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Methods in Go attach to a type via a receiver. The receiver can be a value or a pointer — and choosing wrong is one of Go's classic gotchas.

The two forms

(c Counter) is the value-receiver form. The method gets a copy of the struct. Mutations don't persist.

(c *Counter) is the pointer-receiver form. The method gets a pointer — mutations go through to the original.

Why this matters

Go automatically takes the address when you call a pointer-receiver method on an addressable value.

When to use a pointer receiver

Use *T when:

1. The method needs to mutate the receiver — value receivers can't.
2. The struct is large — copying is expensive.
3. For consistency — if any method on the type uses a pointer receiver, all of them should.

Use value receiver when:

• The type is small (an int, a small struct).
• You don't need to mutate.
• The type is meant to be a value type (Time, Color, Coordinate).

The map element trap

Map values aren't addressable — you can't get a pointer to them, so you can't call pointer-receiver methods on them directly.

Interface satisfaction

Value and pointer methods determine which type satisfies an interface:

If Greet had a pointer receiver, only *Dog (not Dog) would satisfy the interface.

Common mistakes

• Calling a pointer method on a non-addressable value: e.g., someFunc().Method() where someFunc() returns a value, not a pointer.
• Mixing value and pointer receivers on the same type — confusing for readers and risks silent bugs.
• Pointer receivers on tiny types — copying an int via value is faster than dereferencing a pointer.