CODEWITHSUNDEEP
methodsgo
ash
ash

Reputation: 5539

Why can methods in Go only be declared on types defined in the same package?

The Go Tour says the following:

You can only declare a method with a receiver whose type is defined in the same package as the method. You cannot declare a method with a receiver whose type is defined in another package (which includes the built-in types such as int).

Is there a reason for this other than avoiding everyone building their own methods off int and string? I've Googled around, but can't find anything referencing it.

Upvotes: 5

Views: 1986

Answers (1)

joshlf
joshlf

Reputation: 23567

The reason is that if you could define methods on other packages' types, you could modify the behavior of other packages. This is because the method set of a given type can have an effect on how values of that type are used.

Consider, for example, the fmt.Println function. When you pass an argument to fmt.Println, it will print a string representation of that value based on a set of rules. One of those rules is that if the type of the value has a String() string method (that is, it implements the fmt.Stringer interface), then that method will be called in order to obtain the string representation of the value.

Thus, imagine that we have a package, foo, and that package has a type, FooInt, defined as follows:

type FooInt int

Now imagine that this package also has a function, PrintFooInt:

func PrintFooInt(f FooInt) { fmt.Println(f) }

This will print the integer value of f. But let's say that you (in a different package, say main) were able to add methods to FooInt. Then you could do this:

func (f FooInt) String() string { return "foobar!" }

This would actually change the behavior of foo.PrintFooInt, which shouldn't be possible from outside the package.

Upvotes: 10

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