Minor problem messaging C++ wrappers in Objective-C with gcc4.2

Minor problem messaging C++ wrappers in Objective-C with gcc4.2

[John Holdsworth]

Hi,

I've found it very useful to be able to use C++ to extend Objective-C
using a little judicious operator overloading and I notice that gcc-4.2
now automatically "unboxes" wrapper classes with no warning when
they are messaged - provided an appropriate cast is available.

For example:

class AStringClass {
	NSMutaableString *str;
public:
   	AStringClass( NSMutableString *str ) {
		this->str = str;
  	}
	operator NSMutableString * () {
		return str;
	}
};

This class can now be messaged in gcc4.2 as below without a warning:

	AStringClass aString( nil );
	[aString doubleValue];

Unfortunately is can also be sent a message which the compiler
is in a position to know is not available given the only cast
available was to a NSMutableString:

	[aString count]; // not a valid method for NSMutableString *

This happens as internally as there is an implicit cast to type "id" in
${GCCROOT}gcc/objc/objc-act,c, function: objc_finish_message_expr

	  /* APPLE LOCAL begin decay function/array receivers */
#ifndef OBJCPLUS
	  /* In C need to decay array/function receivers so can be converted
	     to id. */
	  struct c_expr exp;
	  exp.value = receiver;
	  exp = default_function_array_conversion (exp);
	  receiver = exp.value;
	/* APPLE LOCAL begin radar 3533972 */
#else
	  if (can_convert_arg (objc_object_type, TREE_TYPE (receiver),
				receiver, LOOKUP_NORMAL))
	    {
	      /* In rare cases, 'receiver' must be converted to type 'id' using
	         user-defined type conversion. 'id' is type of the 1st  
argument to
	         objc_msgSend (id self, SEL op, ...); */
	      tree cnv_rec = perform_implicit_conversion (objc_object_type,  
receiver);
               if (cnv_rec && cnv_rec != error_mark_node)
	        return objc_finish_message_expr (cnv_rec, sel_name,  
method_params);
	    }
	/* APPLE LOCAL end radar 3533972 */
#endif

Would it not be possible for the implicit conversion retain the type
which it was forced into using when searching for a match for "id"
and using this as the type of the receiver of the message rather
than "id". I've looked and the code but can't quite get to where
the change (probably quite a small one) would need to be made.
Perhaps even, this is already fixed...

Can anybody help?

John Holdsworth
objcpp.johnholdsworth.com


On 8 Apr 2009, at 14:34, David Ayers wrote:

> Am Samstag, den 21.03.2009, 11:59 +0100 schrieb John Holdsworth:
>
>> I was wondering if it would be a useful extension to Objective-C
>> expand the [] operator
>> to support array and hash references to NSArray and NSDictionary
>> classes directly to
>> greatly improve the readability of code:
>
> I'm not an ObjC front end maintainer and have no authority but one  
> issue
> I would have with this feature with gcc proper is that the ObjC front
> end would have to learn about the semantics of "NSArray" and
> "NSDictionary" which are actually not part of the language but part of
> an external library.
>
> Now gcc already supports the -fconstant-string-class option as one way
> to embed knowledge about an external library into an executable.   
> But I
> would like adding options with all these semantics from a foreign
> library into the language implementation.
>
> Maybe this could be done more elegantly with plugin infrastructure  
> that
> that es being currently added: http://gcc.gnu.org/wiki/plugins
>
> Cheers,
> David
>
>

Bus error gcc compiler for any for ( x in array ) inside Objective-C++ template

[John Holdsworth]

Hi,

I've encountered a bus error using Apple's gcc in Xcode 3.1, 3.2
compiling  the following code or any containing for( x in y ) is used
inside a template in Objective-C++.

template <typename ETYPE>
class OODictionary {
	void boom() {
		NSArray *keys = nil;
		for ( NSString *key in keys ) {
		}
	}
};

I've not been able to build with a more recent gcc so I can't tell
if it is still present but figure I'd better let you guys know.

Thanks,

John H.
http://objcpp.johnholdsworth.com


On 8 Sep 2009, at 21:30, John Holdsworth wrote:

> Hi,
>
> I've found it very useful to be able to use C++ to extend Objective-C
> using a little judicious operator overloading and I notice that  
> gcc-4.2
> now automatically "unboxes" wrapper classes with no warning when
> they are messaged - provided an appropriate cast is available.
>
> For example:
>
> class AStringClass {
> 	NSMutaableString *str;
> public:
>  	AStringClass( NSMutableString *str ) {
> 		this->str = str;
> 	}
> 	operator NSMutableString * () {
> 		return str;
> 	}
> };
>
> This class can now be messaged in gcc4.2 as below without a warning:
>
> 	AStringClass aString( nil );
> 	[aString doubleValue];
>
> Unfortunately is can also be sent a message which the compiler
> is in a position to know is not available given the only cast
> available was to a NSMutableString:
>
> 	[aString count]; // not a valid method for NSMutableString *
>
> This happens as internally as there is an implicit cast to type "id"  
> in
> ${GCCROOT}gcc/objc/objc-act,c, function: objc_finish_message_expr
>
> 	  /* APPLE LOCAL begin decay function/array receivers */
> #ifndef OBJCPLUS
> 	  /* In C need to decay array/function receivers so can be converted
> 	     to id. */
> 	  struct c_expr exp;
> 	  exp.value = receiver;
> 	  exp = default_function_array_conversion (exp);
> 	  receiver = exp.value;
> 	/* APPLE LOCAL begin radar 3533972 */
> #else
> 	  if (can_convert_arg (objc_object_type, TREE_TYPE (receiver),
> 				receiver, LOOKUP_NORMAL))
> 	    {
> 	      /* In rare cases, 'receiver' must be converted to type 'id'  
> using
> 	         user-defined type conversion. 'id' is type of the 1st  
> argument to
> 	         objc_msgSend (id self, SEL op, ...); */
> 	      tree cnv_rec = perform_implicit_conversion (objc_object_type,  
> receiver);
>              if (cnv_rec && cnv_rec != error_mark_node)
> 	        return objc_finish_message_expr (cnv_rec, sel_name,  
> method_params);
> 	    }
> 	/* APPLE LOCAL end radar 3533972 */
> #endif
>
> Would it not be possible for the implicit conversion retain the type
> which it was forced into using when searching for a match for "id"
> and using this as the type of the receiver of the message rather
> than "id". I've looked and the code but can't quite get to where
> the change (probably quite a small one) would need to be made.
> Perhaps even, this is already fixed...
>
> Can anybody help?
>
> John Holdsworth
> objcpp.johnholdsworth.com
>
>
> On 8 Apr 2009, at 14:34, David Ayers wrote:
>
>> Am Samstag, den 21.03.2009, 11:59 +0100 schrieb John Holdsworth:
>>
>>> I was wondering if it would be a useful extension to Objective-C
>>> expand the [] operator
>>> to support array and hash references to NSArray and NSDictionary
>>> classes directly to
>>> greatly improve the readability of code:
>>
>> I'm not an ObjC front end maintainer and have no authority but one  
>> issue
>> I would have with this feature with gcc proper is that the ObjC front
>> end would have to learn about the semantics of "NSArray" and
>> "NSDictionary" which are actually not part of the language but part  
>> of
>> an external library.
>>
>> Now gcc already supports the -fconstant-string-class option as one  
>> way
>> to embed knowledge about an external library into an executable.   
>> But I
>> would like adding options with all these semantics from a foreign
>> library into the language implementation.
>>
>> Maybe this could be done more elegantly with plugin infrastructure  
>> that
>> that es being currently added: http://gcc.gnu.org/wiki/plugins
>>
>> Cheers,
>> David
>>
>>

Re: Bus error gcc compiler for any for ( x in array ) inside Objective-C++ template

[Ian Lance Taylor]

John Holdsworth <mac@johnholdsworth.com> writes:

> I've encountered a bus error using Apple's gcc in Xcode 3.1, 3.2
> compiling  the following code or any containing for( x in y ) is used
> inside a template in Objective-C++.
>
> template <typename ETYPE>
> class OODictionary {
> 	void boom() {
> 		NSArray *keys = nil;
> 		for ( NSString *key in keys ) {
> 		}
> 	}
> };
>
> I've not been able to build with a more recent gcc so I can't tell
> if it is still present but figure I'd better let you guys know.

I tried your test case with current mainline, and got a bunch of
errors.

/home/iant/foo.mm: In member function ‘void OODictionary<ETYPE>::boom()’:
/home/iant/foo.mm:4:3: error: ‘NSArray’ was not declared in this scope
/home/iant/foo.mm:4:12: error: ‘keys’ was not declared in this scope
/home/iant/foo.mm:4:19: error: ‘nil’ was not declared in this scope
/home/iant/foo.mm:5:9: error: ‘NSString’ was not declared in this scope
/home/iant/foo.mm:5:19: error: ‘key’ was not declared in this scope
/home/iant/foo.mm:5:23: error: expected ‘;’ before ‘in’
/home/iant/foo.mm:7:2: error: expected primary-expression before ‘}’ token
/home/iant/foo.mm:7:2: error: expected ‘;’ before ‘}’ token
/home/iant/foo.mm:7:2: error: expected primary-expression before ‘}’ token
/home/iant/foo.mm:7:2: error: expected ‘)’ before ‘}’ token
/home/iant/foo.mm:7:2: error: expected primary-expression before ‘}’ token
/home/iant/foo.mm:7:2: error: expected ‘;’ before ‘}’ token

Since I don't know Objective C++ at all, I don't know whether this is
expected.

In any case, I encourage you to file a bug report with a
self-contained test case.  Please follow the directions at
http://gcc.gnu.org/bugs.html .  Thanks.

Ian