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D - arrays can't be out parameters?

reply "Sean L. Palmer" <palmer.sean verizon.net> writes:
void foo(out char[32] test)
{
}

DMD gives the error that char[32] can't be an out parameter.

I realize arrays are passed by reference implicitly.  I was after more of
the behavior you get when you return an array, but without the temporary.

char[32] foo()
{
    char[32] result;
    return result;
}

Oh well.  I think it's by design.

Sean
Apr 03 2003
next sibling parent reply "Luna Kid" <lunakid neuropolis.org> writes:
Hmm. Then, how does one modify an array in D
without copying (besides wrapping it in a class)?

Luna Kid

"Sean L. Palmer" <palmer.sean verizon.net> wrote in message
news:b6h0ql$2tva$1 digitaldaemon.com...

 void foo(out char[32] test)
 {
 }

 DMD gives the error that char[32] can't be an out parameter.

 I realize arrays are passed by reference implicitly.  I was after more of
 the behavior you get when you return an array, but without the temporary.

 char[32] foo()
 {
     char[32] result;
     return result;
 }

 Oh well.  I think it's by design.

 Sean
Apr 03 2003
parent reply "Mike Wynn" <mike.wynn l8night.co.uk> writes:
"Luna Kid" <lunakid neuropolis.org> wrote in message
news:b6h9nv$21k$1 digitaldaemon.com...

 Hmm. Then, how does one modify an array in D
 without copying (besides wrapping it in a class)?


arrays are not arrays in the java/C sense, they are slices

int[] foo = new int[40];
equiv to
struct array { T * start; int length; }
foo.start = (int*)calloc( 40, sizeof(int) );
foo.length = 40;

int func( int[] parm ) ...

is equiv to the C
int func( struct array param ) ...

so you get the start and length, which allows modifications to data but not
length (just like delphi open arrays).

arrays can be inout they can not be out I assume because that would cause
implict allocations

you can get out without copying with
char[] func() {
    char[] foo = new char[32];
    return foo[0..32];
}

a work of warning do not be tempted to write

char[] func( ){
    char [32] foo;
    return foo[0..32];
}
will return a slice alllowing access to the stack :)





 Luna Kid

 "Sean L. Palmer" <palmer.sean verizon.net> wrote in message
 news:b6h0ql$2tva$1 digitaldaemon.com...

 void foo(out char[32] test)
 {
 }

 DMD gives the error that char[32] can't be an out parameter.

 I realize arrays are passed by reference implicitly.  I was after more




of

 the behavior you get when you return an array, but without the




temporary.

 char[32] foo()
 {
     char[32] result;
     return result;
 }

 Oh well.  I think it's by design.

 Sean
Apr 03 2003
parent "Luna Kid" <lunakid neuropolis.org> writes:
 arrays can be inout ...


Ahh, I missed that one, sorry...

Thanks!
Luna Kid
Apr 03 2003
prev sibling parent reply Burton Radons <loth users.sourceforge.net> writes:
Sean L. Palmer wrote:

 void foo(out char[32] test)
 {
 }
 
 DMD gives the error that char[32] can't be an out parameter.


Static arrays can't be out parameters; making it a dynamic array will 
work fine.  I think it's a compiler limitation, not lingual.


 I realize arrays are passed by reference implicitly.  I was after more of
 the behavior you get when you return an array, but without the temporary.


Static arrays are passed by content.  I think the optimiser already does 
temporary return value removal.  Walter said something along these lines 
in that thread advocating return variables, and it looks like a fairly 
easy one to pull off.
Apr 03 2003
parent Helmut Leitner <leitner hls.via.at> writes:
Burton Radons wrote:

 
 Sean L. Palmer wrote:

 void foo(out char[32] test)
 {
 }

 DMD gives the error that char[32] can't be an out parameter.


 
 Static arrays can't be out parameters; making it a dynamic array will
 work fine.  I think it's a compiler limitation, not lingual.


                             ---------------------------------

Sorry, could you explain this. Do you think the D specification would
allow it but it is not implemented?
 

 I realize arrays are passed by reference implicitly.  I was after more of
 the behavior you get when you return an array, but without the temporary.


 
 Static arrays are passed by content.  


In any case? Or does this depend on in/out/inout?

Only for small arrays this might make sense. I about 95% of all cases the C way 
of passing arrays by reference is more appropriate with respect to performance.


 I think the optimiser already does
 temporary return value removal.  Walter said something along these lines
 in that thread advocating return variables, and it looks like a fairly
 easy one to pull off.


Could you give me a hint how to find this thread?

-- 
Helmut Leitner    leitner hls.via.at
Graz, Austria   www.hls-software.com
Apr 04 2003