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From: glen herrmannsfeldt <g...@ugcs.caltech.edu>
Newsgroups: comp.lang.fortran
Subject: Re: Function returning defined type?
Date: Mon, 22 Oct 2012 05:12:00 +0000 (UTC)
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jski <john.chludzin...@gmail.com> wrote:

(snip regarding indexing, substringing, function calling and
member selection on the return value of a function.)

> At the risked of being flamed, without simply saying Fortran isn't C,
> is there a rationale for:

> if F(x) returns a type with components a, b, and c, why is F(x)
> %c a "syntax error"?

As mentioned in another post, there are some ambiguous cases, even if
this isn't one. One that I miss more is arrays of pointers, which 
are also not allowed, possibly because of the ambiguities.

> PS>  Actually, in the case of the C++ I'm translating, it turns out
> the function call I'm referring to here is the indexing into an STL
> map<>.  This is done over 2 dozen times in a loop the iterates at
> least 1 million times per run.  Replacing these index "calls" is an
> obvious step to improving its performance.

Are you claiming that the Fortran syntax is slower because it
requires an explicit temporary variable?

It might be that the C or C++ compilers generate a temporary
anyway, and it might be that the Fortran compiler can 
optimize out the store into the variable. It is a syntax
question, not a performance question. Besides, it might
take one extra instruction, and so your 12 million calls
might take a few milliseconds longer.

Now, note that returning a structure requires the function
to return all the elements of the structure, though
you only need one. (Well, possilby there is only one member.)

Same for arrays. Indexing a function returning an array is a
waste, unless the array only has one element.

If performance is a question, write a version of the function
that returns only that element. (Or an ENTRY point into the
existing function.)

-- glen