> In article news.nems.noaa.gov>,
> Paul Van Delst noaa.gov> writes:

>> Steven G. Kargl wrote:

>>> In article news.nems.noaa.gov>,
>>> Paul Van Delst noaa.gov> writes:
>>>

>>>>>
>>>>> The following two line, I believe, are nonconforming.

>>>>
>>>> Really? Huh... I can't be bothered hunting through the standard so
>>>> I'll take your word for it (I have used equivalence about two times
>>>> in the last 20 years I reckon so I have no idea as to how to use it
>>>> in a standard-ly manner.)

>>>
>>>
>>> You need to wade through Section 14 of the Fortran 95 standard.
>>>
>>>
>>> 14.7.6 Events that cause variables to become undefined
>>>
>>> Variables become undefined as follows:
>>> (1) When a variable of a given type becomes defined, all associated
>>> variables of different type become undefined. However, when a
>>> variable of type default real is partially associated with a
>>> variable of type default complex, the complex variable does not
>>> become undefined when the real variable becomes defined and the
>>> real variable does not become undefined when the complex variable
>>> becomes defined. When a variable of type default complex is
>>> partially associated with another variable of type default complex,
>>> definition of one does not cause the other to become undefined.
>>>
>>> Your read statement caused the double precision variable to become
>>> defined. Thereby, i3 is technically undefined. The Standard does
>>> not list this as a constraint, so a compiler can do anything it
>>> wants. This includes the de facto standard of simply using i3 'as if'
>>> had been defined.

>>
>> I read the standard in the section you indicated and, pardon my
>> denseness, but how is an equivalence statement ever to be used
>> to equivalence different types? Or have you never been able to do
>> that?

>
> We're going to need someone like Richard Maine to comment on the
> historical developement of equivalence.
>

>> In 14.7.5 (Events that cause variables to become defined) it states that:
>>
>> (10) When a character storage unit becomes defined, all associated
>> character storage units become defined. When a numeric storage
>> unit becomes defined, all associated numeric storage units of
>> the *same type* [my emphasis] become defined. When an entity of
>> double precision real type becomes defined, all totally associated
>> entities of double precision real type become defined. When an
>> unspecified storage unit becomes defined, all associated unspecified
>> storage units become defined.
>>
>> Fair enough. Associated storage unit of same type == everything gets
>> defined. But the standard also allows equivalencing of different types,
>> and of scalars and arrays.

>
> AFAICT, equivalance of different types is simply storage association
> where different variables can re-use memory. Back when 64 KB of memory
> was considered standard. Re-using memory was very important.
>

>> So, for the case of different types in an equivalence, whenever you
>> define one you automatically undefine the other?

>
> Yep. I believe the idiom would be
>

>> That is,
>> integer :: i
>> real :: x
>> equivalence (i, x)
>> i=10 ! This causes x to become undefined

>
> Do a bunch of computations involving i where x isn't needed.
>

>> x=1.0 ! Whoa. This causes i to become undefined

>
> Now, do a bunch of computations with x where i isn't needed.

>>
>> Wha..?
>>
>> I'm missing something obvious here.

>
> Probably just the 60 year history of a language where memory
> went from ? -> 16 KB -> 64 KB -> 640 KB -> 16 MB -> 4 GB -> 16 GB.