http://d.puremagic.com/issues/show_bug.cgi?id=9489

           Summary: writeln of scoped class instance
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: rejects-valid
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2013-02-09 04:52:58 PST ---
import std.stdio: writeln;
import std.conv: text;
import std.typecons: scoped;
class Foo {
    int x;
    this(int x_) { this.x = x_; }
    override string toString() { return text(x); }
}
void main() {
    auto f = scoped!Foo(100);
    writeln(f.x); // OK
    writeln(f); // Error
    typeof(scoped!Foo(1))[10] foos;
    foreach (i, ref fi; foos)
        fi.x = i * 10;
    writeln(foos); // Error
}



dmd 2.062beta gives:


...\dmd2\src\phobos\std\stdio.d(709): Error: struct std.typecons.scoped!(Foo,
int).scoped.Scoped!(Foo).Scoped is not copyable because it is annotated with
@disable
...\dmd2\src\phobos\std\format.d(414): Error: struct std.typecons.scoped!(Foo,
int).scoped.Scoped!(Foo).Scoped is not copyable because it is annotated with
@disable
...\dmd2\src\phobos\std\format.d(437): Error: struct std.typecons.scoped!(Foo,
int).scoped.Scoped!(Foo).Scoped is not copyable because it is annotated with
@disable
...\dmd2\src\phobos\std\format.d(451): Error: struct std.typecons.scoped!(Foo,
int).scoped.Scoped!(Foo).Scoped is not copyable because it is annotated with
@disable
...\dmd2\src\phobos\std\format.d(463): Error: struct std.typecons.scoped!(Foo,
int).scoped.Scoped!(Foo).Scoped is not copyable because it is annotated with
@disable
...\dmd2\src\phobos\std\format.d(475): Error: struct std.typecons.scoped!(Foo,
int).scoped.Scoped!(Foo).Scoped is not copyable because it is annotated with
@disable
...\dmd2\src\phobos\std\format.d(489): Error: struct std.typecons.scoped!(Foo,
int).scoped.Scoped!(Foo).Scoped is not copyable because it is annotated with
@disable
...\dmd2\src\phobos\std\format.d(498): Error: struct std.typecons.scoped!(Foo,
int).scoped.Scoped!(Foo).Scoped is not copyable because it is annotated with
@disable
...\dmd2\src\phobos\std\stdio.d(714): Error: template instance
std.format.formattedWrite!(LockingTextWriter, char, Scoped!(Foo)) error
instantiating
...\dmd2\src\phobos\std\stdio.d(1620):        instantiated from here:
write!(Scoped!(Foo),char)
temp.d(12):        instantiated from here: writeln!(Scoped!(Foo))
...\dmd2\src\phobos\std\stdio.d(714): Error: struct std.typecons.scoped!(Foo,
int).scoped.Scoped!(Foo).Scoped is not copyable because it is annotated with
@disable
...\dmd2\src\phobos\std\stdio.d(1620): Error: template instance
std.stdio.File.write!(Scoped!(Foo),char) error instantiating
temp.d(12):        instantiated from here: writeln!(Scoped!(Foo))
...\dmd2\src\phobos\std\stdio.d(1620): Error: struct std.typecons.scoped!(Foo,
int).scoped.Scoped!(Foo).Scoped is not copyable because it is annotated with
@disable
temp.d(12): Error: template instance std.stdio.writeln!(Scoped!(Foo)) error
instantiating
temp.d(12): Error: struct std.typecons.scoped!(Foo,
int).scoped.Scoped!(Foo).Scoped is not copyable because it is annotated with
@disable
...\dmd2\src\phobos\std\format.d(1821): Error: static assert
(isInputRange!(Scoped!(Foo)[])) is false
...\dmd2\src\phobos\std\format.d(1789):        instantiated from here:
formatValue!(LockingTextWriter, Scoped!(Foo)[], char)
...\dmd2\src\phobos\std\format.d(2953):        instantiated from here:
formatValue!(LockingTextWriter, Scoped!(Foo)[10u], char)
...\dmd2\src\phobos\std\format.d(416):        instantiated from here:
formatGeneric!(LockingTextWriter, Scoped!(Foo)[10u], char)
...\dmd2\src\phobos\std\stdio.d(735):        ... (1 instantiations, -v to show)
...
...\dmd2\src\phobos\std\stdio.d(1620):        instantiated from here:
write!(Scoped!(Foo)[10u],char)
temp.d(16):        instantiated from here: writeln!(Scoped!(Foo)[10u])

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Andrej Mitrovic <andrej.mitrovich@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrej.mitrovich@gmail.com
            Summary|writeln of scoped class     |writeln of struct with
                   |instance                    |disabled copy ctor


--- Comment #1 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-02-09 05:44:27 PST ---
Unrelated to scoped classes, the root cause is that writeln() takes its
arguments by value:

import std.stdio;

struct S
{
    @disable this(this);
}

void main()
{
    S s;
    writeln(s);  // error
}

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--- Comment #2 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-02-18 20:10:35 PST ---
As a workaround you can use this:

import std.stdio;
import std.string;

struct Wrap(T)
{
    T* t;
    auto ref opDispatch(string s, T...)(T args)
    {
        mixin(format("return t.%s(args);", s));
    }
}

auto wrap(T)(T* t)
{
    return Wrap!T(t);
}

struct S
{
    @disable this(this);
    int x;
    float y;

    string toString()
    {
        return format("S { x: %s, y: %s }", x, y);
    }
}

void main()
{
    S s = S(1, 2.0);
    writeln(wrap(&s));
}

(std.typecons.Proxy doesn't seem to help here, it won't work with pointers)

Generally speaking this is a gigantic problem to fix. Pretty much everything in Phobos expects structs to be copyable. Phobos could try to take arguments by ref (or rather auto ref for compatibility), but this would have to be applied to all functions.

I think the safest thing we can do is ask the user to use this sort of wrapper type.

Maybe writeln/format could be the exception to the rule and take all arguments by auto ref and then internally use wrappers for @disable structs to pass them to other internal formatting functions. This would make it convenient to print @disable structs since it's common to print things..

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hsteoh@quickfur.ath.cx changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hsteoh@quickfur.ath.cx


--- Comment #3 from hsteoh@quickfur.ath.cx 2013-03-15 10:58:55 PDT ---
This issue is not just limited to printing. Structs that have @disabled
this(this) are currently highly-crippled, because almost everything in Phobos
assumes that structs can be passed by value. A struct range with @disabled
this(this) is unusable with any range function unless you do something really
ugly like (&range).cycle(), etc., which is a kind of hack (taking advantage of
D's automatic dereference when a pointer is used with the . operator).

Auto ref would solve a lot of these problems.

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