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digitalmars.D.learn - What does ref means

reply malio <youdontwanttoknow unknown.com> writes:
Hi guys,

I'm a bit confused what exactly ref means and in which cases I definitely need
this keyword.


Thanks for any reply!
Sep 06 2011
next sibling parent reply bearophile <bearophileHUGS lycos.com> writes:
malio:


 I'm a bit confused what exactly ref means and in which cases I definitely need
this keyword.


ref is not too much hard to understand. This is a simple usage example:


import std.stdio;
void inc(ref int x) {
    x++;
}
void main() {
    int x = 10;
    writeln(x);
    inc(x);
    writeln(x);
}


It is almost syntax sugar for:

import std.stdio;
void inc(int* x) {
    (*x)++;
}
void main() {
    int x = 10;
    writeln(x);
    inc(&x);
    writeln(x);
}


But a pointer can be null too.

Beside allowing that mutation of variables, in D you are allowed to use "const
ref" too (or immutable ref), this is useful if your value is many bytes long,
to avoid a costly copy:

import std.stdio;
struct Foo { int[100] a; }
void showFirst(const ref Foo f) {
    writeln(f.a[0]);
}
void main() {
    Foo f;
    f.a[] = 5;
    showFirst(f);
}


Another use for ref is on the return argument:

import std.stdio;
struct Foo {
    double[3] a;
    ref double opIndex(size_t i) {
        return a[i];
    }
}
void main() {
    Foo f;
    f.a[] = 5;
    writeln(f.a);
    f[1] = 10;
    writeln(f.a);
}

Bye,
bearophile
Sep 06 2011
parent reply malio <youdontwanttoknow unknown.com> writes:
== Auszug aus bearophile (bearophileHUGS lycos.com)'s Artikel

 malio:

 I'm a bit confused what exactly ref means and in which cases I definitely need
this keyword.


 ref is not too much hard to understand. This is a simple usage example:
 import std.stdio;
 void inc(ref int x) {
     x++;
 }
 void main() {
     int x = 10;
     writeln(x);
     inc(x);
     writeln(x);
 }
 It is almost syntax sugar for:
 import std.stdio;
 void inc(int* x) {
     (*x)++;
 }
 void main() {
     int x = 10;
     writeln(x);
     inc(&x);
     writeln(x);
 }
 But a pointer can be null too.
 Beside allowing that mutation of variables, in D you are allowed to use "const
ref" too (or immutable ref), this is useful if your value is many bytes long,


to avoid a costly copy:

 import std.stdio;
 struct Foo { int[100] a; }
 void showFirst(const ref Foo f) {
     writeln(f.a[0]);
 }
 void main() {
     Foo f;
     f.a[] = 5;
     showFirst(f);
 }
 Another use for ref is on the return argument:
 import std.stdio;
 struct Foo {
     double[3] a;
     ref double opIndex(size_t i) {
         return a[i];
     }
 }
 void main() {
     Foo f;
     f.a[] = 5;
     writeln(f.a);
     f[1] = 10;
     writeln(f.a);
 }
 Bye,
 bearophile


Okay, thanks bearophile. But I currently doesn't exactly understand what's the
difference between "ref" and "const ref"/"immutable ref". If "ref" is syntactic
sugar for pointers only (like your first example), does it also create a copy
of the parameters which are marked as "ref"? I thought that pointers (and in
this context also "ref") avoid the creation of costly copies?!?

Thanks!
Sep 06 2011
parent reply bearophile <bearophileHUGS lycos.com> writes:
malio:


 Okay, thanks bearophile. But I currently doesn't exactly understand what's the
difference between "ref" and "const ref"/"immutable ref". If "ref" is syntactic
 sugar for pointers only (like your first example), does it also create a copy
of the parameters which are marked as "ref"? I thought that pointers (and in
 this context also "ref") avoid the creation of costly copies?!?


"ref" just passes a reference to something, so it doesn't perform copies.

"const ref" or "immutable ref" just means that you can't change the value (with
the usual semantic differences between const and immutable, that are both
transitive).

For the programmer that reads your code, "ref" means the function you have
written will usually modify the given argument, while "const ref" means it will
not modify it.

Bye,
bearophile
Sep 06 2011
next sibling parent malio <youdontwantknow unknown.com> writes:
== Auszug aus Steven Schveighoffer (schveiguy yahoo.com)'s Artikel

 On Tue, 06 Sep 2011 05:28:22 -0400, malio <youdontwanttoknow unknown.com>
 wrote:

 Hi guys,

 I'm a bit confused what exactly ref means and in which cases I
 definitely need this keyword.


 ref is simple.  It's a pointer, but without the messy pointer syntax.
 These two programs are exactly the same (will generate the same code):
 void foo(int *i)
 {
     *i = 5;
 }
 void main()
 {
     int x = 2;
     foo(&x);
 }
 ----------------------
 void foo(ref int i)
 {
     i = 5;
 }
 void main()
 {
     int x = 2;
     foo(x); // note, there's no need to use &, the compiler does it for you
 }
 -Steve



 malio:

 Okay, thanks bearophile. But I currently doesn't exactly understand what's




the difference between "ref" and "const ref"/"immutable ref". If "ref" is
syntactic

 sugar for pointers only (like your first example), does it also create a




copy of the parameters which are marked as "ref"? I thought that pointers (and
in

 this context also "ref") avoid the creation of costly copies?!?


 "ref" just passes a reference to something, so it doesn't perform copies.
 "const ref" or "immutable ref" just means that you can't change the value


(with the usual semantic differences between const and immutable, that are both
transitive).

 For the programmer that reads your code, "ref" means the function you have


written will usually modify the given argument, while "const ref" means it will
not modify it.

 Bye,
 bearophile


Ah, okay - thanks in advance bearophile and Steve :)
Sep 06 2011
prev sibling parent reply Johannes Totz <johannes jo-t.de> writes:
On 06/09/2011 12:00, bearophile wrote:

 malio:
 

 Okay, thanks bearophile. But I currently doesn't exactly understand
 what's the difference between "ref" and "const ref"/"immutable
 ref". If "ref" is syntactic sugar for pointers only (like your
 first example), does it also create a copy of the parameters which
 are marked as "ref"? I thought that pointers (and in this context
 also "ref") avoid the creation of costly copies?!?


 
 "ref" just passes a reference to something, so it doesn't perform
 copies.
 
 "const ref" or "immutable ref" just means that you can't change the
 value (with the usual semantic differences between const and
 immutable, that are both transitive).


So if a parameter is immutable (without ref) the compiler could infer a
ref to avoid copy because it can't be modified?


 For the programmer that reads your code, "ref" means the function you
 have written will usually modify the given argument, while "const
 ref" means it will not modify it.
Sep 07 2011
next sibling parent "Simen Kjaeraas" <simen.kjaras gmail.com> writes:
On Wed, 07 Sep 2011 20:50:04 +0200, Johannes Totz <johannes jo-t.de> wrote:


 On 06/09/2011 12:00, bearophile wrote:

 malio:


 Okay, thanks bearophile. But I currently doesn't exactly understand
 what's the difference between "ref" and "const ref"/"immutable
 ref". If "ref" is syntactic sugar for pointers only (like your
 first example), does it also create a copy of the parameters which
 are marked as "ref"? I thought that pointers (and in this context
 also "ref") avoid the creation of costly copies?!?


 "ref" just passes a reference to something, so it doesn't perform
 copies.

 "const ref" or "immutable ref" just means that you can't change the
 value (with the usual semantic differences between const and
 immutable, that are both transitive).


 So if a parameter is immutable (without ref) the compiler could infer a
 ref to avoid copy because it can't be modified?


Theoretically at least. I don't believe such an optimization is actually
performed in current D compilers.


-- 
   Simen
Sep 07 2011
prev sibling parent Timon Gehr <timon.gehr gmx.ch> writes:
On 09/07/2011 08:50 PM, Johannes Totz wrote:

 On 06/09/2011 12:00, bearophile wrote:

 malio:


 Okay, thanks bearophile. But I currently doesn't exactly understand
 what's the difference between "ref" and "const ref"/"immutable
 ref". If "ref" is syntactic sugar for pointers only (like your
 first example), does it also create a copy of the parameters which
 are marked as "ref"? I thought that pointers (and in this context
 also "ref") avoid the creation of costly copies?!?


 "ref" just passes a reference to something, so it doesn't perform
 copies.

 "const ref" or "immutable ref" just means that you can't change the
 value (with the usual semantic differences between const and
 immutable, that are both transitive).


 So if a parameter is immutable (without ref) the compiler could infer a
 ref to avoid copy because it can't be modified?


In theory it could. I don't think the current D compilers do that. To 
allow it, clear rules would have to be fixed when to apply the 
optimization and when not to apply the optimization.
(that is not an issue if the compiler compiles the whole project in one 
pass though.)

But probably having to write 'ref' yourself to make calls faster is good 
enough, it also does not influence inline assembly in strange and 
non-visible ways.



 For the programmer that reads your code, "ref" means the function you
 have written will usually modify the given argument, while "const
 ref" means it will not modify it.
Sep 07 2011
prev sibling parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Tue, 06 Sep 2011 05:28:22 -0400, malio <youdontwanttoknow unknown.com>  
wrote:


 Hi guys,

 I'm a bit confused what exactly ref means and in which cases I  
 definitely need this keyword.


ref is simple.  It's a pointer, but without the messy pointer syntax.   
These two programs are exactly the same (will generate the same code):

void foo(int *i)
{
    *i = 5;
}

void main()
{
    int x = 2;
    foo(&x);
}

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

void foo(ref int i)
{
    i = 5;
}

void main()
{
    int x = 2;
    foo(x); // note, there's no need to use &, the compiler does it for you
}

-Steve
Sep 06 2011