On Jul 4, 2012, at 12:40 AM, Graham Cox wrote:
> Why doesn't this compile?
>
>
> NSComparisonResult (^comp)( id<DKStorableObject>, id<DKStorableObject>
> ) = ^( id<DKStorableObject> a, id<DKStorableObject> b )
> {
> if( a.index < b.index )
> return NSOrderedAscending;
> else if ( a.index > b.index )
> return NSOrderedDescending;
> else
> return NSOrderedSame;
> };
>
> error: incompatible block pointer types initializing 'NSComparisonResult
> (^)(id<DKStorableObject>, id<DKStorableObject>)' with an expression of type
> 'int (^)(id<DKStorableObject>, id<DKStorableObject>)'
>
> Why does it assume that the return type is 'int"? There is nothing here that
> appears to suggest it is one.
Yes, there is. NSOrderedAscending, NSOrderedDescending, and NSOrderedSame are
elements of an enum. They are effectively ints.
> NSComparisonResult is a typedef of NSInteger.
Right, but the elements of the enum are not of type NSComparisonResult. In
traditional C, there's no way to declare the type of elements of an enum.
There's a coming extension to the language in clang that addresses this.
<http://clang.llvm.org/docs/LanguageExtensions.html#objc_fixed_enum> I don't
know what version of Xcode includes or will include this. Even when it arrives
in the compiler, the SDK will have to be updated to take advantage of it.
> If I attempt to cast the result, I get an equally baffling error:
Cast what result? I bet if you cast the value in each return statement, that
would work.
> error: invalid block pointer conversion initializing 'NSComparisonResult
> (^)(id<DKStorableObject>, id<DKStorableObject>)' with an expression of type
> 'NSComparisonResult' (aka 'long')
Sounds like you tried to cast the block reference to an NSComparisonResult.
Roland has suggested the syntax for explicitly declaring the return type of the
block, which is not a cast as such. It just means the compiler doesn't have to
/ get to infer it from the return statements within the block.
> Could the stupid block syntax be any less intuitive?
It follows the general form for C declarations, such as for function pointers.
However, blocks can be defined in different contexts than functions, which
makes it seem out of place. In general, try writing it exactly as though you
were writing a function. Then replace the function name with either "^" or,
when defining a variable or typedef, "(^name_of_variable_or_typedef)".
Regards,
Ken
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