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digitalmars.D.learn - Why can't rvalues be passed to a 'ref' parameter?

reply thebluepandabear <therealbluepandabear protonmail.com> writes:
Hello,

I am not really understanding why rvalues cannot be passed to a 
'ref' parameter, the explanation in the book about D I am reading 
was not clear:

"The main reason for this limitation is the fact that a function 
taking a ref
parameter can hold on to that reference for later use, at a time 
when the rvalue
would not be available."

I didn't really understand what Ali meant by this statement, any 
help would be appreciated so this can be clarified.

Regards,
thebluepandabear
Dec 10 2022
next sibling parent reply zjh <fqbqrr 163.com> writes:
On Sunday, 11 December 2022 at 04:36:45 UTC, thebluepandabear 
wrote:


 "The main reason for this limitation is the fact that a 
 function taking a ref
 parameter can hold on to that reference for later use, at a 
 time when the rvalue
 would not be available."



I only know that `rvalue` is a temporary value that will not be 
used, while `ref` is omitting a pointer.
Dec 11 2022
parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 12/11/22 01:25, zjh wrote:

 On Sunday, 11 December 2022 at 04:36:45 UTC, thebluepandabear wrote:


 "The main reason for this limitation is the fact that a function
 taking a ref
 parameter can hold on to that reference for later use, at a time when
 the rvalue
 would not be available."



 I only know that `rvalue` is a temporary value that will not be used,


Temporary values can be used but they should not be stored for later. In 
this context, if the ref parameter were also 'scope', we should be able 
to pass rvalues. (See below.)


 while `ref` is omitting a pointer.


Yes, although it is still a pointer behind the scenes, 'ref' is syntax 
that avoids pointers.

However, the situation has changed in D: It has been possible to pass 
rvalues by reference through the magic of 'in' parameters. This 
currently requires the -preview=in compiler switch, which makes 'in' 
parameters imply 'const scope':

   https://dlang.org/spec/function.html#in-params

import std.stdio;

struct S {
     string id;
}

void foo(in S s) {
     writeln("foo received ", s.id);
}

void main() {
     auto s = S("lvalue");
     foo(s);
     foo(S("rvalue"));
}

Prints

foo received lvalue
foo received rvalue

On can add a copy constructor to S to see whether 'in' is by-copy or 
by-value.

Ali
Dec 11 2022
parent zjh <fqbqrr 163.com> writes:
On Sunday, 11 December 2022 at 16:48:57 UTC, Ali Çehreli wrote:


 However, the situation has changed in D: It has been possible 
 to pass rvalues by reference through the magic of 'in' 
 parameters. This currently requires the -preview=in compiler 
 switch, which makes 'in' parameters imply 'const scope':

   https://dlang.org/spec/function.html#in-params

 [...]


Thank you for your detailed `explanation`.
Dec 11 2022
prev sibling parent ryuukk_ <ryuukk.dev gmail.com> writes:
On Sunday, 11 December 2022 at 04:36:45 UTC, thebluepandabear 
wrote:

 Hello,

 I am not really understanding why rvalues cannot be passed to a 
 'ref' parameter, the explanation in the book about D I am 
 reading was not clear:

 "The main reason for this limitation is the fact that a 
 function taking a ref
 parameter can hold on to that reference for later use, at a 
 time when the rvalue
 would not be available."

 I didn't really understand what Ali meant by this statement, 
 any help would be appreciated so this can be clarified.

 Regards,
 thebluepandabear



You can if you compile with: ``-preview=rvaluerefparam``

I personally think this preview should be enabled by default

I use it all the time


```D
struct vec2{ float x; float y;}

void pass(const ref vec2 pos)
{}

void main()
{
     pass( vec2(1,1) );
}
```


If you use dub:

```json
            "dflags": [
                 "-preview=rvaluerefparam",
             ],
```
Dec 11 2022