Re: Why can't rvalues be passed to a 'ref' parameter?
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`.
Re: Why can't rvalues be passed to a 'ref' parameter?
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",
],
```
Re: Why can't rvalues be passed to a 'ref' parameter?
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
Re: Why can't rvalues be passed to a 'ref' parameter?
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.
Why can't rvalues be passed to a 'ref' parameter?
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
