How do I use generics with custom types?

October 29, 2023

I’m really confused about how the rules around generics work. I just want to have a function accept multiple custom types and have different logic for each one.

type myObjOne struct {
    myfield        string
}

type myObjTwo struct {
    myObjTwo
    myOtherField   string
}

type Generic interface {
    myObjOne | myObjTwo
}

func derp[T Generic](c T) {
    switch any(c).(type) {
    case myObjOne:
        // I can't access the fields of my types like this
        c.myfield = "sdfsfsdfdsf"
    case myObjTwo:
        c.myOtherField = "sdfsfsdfdsf"
    }
    return
}

Why is c.myfield throwing c.myfield undefined (type T has no field or method myfield) compilerMissingFieldOrMethod?

>Solution :

c is type T, not the result of the type assertion. Fix by assigning the result of the type assertion to a variable.

func derp[T Generic](c T) {
    switch c := any(c).(type) {
    case myObjOne:
        // I can't access the fields of my types like this
        c.myfield = "sdfsfsdfdsf"
    case myObjTwo:
        c.myOtherField = "sdfsfsdfdsf"
    }
    return
}

This code declares a second variable c in the type switch. The type of c depends on the branch of the switch.