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digitalmars.D - scope ref const(T) --> error?!

reply "Mehrdad" <wfunction hotmail.com> writes:
What's wrong with passing a struct as scope ref const?

I want to avoid copying the struct, and its information is only 
read inside the function...
May 03 2012
next sibling parent Manu <turkeyman gmail.com> writes:
You're looking for 'in ref T'? I was also trying to do this today funnily
enough, and it seems a shame this doesn't work. Is this a bug, or is it
intended that it's not supported?

On 3 May 2012 21:28, Mehrdad <wfunction hotmail.com> wrote:


 What's wrong with passing a struct as scope ref const?

 I want to avoid copying the struct, and its information is only read
 inside the function...
May 03 2012
prev sibling parent reply "Tove" <tove fransson.se> writes:
On Thursday, 3 May 2012 at 18:28:19 UTC, Mehrdad wrote:

 What's wrong with passing a struct as scope ref const?

 I want to avoid copying the struct, and its information is only 
 read inside the function...


ref scope? hm? What additional semantics do you desire from that 
construct, which 'const ref' doesn't provide?

A a;

void fun(const scope ref A x)
{
   // x goes out of scope, destroy it... oops it's a global 
variable!?
}

fun(a);
May 03 2012
parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 05/03/2012 03:21 PM, Tove wrote:

 On Thursday, 3 May 2012 at 18:28:19 UTC, Mehrdad wrote:

 What's wrong with passing a struct as scope ref const?

 I want to avoid copying the struct, and its information is only read
 inside the function...


 ref scope? hm? What additional semantics do you desire from that
 construct, which 'const ref' doesn't provide?


scope is a not-yet-implemented promise of the function. It says: "trust 
me, I will not use this reference outside of the function."


 A a;

 void fun(const scope ref A x)
 {
 // x goes out of scope, destroy it... oops it's a global variable!?


scope does not destroy. scope makes this illegal (assuming that A is a 
class type):

     a = x;

There is deprecated use of the 'scope' keyword but this is not it.


 }

 fun(a);


Ali
May 03 2012
parent reply "Tove" <tove fransson.se> writes:
On Thursday, 3 May 2012 at 22:25:45 UTC, Ali Çehreli wrote:

 On 05/03/2012 03:21 PM, Tove wrote:

 On Thursday, 3 May 2012 at 18:28:19 UTC, Mehrdad wrote:

 What's wrong with passing a struct as scope ref const?

 I want to avoid copying the struct, and its information is




 only read

 inside the function...


 ref scope? hm? What additional semantics do you desire from


 that

 construct, which 'const ref' doesn't provide?


 scope is a not-yet-implemented promise of the function. It 
 says: "trust me, I will not use this reference outside of the 
 function."


 A a;

 void fun(const scope ref A x)
 {
 // x goes out of scope, destroy it... oops it's a global


 variable!?

 scope does not destroy. scope makes this illegal (assuming that 
 A is a class type):

     a = x;

 There is deprecated use of the 'scope' keyword but this is not 
 it.


 }

 fun(a);


 Ali


right, thanks. I forgot about that, since it was never 
implemented I didn't use it.

But nevertheless... the actual implemented semantics is the same 
for parameters as for the deprecated function body case, at the 
end of the function the parameter goes out of scope too! i.e. 
destructor should be called.
May 03 2012
next sibling parent "Tove" <tove fransson.se> writes:
On Thursday, 3 May 2012 at 22:43:16 UTC, Tove wrote:

 scope does not destroy. scope makes this illegal (assuming 
 that A is a class type):

    a = x;

 There is deprecated use of the 'scope' keyword but this is not 
 it.


 }

 fun(a);


 Ali


 right, thanks. I forgot about that, since it was never 
 implemented I didn't use it.

 But nevertheless... the actual implemented semantics is the 
 same for parameters as for the deprecated function body case, 
 at the end of the function the parameter goes out of scope too! 
 i.e. destructor should be called.


Hmmm sorry for the confusion, I was living under the delusion 
that:

scope class A{}
void fun(scope A x){}
fun(new A());

did something, but it doesn't. ;)
May 03 2012
prev sibling parent reply "Jonathan M Davis" <jmdavisProg gmx.com> writes:
On Friday, May 04, 2012 00:43:14 Tove wrote:

 But nevertheless... the actual implemented semantics is the same
 for parameters as for the deprecated function body case, at the
 end of the function the parameter goes out of scope too! i.e.
 destructor should be called.


No, it's not the same semantics. scope on parameters is all about preventing 
escaping - and in the case of delegates, provides the the additional benefit of 
indicating that allocating a closure is unnecessary. If you did something like

MyClass foo(scope MyClass c)
{
 ...
}

c would _not_ have its destructor called. Rather, the compiler would guarantee 
that no reference to c had escaped the function (e.g. by being returned). This 
is _completely_ different from doing

void foo()
{
 scope c = new MyClass;
}

which is being replaced by std.typecons.scoped.

- Jonathan M Davis
May 03 2012
parent "Mehrdad" <wfunction hotmail.com> writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8121
May 19 2012