>> Ah, yes. So we should share the parsing of the decl-specifier-seq with the
>> C-style for loop, which allows us to avoid the tentative parsing.
>
> That was my original idea, but the C-style loop calls
> cp_parser_simple_declaration(), that shouts at the ':'. So we should
> either modify it to accept the ':' or split it in two. Both options
> are well beyond my intentions.
I just implemented "fast enumeration" (ie, "for (object in array) { ... }")
for Objective-C, and I was now planning on doing it for Objective-C++ too. ;-)
If you're doing range-based for-loops for C++, we may as well share ideas ;-)
What I've done for Objective-C fast enumeration (and what Apple did before me
as well)
is implemented your idea of modifying the declaration parsing to accept ':'
(well, 'in'
in the ObjC case) to terminate a declaration in that context:
* when you start parsing the 'for', a flag is turned on in the parser that
means that
we are "maybe in a fast enumeration"
* then, you go on parsing your usual C for loop, but the code that parses
expressions/declarations
has a special case for when it's "maybe in a fast enumeration", in which case
it accepts
"in" as terminating the expression/declaration (as well as ";"), and moreover
if it finds "in" in
there, it will save the declaration specially and make sure to communicate back
to the for
parsing code that we are "really" in a fast enumeration
* when the initial expression/declaration has been parsed, the for parsing
code turns off the
"maybe in a fast enumeration" flag; it now knows whether it's a normal for loop
or a fast enumeration
one and everything is easy from there on
It sounds like the same could apply to C++ range-based for-loops ? Except you
use ':' instead
of 'in' ? We could almost share the code - ie, set a single flag in the
C++/ObjC++ parser to say we are
at the beginning of a 'for' loop, and have that flag turn on recognizing ':'
(for range-based loops)
and 'in' (only if (dialect_objc()), for fast ObjC++ enumeration) as terminating
declarations.
The main disadvantage of doing things this way is making sure you won't
misunderstand a ':' (or 'in')
as meaning a range-based for-loop (or fast enumeration loop) when it isn't.
The best defense IMO is to
make sure ":" is only recognized in this way at the end of a declaration (where
normally a ';' would be)
(this is harder in Objective-C, but should be easier in Objective-C++).
It's worth trying to think of valid cases where you'd use a ":" inside a C for
loop though.
Anyway, let me know if this makes any sense or if I missed everything. Am I
right that the beginning
of a C++ range-based for-loop is identical to a standard C for loop up until
the ':' ? If not, obviously
this technique won't work. ;-)
Thanks
