Re: [C++11] Reclaiming fixed-point suffixes for user-defined literals.

[David Brown <david dot brown at hesbynett dot no>]

On 04/11/11 20:35, 3dw4rd@verizon.net wrote:
> Greetings,
>
> Now that C++11 user-defined literals are in trunk I was thinking
> about reclaiming some of the numeric suffixes that are currently
> recognized by gcc in the preprocessor.  The C++11 spec stipulates
> that any suffix that is recognized by the implementation is not
> allowable as a user-defined literal suffix in c++.  This prevents C++
> from hijacking 123LL, 1.234L, etc.  On the other hand, there are
> several numeric literal suffixes that are recognized and used as gnu
> extensions.
>
> One class of suffixes stands out in this connection: fixed-point
> literals.  These end in 'k' or 'r' and can optionally be unsigned by
> starting with 'u' and optionally have a size 'h', 'l', 'll'.  The
> difference for these suffixes is that fixed-point literals are
> explicitly rejected by the c++ front end.  Attempts to use such
> literals: int i= 1.23k; results in 'error: fixed-point types not
> supported in C++'.
>
> So I ask the question:  Should I make a simple change to libcpp to
> allow fixed-point literal suffixes to pass to the user-defined
> literal code in c++11 mode?
>
> Thanks,
>
> Ed Smith-Rowland
>
> P.S. There are other suffixes that might be reclaimed as well such as
> 'i', 'I', 'j', 'J' for complex integral or floating point imaginary
> numbers and others.  These might be more difficult or impossible to
> reclaim for C++11 because these might be allowed and used in gnu-C++
> and it might break existing code.

gcc has a tradition of allowing C-compatible features from C++ to be 
supported in C, and useful features from C to be supported in C++. 
Typically these being life as gcc extensions, but they sometimes move 
into the standards (an easy example being gcc's support for C++ comments 
in C from long before C99 came out).  An example is gcc's support for 
C99-style "_Complex" floating point types in C++.

Is there a good reason why the fixed point types (and decimal types, and 
extended float types) are not supported in C++ as a gcc extension?  It 
strikes me that from the users' viewpoint it would be best for these 
features to be available in C++ as well.  If the actual implementation 
of this would be difficult (I have no idea of the effort involved here), 
then the next best thing is to reserve the suffixes to avoid breaking 
things in the future, and to avoid user confusion.