Thread: Question on multiple return values

Originally Posted by nonoob

I'd be more comfortable with calling menu like: menu(&principal, &rate, &months);, with those variables being defined in main.

As am I.
I merely want to make it clear, that yes, you can return data via an array, as distasteful to some as it may be.

Originally Posted by Steve A.

Not true.
It's clearly in the code.

No. What you're not saying is that your trick --and it is a trick-- only works for static types. I saw the declaration and that is why I pointed it out.
Without the static modifier the array is disposed of, released from the stack, the instant the program returns... any pointer retuned to that variable is immediately invalid and very likely to produce incorrect results.

No, it is an array.
It is clearly declared as an array.

But you are NOT returning the array... C can't do that. What you are doing is what I said; returning a pointer and choosing to treat it as an array.

All that is clearly indicated within the code.
I'm not keeping any secrets here.
The beginner is going to learn about scope eventually.

Yes they will. But unless you tell them the limitations and pitfalls of of the scheme you're using, you are indeed misleading them about what the language can and cannot do.

No, actually, its because an array doesn't fit into the EAX register, but, a pointer to the array does.
But, that's being too technical, isn't it.

Yes that's part of it... but even a static array is an entity of unknown size. The language doesn't have a list someplace that says "this array is this big, that array is that big"... that information is simply not stored anywhere. You can declare a static int array of 3 elements inside your function, return a pointer to it then treat that pointer as a pointer to a 2 dimensional array of 30 x 50... neither the the compiler nor the runtime will know or care. Thus it is up to you to treat these things correctly. In the end it has very little to do with CPU registers and a lot more to do with careful programming, which these guys won't do unless they have appropriate information... Telling them C can return arrays because you know a trick that makes it look that way is simply wrong.

No, actually Tater, I did think about it. And I knew you would bring it up.
I'm well aware of the consequences of static variables.
However, they are an established part of the C programming language.
And, nearly all programming languages.
You cannot ignore that they do exist.

Nobody said any such thing... do not put words in my mouth to further your own arguments.

Explain then why the text books and documentation go to lengths to describe the usage of these "bad" features.
Why then, are they part of the ANSI Standard ?

Nobody said it was a bad feature... you're putting words in my mouth again!

What I did say is that showing them stuff like this without explaining why and how it works is likely to lead to them misunderstanding how the C language actually works.

Originally Posted by CommonTater

No. What you're not saying is that your trick --and it is a trick-- only works for static types.

What trick ??

Originally Posted by Wikipedia

Static variable:
From Wikipedia, the free encyclopedia:
In computer programming , a static variable is a variable that has been allocated statically — whose lifetime extends across the entire run of the program. This is in contrast to the more ephemeral automatic variables (local variables), whose storage is allocated and deallocated on the call stack; and in contrast to objects whose storage is dynamically allocated.
In many programming languages, such as Pascal, all local variables are automatic and all global variables are allocated statically. In these languages, the term "static variable" is generally not used, since "local" and "global" suffice to cover all the possibilities.

In the C programming language, the function of static variables can be illustrated as such:

Code:

#include <stdio.h>
 
void func() {
        static int x = 0; // x is initialized only once across three calls of func()

        printf("%d\n", x); // outputs the value of x
        x = x + 1;
}
 
int main(int argc, char * const argv[]) {
        func(); // prints 0
        func(); // prints 1
        func(); // prints 2
        return 0;
}

C and related languages:
In the C programming language (and its close descendants such as C++ and Objective-C), static is a storage class (not to be confused with classes in object-oriented programming), along with auto (for automatic variables), register (a variant of auto) and extern (for static variables and functions which are visible in other source files). Every variable in a C program is in exactly one of these storage classes.

Every C variable declared as static has static lifetime and is not visible outside its own translation unit. There are other effects depending on where the variable is defined:
Static global variables: variables declared as static at the top level of a source file (outside any function definitions) are only visible throughout that file ("file scope").
Static local variables: variables declared as static inside a function are statically allocated while having the same scope as automatic local variables. Hence whatever values the function puts into its static local variables during one call will still be present when the function is called again.
static member variables: in C++, member variables declared as static inside class definitions are class variables (shared between all class instances, as opposed to instance variables).

Nowhere is "static" equated with "trick".

But you are NOT returning the array... C can't do that. What you are doing is what I said; returning a pointer and choosing to treat it as an array.

Because that's how you do it, Tater.

Nobody said any such thing... do not put words in my mouth to further your own arguments.

What are you referring to here ??

Nobody said it was a bad feature... you're putting words in my mouth again!

Maybe...by your response ...? I don't know...

Originally Posted by Steve A.

Because that's how you do it, Tater.

Well, actually no... because that's how the LANGUAGE does it.
Like you said, you can't return an array because it won't fit in the register.

Ok... since you seem far more interested in the argument than the facts... one more try at this before I ignore you...

I did not say that what you are doing is wrong. I've used it myself, when it's appropriate. ... ok, got that?

What I did point out is that although it appears to return an array, all it's really doing is returning a pointer that you are choosing to treat as an array in your code. The point was not what you are doing... the point was that by not providing an explaination of how it works, people will mistakenly believe that C can return an actual array and until they learn about static modifiers and rules of scope, your unexplained trick is likely to cause them some misunderstaning of how C actually works... Get it? I didn't say your code was wrong... you simply should have explained it far enough to avoid misunderstandings.

Or in more direct terms... Don't be telling people C does stuff it does not know how to do.

(oh yeah... and welcome to my ignore list!)

???...listen...
is that the sound of a bicycle being peddled backwards ?

Originally Posted by CommonTater

Why using static arrays and returning pointers is a very bad idea...

Originally Posted by CommonTater

Nobody said it was a bad feature... you're putting words in my mouth again!

Why does everything here (this forum) have to turn into a pi_ss_ing contest ?
Tater, you and I both know your example is from page-1, chapter-1, of Pointers-101.

Everyone who learns about pointers learns about the side-effects of using them. Big-wup.
Anybody can take just about any C method and illustrate a worst case senario.
Your example is simply that.
Here I thought that maybe you really had something to show us.
I'm a bit disappointed that you didn't have something more interesting.

Just because I don't share the same religious ferver about C that you do, it doesn't automatically make me wrong and like wise the opposite.
We simply have differing opinions on certain subjects.

It's my belief that a student of C should be exposed to every aspect of the language and not be told that "no you can't use that, because I don't like it".

Your example could be better served, instead, by saying: "if you do decide to use pointers to return values, be aware of the side-effects" and then show your example. Just like the textbooks do on the very same subject.

Originally Posted by Steve A.

No, actually, its because an array doesn't fit into the EAX register, but, a pointer to the array does.
But, that's being too technical, isn't it.

Um. The size of EAX register has very little to do with what can and can not be returned.
In fact, functions can return any data type including a huge struct which may contain arrays. You will see memory-move instructions generated for such a case.

Originally Posted by CommonTater

You tried to tell people C can do something it cannot.

Clearly, I supplied examples of how it does.

You got tagged for it.

???
More like I got flamed for treading on your turf.

Originally Posted by nonoob

In fact, functions can return any data type including a huge struct which may contain arrays.

That is exactly my point.

Originally Posted by Steve A.

Clearly, I supplied examples of how it does.

C DOES NOT return arrays. It cannot because it does not know their size. You used a clever programming trick to make it seem like it does... and I provided a simple, clear, example of why it's a bad idea to use your trick in real programs. Simply because returning pointers couples all arrays back to the static one in the function... use that thing twice and you end up with array values that change "by magic" whenever you call the function. On it's surface it looks real clever... in reality it's a patently stupid thing to do unless you know exactly the consequences of doing it... which you either did not or could not explain. Frankly I'm guessing you discovered this trick of yours and thought yourself ever so clever, remaining blissfully unaware of it's consequences.

More like I got flamed for treading on your turf.

No sir... I have no turf. Such stupid notions are the premise of an immature mind.