generic-programming - Eiffel：有没有办法在没有任何附加实例的情况下测试给定类的通用参数？

Question

有没有办法在没有任何附加实例的情况下测试给定类的通用参数？

class BAG[G -> MOUSE]

feature -- 

    discriminate
        do
            if G.conforms_to (MAMMAL) then
                io.putstring ("you gave me a real animal")
            elseif G.conforms_to (COMPUTER_ACCESSORY) then
                io.putstring ("Seems you don't like animals such as computers")
            else
                io.pustring ("Still dont know what you are dealing with")
            end
        end

标签： generic-programmingeiffel

Answer 1

你几乎成功了。缺少的部分是花括号和括号：

        if ({G}).conforms_to ({MAMMAL}) then
            io.put_string ("You gave me a real animal.")
        elseif ({G}).conforms_to ({COMPUTER_ACCESSORY}) then
            io.put_string ("Seems you don't like animals such as computers.")
        else
            io.put_string ("Still don't know what you are dealing with.")
        end

解释：

1. {FOO}，其中FOO是类型名称，代表类型对象。它适用于任何类型，包括正式的泛型，因此{G}和{MAMMAL}.
2. 该语法{FOO}.bar是为非对象调用保留的。但是这里我们想要对类型对象进行对象调用。因此，{G}用括号括起来：（({G}).conforms_to而不是{G}.conforms_to）。