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digitalmars.D - "ubyte[size] store = void" in std.variant

reply Graham Fawcett <fawcett uwindsor.ca> writes:
Hi folks,

The following statement appears in std.variant:

190    union
191    {
192        ubyte[size] store = void;
193        // conservatively mark the region as pointers
194        static if (size >= (void*).sizeof)
195            void* p[size / (void*).sizeof];
196    }

The '= void' on line 192 sometimes leads to 'Error: void initializer
has no value' errors in application code. For example, this fails to
compile on DMD 2.047:

  foreach (int v; map! "a.get!int" (variantArray(1,2,3)))
    writeln(v);

Changing line 192 to 'ubyte[size] store;' resolves the issue.

My question is: what is the point of the '= void' initializer here?
Would std.variant be broken if '= void' were removed?

Thanks,
Graham
Jun 15 2010
next sibling parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Tue, 15 Jun 2010 09:04:23 -0400, Graham Fawcett <fawcett uwindsor.ca>  
wrote:


 Hi folks,

 The following statement appears in std.variant:

 190    union
 191    {
 192        ubyte[size] store = void;
 193        // conservatively mark the region as pointers
 194        static if (size >= (void*).sizeof)
 195            void* p[size / (void*).sizeof];
 196    }

 The '= void' on line 192 sometimes leads to 'Error: void initializer
 has no value' errors in application code. For example, this fails to
 compile on DMD 2.047:

   foreach (int v; map! "a.get!int" (variantArray(1,2,3)))
     writeln(v);

 Changing line 192 to 'ubyte[size] store;' resolves the issue.

 My question is: what is the point of the '= void' initializer here?
 Would std.variant be broken if '= void' were removed?



= void means don't initialize the data.  Otherwise, the compiler/runtime  
will fill in the data will all 0s.  However, I'm not sure how that works  
with a union, since you may have conflicting requirements for  
initialization.

Is there a place in the spec that covers this?

-Steve
Jun 15 2010
parent bearophile <bearophileHUGS lycos.com> writes:
Graham Fawcett:

   struct foo {
     enum N = 10; // or whatever
     ubyte[N] store = void;
   }
   foo z = foo();



I think there's a bug there, you can add it to Bugzilla (if not already
present):

struct Foo {
    int[1] a = void;
}
void main() {
    Foo f = Foo();
}

Bye,
bearophile
Jun 15 2010
prev sibling next sibling parent Graham Fawcett <fawcett uwindsor.ca> writes:
On Tue, 15 Jun 2010 10:29:56 -0400, Steven Schveighoffer wrote:


 On Tue, 15 Jun 2010 09:04:23 -0400, Graham Fawcett <fawcett uwindsor.ca>
 wrote:
 

 Hi folks,

 The following statement appears in std.variant:

 190    union
 191    {
 192        ubyte[size] store = void;
 193        // conservatively mark the region as pointers 194       
 static if (size >= (void*).sizeof) 195            void* p[size /
 (void*).sizeof]; 196    }

 The '= void' on line 192 sometimes leads to 'Error: void initializer
 has no value' errors in application code. For example, this fails to
 compile on DMD 2.047:

   foreach (int v; map! "a.get!int" (variantArray(1,2,3)))
     writeln(v);

 Changing line 192 to 'ubyte[size] store;' resolves the issue.

 My question is: what is the point of the '= void' initializer here?
 Would std.variant be broken if '= void' were removed?



 = void means don't initialize the data.  Otherwise, the compiler/runtime
 will fill in the data will all 0s.  However, I'm not sure how that works
 with a union, since you may have conflicting requirements for
 initialization.



I'm not sure if it's in the spec, but a quick test results in a
compiler error if I declare a union with overlapping initializers.

Simplifying the 'std.variant' case, I get the same 'void initializer
has no value' error like this:

    struct foo {
      ubyte[] store = void;
    }
    foo z = foo();

Is this a compiler bug?

Graham
Jun 15 2010
prev sibling next sibling parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Tue, 15 Jun 2010 11:18:07 -0400, Graham Fawcett <fawcett uwindsor.ca>  
wrote:


 On Tue, 15 Jun 2010 10:29:56 -0400, Steven Schveighoffer wrote:


 On Tue, 15 Jun 2010 09:04:23 -0400, Graham Fawcett <fawcett uwindsor.ca>
 wrote:


 Hi folks,

 The following statement appears in std.variant:

 190    union
 191    {
 192        ubyte[size] store = void;
 193        // conservatively mark the region as pointers 194
 static if (size >= (void*).sizeof) 195            void* p[size /
 (void*).sizeof]; 196    }

 The '= void' on line 192 sometimes leads to 'Error: void initializer
 has no value' errors in application code. For example, this fails to
 compile on DMD 2.047:

   foreach (int v; map! "a.get!int" (variantArray(1,2,3)))
     writeln(v);

 Changing line 192 to 'ubyte[size] store;' resolves the issue.

 My question is: what is the point of the '= void' initializer here?
 Would std.variant be broken if '= void' were removed?



 = void means don't initialize the data.  Otherwise, the compiler/runtime
 will fill in the data will all 0s.  However, I'm not sure how that works
 with a union, since you may have conflicting requirements for
 initialization.



 I'm not sure if it's in the spec, but a quick test results in a
 compiler error if I declare a union with overlapping initializers.

 Simplifying the 'std.variant' case, I get the same 'void initializer
 has no value' error like this:

     struct foo {
       ubyte[] store = void;
     }
     foo z = foo();

 Is this a compiler bug?



Note, you can only use = void on a value type, not a dynamic array.  The  
variant union member is a static array, not a dynamic one.

But actually, now that I think about it, =void is generally used in a  
function, not in a type definition.  I'm not sure = void should be allowed  
in that context.  So maybe the compiler is right to complain...

-Steve
Jun 15 2010
prev sibling next sibling parent Graham Fawcett <fawcett uwindsor.ca> writes:
On Tue, 15 Jun 2010 11:24:48 -0400, Steven Schveighoffer wrote:


 On Tue, 15 Jun 2010 11:18:07 -0400, Graham Fawcett <fawcett uwindsor.ca>
 wrote:
 

 On Tue, 15 Jun 2010 10:29:56 -0400, Steven Schveighoffer wrote:


 On Tue, 15 Jun 2010 09:04:23 -0400, Graham Fawcett
 <fawcett uwindsor.ca> wrote:


 Hi folks,

 The following statement appears in std.variant:

 190    union
 191    {
 192        ubyte[size] store = void;
 193        // conservatively mark the region as pointers 194 static
 if (size >= (void*).sizeof) 195            void* p[size /
 (void*).sizeof]; 196    }

 The '= void' on line 192 sometimes leads to 'Error: void initializer
 has no value' errors in application code. For example, this fails to
 compile on DMD 2.047:

   foreach (int v; map! "a.get!int" (variantArray(1,2,3)))
     writeln(v);

 Changing line 192 to 'ubyte[size] store;' resolves the issue.

 My question is: what is the point of the '= void' initializer here?
 Would std.variant be broken if '= void' were removed?



 = void means don't initialize the data.  Otherwise, the
 compiler/runtime will fill in the data will all 0s.  However, I'm not
 sure how that works with a union, since you may have conflicting
 requirements for initialization.



 I'm not sure if it's in the spec, but a quick test results in a
 compiler error if I declare a union with overlapping initializers.

 Simplifying the 'std.variant' case, I get the same 'void initializer
 has no value' error like this:

     struct foo {
       ubyte[] store = void;
     }
     foo z = foo();

 Is this a compiler bug?



 Note, you can only use = void on a value type, not a dynamic array.  The
 variant union member is a static array, not a dynamic one.



Oops, thanks for catching that. I was trying various combinations of
types and array-sizes to look for a pattern. For the record,

  struct foo {
    enum N = 10; // or whatever
    ubyte[N] store = void;
  }
  foo z = foo();

...returns the same error.

Graham


 But actually, now that I think about it, =void is generally used in a
 function, not in a type definition.  I'm not sure = void should be
 allowed in that context.  So maybe the compiler is right to complain...
 
 -Steve
Jun 15 2010
prev sibling next sibling parent Graham Fawcett <fawcett uwindsor.ca> writes:
On Tue, 15 Jun 2010 13:24:07 -0400, bearophile wrote:


 Graham Fawcett:

   struct foo {
     enum N = 10; // or whatever
     ubyte[N] store = void;
   }
   foo z = foo();



 I think there's a bug there, you can add it to Bugzilla (if not already
 present):
 
 struct Foo {
     int[1] a = void;
 }
 void main() {
     Foo f = Foo();
 }
 
 Bye,
 bearophile



OK, added.

http://d.puremagic.com/issues/show_bug.cgi?id=4322

"void initializer has no value" on struct/union members initialized to 
"void"

Thanks,
Graham
Jun 15 2010
prev sibling parent Graham Fawcett <fawcett uwindsor.ca> writes:
On Tue, 15 Jun 2010 17:39:50 +0000, Graham Fawcett wrote:


 On Tue, 15 Jun 2010 13:24:07 -0400, bearophile wrote:
 

 Graham Fawcett:

   struct foo {
     enum N = 10; // or whatever
     ubyte[N] store = void;
   }
   foo z = foo();



 I think there's a bug there, you can add it to Bugzilla (if not already
 present):
 
 struct Foo {
     int[1] a = void;
 }
 void main() {
     Foo f = Foo();
 }
 
 Bye,
 bearophile



 OK, added.
 
 http://d.puremagic.com/issues/show_bug.cgi?id=4322
 
 "void initializer has no value" on struct/union members initialized to
 "void"
 
 Thanks,
 Graham



FWIW, removing line 100 in init.c allows our simple examples to
compile and run correctly. Of course I'm not 100% clear on why the
error message was there in the first place. :)

diff --git a/trunk/src/init.c b/trunk/src/init.c
index ed3a091..cf9bc72 100644
--- a/trunk/src/init.c
+++ b/trunk/src/init.c
   -97,7 +97,6    Initializer *VoidInitializer::semantic(Scope *sc, Type 
*t)
 
 Expression *VoidInitializer::toExpression()
 {
-    error(loc, "void initializer has no value");
     return new IntegerExp(0);
 }

best,
Graham
Jun 15 2010