explicit creation type not conforming to type of target

问题

I really don't understand as it seems to me the basics of type conformance. I have a Creation instruction lists explicit creation type which does not conform to type of target on create {JANITZA_DEVICE} l_device.make_from_file_path (a_file_path) with eiffel studio 19.5 enterprise.

SMA_INVERTER_MANAGER_CSV

class
    SMA_INVERTER_MANAGER_CSV

inherit
    SUNSPEC_DEVICE_CSV[SMA_INVERTER_MANAGER_DEVICE]

create
    make

SUNSPEC_DEVICE_CSV

deferred class
    SUNSPEC_DEVICE_CSV[G -> SUNSPEC_DEVICE create make_from_file_path end]

inherit
    CONSUMPTION_SECTOR_CSV[G]
        redefine
            process_file,
            set_header_csv
        end

feature --

    process_file (a_file_path: PATH)
        require else
            attached a_file_path.entry
            attached consumption_sector
        local
            l_device: like devices.item
        do
            check
                attached_consumption_sector: attached consumption_sector
            then
                if is_valid_file_path (a_file_path) then
                    if attached a_file_path.utf_8_name.has_substring ("janitza_UMG604") then
                        create {JANITZA_DEVICE} l_device.make_from_file_path (a_file_path) -- The compiler doesn't agree!
                    else
                        create l_device.make_from_file_path (a_file_path)
                    end
                    l_device.load_measuring_point (create_measuring_points, measuring_point_db_service, consumption_sector)
                    devices.extend (l_device)
                    Precursor (a_file_path) -- load measure_units from csv_row
                    devices.wipe_out
                end
            end
        ensure then
            devices.is_empty
        end

CONSUMPTION_SECTOR_CSV[G]

deferred class
    CONSUMPTION_SECTOR_CSV[G -> MEASURING_POINT_DEVICE]

feature -- Access

    devices: LINKED_SET[G]

SUNSPEC_DEVICE

class
    SUNSPEC_DEVICE

inherit
    MEASURING_POINT_DEVICE
        redefine
            default_create,
            set_measuring_point,
            out
        select
            serial
        end

    MODBUS_DEVICE
        rename
            serial as modbus_serial,
            set_serial as set_modbus_serial
        undefine
            make
        redefine
            default_create,
            make_from_file_path,
            name_from_file_path,
            out
        select
            set_modbus_serial
        end

create
    make_from_file_path

JANITZA_DEVICE

class
    JANITZA_DEVICE

inherit
    SUNSPEC_DEVICE
        redefine
            set_measure_units,
            name_from_file_path
        end

create
    make_from_file_path

回答1:

Here is a simplified case:

class ANIMAL

class CAT inherit ANIMAL

class HOUSE_CAT inherit CAT

class DOG inherit ANIMAL

class
    ENCLOSURE [G -> ANIMAL]
feature
    specimens: LIST [G] --> The actual type will vary with the generic parameter

    describe
        do
            print(specimens.generating_type)
        end

class
    APPLICATION

feature

    test
        local
            l_cat: CAT
            l_animal_enclosure: ENCLOSURE [ANIMAL]
            l_cat_enclosure: ENCLOSURE [CAT]
            l_house_cat_enclosure: ENCLOSURE [HOUSE_CAT]
            l_dog_enclosure: ENCLOSURE [DOG]
        do
            create l_specimen

            create l_animal_enclosure
            l_animal_enclosure.describe --> LIST [ANIMAL]
            l_animal_enclosure.specimens.add (l_cat) --> Fine, CAT conforms to ANIMAL

            create l_cat_enclosure
            l_cat_enclosure.describe --> LIST [CAT]
            l_cat_enclosure.specimens.add (l_cat) --> Fine, CAT conforms to CAT

            create l_house_cat_enclosure
            l_house_cat_enclosure.describe --> LIST [HOUSE_CAT]
            l_house_cat_enclosure.specimens.add (l_cat) --> ERROR, CAT does not conform to HOUSE_CAT


            create l_dog_enclosure
            l_dog_enclosure.describe --> LIST [DOG]
            l_dog_enclosure.specimens.add (l_cat) --> ERROR, CAT does not conform to CAT
        end

In your case, devices: LINKED_SET [G] is too vague, nothing proves that JANITZA_DEVICE is a valid type because G might end up being lower down the hierarchy (like HOUSE_CAT -> CAT; you cannot substitute a CAT to a HOUSE_CAT, thus a list of HOUSE_CAT cannot accommodate a CAT) or in a separate branch of the hierarchy (like DOG -> ANIMAL; a dog is not a cat, they only share a common ancestor).

If {ENCLOSURE}.specimens was declared as LIST [ANIMAL] instead of LIST [G], describe would always print LIST [ANIMAL] regardless of the actual generic parameter because the type would not vary, thus the previous code would compile and run just fine. Similarly, if {CONSUMPTION_SECTOR_CSV}.devices was declared as LINKED_SET [SUNSPEC_DEVICE] instead of LINKED_SET [G], it could accommodate all descendants of SUNSPEC_DEVICE regardless of the actual type of G.

Alternatively, you could move the parts specific to JANITZA_DEVICE from CONSUMPTION_SECTOR_CSV to a descendant of CONSUMPTION_SECTOR_CSV where G is closed, as in

class
    JANITZA_CONSUMPTION_SECTOR_CSV -- No generic here!
inherit
    CONSUMPTION_SECTOR_CSV [JANITZA_DEVICE]
        redefine
            process_file -- Add the parts specific to `JANITZA_DEVICE` in the implementation
        end

which would ensure that devices can hold instances JANITZA_DEVICE.

回答2:

I think that user10481525 has explained the reason for the error. Your code doesn't guarantee that JANITZA_DEVICE will conform to G in all potential descendants of SUNSPEC_DEVICE_CSV.

You have declared the attribute devices: LINKED_SET [G]. Therefore, the local variable l_device: like devices.item would be of type G. Attaching a JANITZA_DEVICE to it might be invalid in a descendant of SUNSPEC_DEVICE_CSV. Why? Because G could be any descendant of SUNSPEC_DEVICE.

For example, suppose you had a descendant class FOO_DEVICE_CSV inherit SUNSPEC_DEVICE_CSV [FOO_DEVICE]; where class FOO_DEVICE inherit SUNSPEC_DEVICE. Your local variable would effectively resolve to local l_device: FOO_DEVICE. Therefore, the descendant would be trying to attach an object of type JANITZA_DEVICE to it. But that's invalid, because JANITZA_DEVICE doesn't conform to FOO_DEVICE:

feature devices: LINKED_SET [FOO_DEVICE] ... local l_device: FOO_DEVICE ... create {JANITZA_DEVICE} l_device.make_from_file_path (a_file_path) -- Invalid!

来源：https://stackoverflow.com/questions/58368872/explicit-creation-type-not-conforming-to-type-of-target

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polymorphism

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