Passing a reference to a value returned by a function

Passing a reference to a value returned by a function

[Mihnea Balta]

Hi.

I have this piece of code (excuse the long function names, it's example code 
:) ):

#include <stdio.h>

struct type_t{
    int a, b, c;
};

type_t function_returning_struct(){
    type_t  a;

    a.a = 1;
    return a;
}

int function_taking_reference(type_t& a){
    if(a.a == 1)
        return 1;
    else
        return 0;
}

int main(){
    printf("%d\n", function_taking_reference(function_returning_struct()));
    return 1;
}

I'm trying to port code written like this from visualc to gcc. I suppose that 
visualc compiles the above piece of code because it creates a temporary 
type_t that holds the return of function_returning_struct(), and passes a 
reference to that to function_taking_reference(). However, gcc 3.2.2 won't 
compile that kind of code.

Is there any way to make gcc work with such code? I'm asking because it's in a 
lot of places, and adding a temp var by hand to all the places where this 
stuff occurs would be a pain.

Thanks.

Re: Passing a reference to a value returned by a function

[Oliver Kullmann]

On Tue, Mar 11, 2003 at 05:56:34PM +0200, Mihnea Balta wrote:
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> From: Mihnea Balta <dark_lkml@mymail.ro>
> To: gcc-help@gcc.gnu.org
> Date: Tue, 11 Mar 2003 17:56:34 +0200
> Subject: Passing a reference to a value returned by a function
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> 
> Hi.
> 
> I have this piece of code (excuse the long function names, it's example code 
> :) ):
> 
> #include <stdio.h>
> 
> struct type_t{
>     int a, b, c;
> };
> 
> type_t function_returning_struct(){
>     type_t  a;
> 
>     a.a = 1;
>     return a;
> }
> 
> int function_taking_reference(type_t& a){
>     if(a.a == 1)
>         return 1;
>     else
>         return 0;
> }
> 
> int main(){
>     printf("%d\n", function_taking_reference(function_returning_struct()));
>     return 1;
> }
> 
> I'm trying to port code written like this from visualc to gcc. I suppose that 
> visualc compiles the above piece of code because it creates a temporary 
> type_t that holds the return of function_returning_struct(), and passes a 
> reference to that to function_taking_reference(). However, gcc 3.2.2 won't 
> compile that kind of code.
> 
> Is there any way to make gcc work with such code? I'm asking because it's in a 
> lot of places, and adding a temp var by hand to all the places where this 
> stuff occurs would be a pain.
> 
> Thanks.

Hi,

I hope very much that there is no way to make gcc to work with such code,
since it doesn't make sense to pass temporary values by reference, and the
standards explicitely forbids it. But you don't have to add variables ---
just add the const qualifier to the argument type of function_taking_reference,
and you have perfectly legal C++ code. Doing so doesn't change your code,
but just increases safety.

Oliver

Re: Passing a reference to a value returned by a function

[Mihnea Balta]

Thanks, it works with the const qualifier.
My next question would have been about how much visualc's behavior respects 
the standard, thanks for answering that in advance ;)

On Tuesday 11 March 2003 18:16, Oliver Kullmann wrote:
> On Tue, Mar 11, 2003 at 05:56:34PM +0200, Mihnea Balta wrote:
> > Mailing-List: contact gcc-help-help@gcc.gnu.org; run by ezmlm
> > Precedence: bulk
> > List-Unsubscribe:
> > <mailto:gcc-help-unsubscribe-O.Kullmann=swansea.ac.uk@gcc.gnu.org>
> > List-Archive: <http://gcc.gnu.org/ml/gcc-help/>
> > List-Post: <mailto:gcc-help@gcc.gnu.org>
> > List-Help: <mailto:gcc-help-help@gcc.gnu.org>
> > Delivered-To: mailing list gcc-help@gcc.gnu.org
> > From: Mihnea Balta <dark_lkml@mymail.ro>
> > To: gcc-help@gcc.gnu.org
> > Date: Tue, 11 Mar 2003 17:56:34 +0200
> > Subject: Passing a reference to a value returned by a function
> > X-Spam-Status: No, hits=0.0 required=8.0
> > 	tests=none
> > 	version=2.50
> > X-Spam-Level:
> > X-Spam-Checker-Version: SpamAssassin 2.50 (1.173-2003-02-20-exp)
> > X-MIME-Autoconverted: from quoted-printable to 8bit by cs-svr1.swan.ac.uk
> > id h2BFw0uE014171
> >
> > Hi.
> >
> > I have this piece of code (excuse the long function names, it's example
> > code
> >
> > :) ):
> >
> > #include <stdio.h>
> >
> > struct type_t{
> >     int a, b, c;
> > };
> >
> > type_t function_returning_struct(){
> >     type_t  a;
> >
> >     a.a = 1;
> >     return a;
> > }
> >
> > int function_taking_reference(type_t& a){
> >     if(a.a == 1)
> >         return 1;
> >     else
> >         return 0;
> > }
> >
> > int main(){
> >     printf("%d\n",
> > function_taking_reference(function_returning_struct())); return 1;
> > }
> >
> > I'm trying to port code written like this from visualc to gcc. I suppose
> > that visualc compiles the above piece of code because it creates a
> > temporary type_t that holds the return of function_returning_struct(),
> > and passes a reference to that to function_taking_reference(). However,
> > gcc 3.2.2 won't compile that kind of code.
> >
> > Is there any way to make gcc work with such code? I'm asking because it's
> > in a lot of places, and adding a temp var by hand to all the places where
> > this stuff occurs would be a pain.
> >
> > Thanks.
>
> Hi,
>
> I hope very much that there is no way to make gcc to work with such code,
> since it doesn't make sense to pass temporary values by reference, and the
> standards explicitely forbids it. But you don't have to add variables ---
> just add the const qualifier to the argument type of
> function_taking_reference, and you have perfectly legal C++ code. Doing so
> doesn't change your code, but just increases safety.
>
> Oliver
>
>
> ------------------------------------------------------------------------
> http://felicitari.mymail.ro/

Re: Passing a reference to a value returned by a function

[Mihnea Balta]

Yes, that's what I meant with "adding a temp var by hand to all the places 
where this stuff occurs" :). I'd like to have as little modification as 
possible to the original code, since that kind of construct occurs very 
often.

I can't make the functions take structs instead of references either, because 
it's a critical section of the code, and the strctures are pretty big.

On Tuesday 11 March 2003 18:05, Ajay Bansal wrote:
> One way id to create a variable & store the return value of
> function_returning_struct in this variable as
>
>  type_t a = function_returning_struct();
>     printf("%d\n", function_taking_reference(a));
>     return 1;
>
> -----Original Message-----
> From: Mihnea Balta [mailto:dark_lkml@mymail.ro]
> Sent: Tuesday, March 11, 2003 9:27 PM
> To: gcc-help@gcc.gnu.org
> Subject: Passing a reference to a value returned by a function
>
>
> Hi.
>
> I have this piece of code (excuse the long function names, it's example
> code
>
> :) ):
>
> #include <stdio.h>
>
> struct type_t{
>     int a, b, c;
> };
>
> type_t function_returning_struct(){
>     type_t  a;
>
>     a.a = 1;
>     return a;
> }
>
> int function_taking_reference(type_t& a){
>     if(a.a == 1)
>         return 1;
>     else
>         return 0;
> }
>
> int main(){
>     printf("%d\n",
> function_taking_reference(function_returning_struct()));
>     return 1;
> }
>
> I'm trying to port code written like this from visualc to gcc. I suppose
> that
> visualc compiles the above piece of code because it creates a temporary
> type_t that holds the return of function_returning_struct(), and passes
> a
> reference to that to function_taking_reference(). However, gcc 3.2.2
> won't
> compile that kind of code.
>
> Is there any way to make gcc work with such code? I'm asking because
> it's in a
> lot of places, and adding a temp var by hand to all the places where
> this
> stuff occurs would be a pain.
>
> Thanks.
>
>
> ------------------------------------------------------------------------
> http://felicitari.mymail.ro/

RE: Passing a reference to a value returned by a function

[Ajay Bansal]

One way id to create a variable & store the return value of
function_returning_struct in this variable as

 type_t a = function_returning_struct();
    printf("%d\n", function_taking_reference(a));
    return 1;

-----Original Message-----
From: Mihnea Balta [mailto:dark_lkml@mymail.ro] 
Sent: Tuesday, March 11, 2003 9:27 PM
To: gcc-help@gcc.gnu.org
Subject: Passing a reference to a value returned by a function


Hi.

I have this piece of code (excuse the long function names, it's example
code 
:) ):

#include <stdio.h>

struct type_t{
    int a, b, c;
};

type_t function_returning_struct(){
    type_t  a;

    a.a = 1;
    return a;
}

int function_taking_reference(type_t& a){
    if(a.a == 1)
        return 1;
    else
        return 0;
}

int main(){
    printf("%d\n",
function_taking_reference(function_returning_struct()));
    return 1;
}

I'm trying to port code written like this from visualc to gcc. I suppose
that 
visualc compiles the above piece of code because it creates a temporary 
type_t that holds the return of function_returning_struct(), and passes
a 
reference to that to function_taking_reference(). However, gcc 3.2.2
won't 
compile that kind of code.

Is there any way to make gcc work with such code? I'm asking because
it's in a 
lot of places, and adding a temp var by hand to all the places where
this 
stuff occurs would be a pain.

Thanks.