How Can I Resolve the 'cannot use variable of type *T as type stringer' Error in Go Generics?

Type Safety Considerations in Generics

In Go's generics, the type parameter T is distinct from the constraint interface FooBar. However, this distinction raises a question: how can you pass an instance of T to a function expecting a type that implements stringer (like do)?

Understanding the Error

The error message "cannot use variable of type *T as type stringer in argument to do" suggests that the parameter t of blah is being misidentified as a type that implements stringer. However, T does not inherently possess this method.

Resolving the Issue

To enable the conversion of t to stringer, the following steps are necessary:

1. Introduce the stringer interface to FooBar: Add stringer to the FooBar constraint, allowing all types that satisfy FooBar to have a stringer method.
2. Change the types in the union: Modify the FooBar constraint to accept pointers to foo or bar, as the receiver type of stringer is a pointer.
3. Declare composite literals as pointers: Since you can't declare composite literals on constraints without a core type, you need to declare t as a pointer type.
4. Introduce a type parameter for FooBar: Extract the type union from FooBar into a new type parameter to ensure that U and T have compatible type sets.

Final Code

type FooBar[T foo | bar] interface {
    *T
    stringer
}

func blah[T foo | bar, U FooBar]() {
    var t T
    do(U(&t))
}

func main() {
    blah[foo]()
}

Explanation

Now, when blah is called, a variable t of type T is declared as a pointer. FooBar[T] constrains U to be a type that contains a pointer to T. Converting &t to U is valid, and the methods of FooBar[T] (which includes stringer) are available to the receiver t.

Note:

Using type parameters for FooBar allows for passing arguments and removes the need for intermediate trickery. For instance, you could modify blah to:

func blah[T FooBar](t T) {
    do(t)
}

And call it with blah(&foo{}).

The above is the detailed content of How Can I Resolve the 'cannot use variable of type *T as type stringer' Error in Go Generics?. For more information, please follow other related articles on the PHP Chinese website!