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digitalmars.D.bugs - [Issue 8850] New: Nested struct creation by a template

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8850

           Summary: Nested struct creation by a template
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: Jesse.K.Phillips+D gmail.com


--- Comment #0 from Jesse Phillips <Jesse.K.Phillips+D gmail.com> 2012-10-18
14:15:06 PDT ---
I'm not sure the intended behavior but currently is inconsistent. The following
code fails to compile with:

bad.d(2): Error: function D main is a nested function and cannot be accessed
from bad.fun!(R).fun

T fun(T)() if(is(T == struct)) {
    T s;
    return s;
}

void main() {
    struct R {
        void f() { }
    }

    auto m = fun!R();
}

However removing the function from the struct definition (include other values
if desired) then it will compile. I'd think we'd want templates to have the
ability to create a nested struct.

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next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8850


Jesse Phillips <Jesse.K.Phillips+D gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Jesse.K.Phillips+D gmail.co
                   |                            |m


--- Comment #1 from Jesse Phillips <Jesse.K.Phillips+D gmail.com> 2012-10-19
11:24:26 PDT ---
May be duplicating or related to:
http://d.puremagic.com/issues/show_bug.cgi?id=8542

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Oct 19 2012
prev sibling next sibling parent d-bugmail puremagic.com writes:
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monarchdodra gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |monarchdodra gmail.com


--- Comment #2 from monarchdodra gmail.com 2012-10-19 13:14:58 PDT ---
(In reply to comment #0)

 I'm not sure the intended behavior but currently is inconsistent. The following
 code fails to compile with:
 
 bad.d(2): Error: function D main is a nested function and cannot be accessed
 from bad.fun!(R).fun
 
 T fun(T)() if(is(T == struct)) {
     T s;
     return s;
 }
 
 void main() {
     struct R {
         void f() { }
     }
 
     auto m = fun!R();
 }
 
 However removing the function from the struct definition (include other values
 if desired) then it will compile. I'd think we'd want templates to have the
 ability to create a nested struct.


Nested structs keep a frame pointer (or something alike) to be able to access
anything inside main, from outside of main. As such, you can't use them (as is)
as a template parameter.

HOWEVER, you can declare your struct as "static" explicitly stating that the
struct does not keep any extra info, at which point it becomes useable:

T fun(T)() if(is(T == struct)) {
   T s;
   return s;
}

void main() {
   static struct R {
       void f() { }
   }

   auto m = fun!R();
}

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timon.gehr gmx.ch changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |timon.gehr gmx.ch


--- Comment #3 from timon.gehr gmx.ch 2012-10-19 15:30:32 PDT ---
I think it is supposed to work, the following does work:

T fun(T, alias f)(){
    T s;
    return s;
}

void main() {
    int x=2;
    void f(){}
    struct R { int f() { return x; } }
    auto m = fun!(R,f)();
}

(The reason why it works is that the alias function parameter forces local
template instantiation.)

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--- Comment #4 from Kenji Hara <k.hara.pg gmail.com> 2012-11-07 04:50:03 PST ---
(In reply to comment #0)

 T fun(T)() if(is(T == struct)) {
     T s;    // [a]
     return s;
 }
 void main() {
     struct R {
         void f() { }
     }
     auto m = fun!R();
 }


In this case, R in main is a nested struct, and fun!T cannot access main's
frame pointer to construct R at the point [a], then the code correctly fail to
compile.
This behavior is introduced by fixing bug 8339, so it is intended behavior.


 However removing the function from the struct definition (include other values


if desired) then it will compile. I'd think we'd want templates to have the
ability to create a nested struct.

If you remove member function f() from R, R is implicitly treated as static,
because no code would access to the frame of main. Then, the need to access
frame of main from fun!R is eliminated, and the compilation will succeed.

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--- Comment #5 from Kenji Hara <k.hara.pg gmail.com> 2012-11-07 04:55:54 PST ---
(In reply to comment #3)

 I think it is supposed to work, the following does work:


In Currently, I answer: No.
Even if TemplateTypeParameter gets nested struct type, a template instantiation
will not capture the enclosing scope frame of the nested struct. It is intended
behavior.


 T fun(T, alias f)(){
     T s;
     return s;
 }
 
 void main() {
     int x=2;
     void f(){}
     struct R { int f() { return x; } }
     auto m = fun!(R,f)();
 }
 
 (The reason why it works is that the alias function parameter forces local
 template instantiation.)


In this case, as you say, func!(R, f) captures main's frame pointer (In other
words, func!(R, f) will be instantiated as a _nested template_ in main).

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Kenji Hara <k.hara.pg gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID


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timon.gehr gmx.ch changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|INVALID                     |


--- Comment #6 from timon.gehr gmx.ch 2012-11-07 09:55:21 PST ---
(In reply to comment #5)

 (In reply to comment #3)

 I think it is supposed to work, the following does work:


 
 In Currently, I answer: No.
 Even if TemplateTypeParameter gets nested struct type, a template instantiation
 will not capture the enclosing scope frame of the nested struct. It is intended
 behavior.
 


On second thought, you are right.


 T fun(T, alias f)(){
     T s;
     return s;
 }
 
 void main() {
     int x=2;
     void f(){}
     struct R { int f() { return x; } }
     auto m = fun!(R,f)();
 }
 
 (The reason why it works is that the alias function parameter forces local
 template instantiation.)


 
 In this case, as you say, func!(R, f) captures main's frame pointer (In other
 words, func!(R, f) will be instantiated as a _nested template_ in main).


I still think it might be sub-optimal that this has an influence on whether the
context pointer for R is available, but it is not a big deal.

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--- Comment #7 from Kenji Hara <k.hara.pg gmail.com> 2012-11-07 16:23:43 PST ---
(In reply to comment #6)

 (In reply to comment #5)
 I still think it might be sub-optimal that this has an influence on whether the
 context pointer for R is available, but it is not a big deal.


If you think that should work, you should open a new issue instead of reopen
this,  because it is an enhancement.

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