored in a map, as in the following example:

<code class="go">res := TMap["key"].pointerMethod()</code>

In such cases, obtaining a pointer to the variable is impractical, as this would impose constraints on the runtime's implementation of data structures like maps.

Consequences of Design

This design decision has both advantages and disadvantages:

• Advantages:

  • Allows for value semantics for T types, ensuring that method calls do not modify the original value.
  • Prevents accidental modification of values through pointer references.

• Disadvantages:

  • In certain situations, it can be inconvenient to obtain a pointer to a T variable to call a pointer method.
  • Requires careful consideration when designing APIs that expect both value and pointer receivers to avoid ambiguity.

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