As mentioned in a post of comp.std.c++: http://preview.tinyurl.com/yaqvnnq
there's a need for some way to get the nth element of a pack expansion. For example, boost::mpl::arg: http://www.boost.org/doc/libs/1_40_0/libs/mpl/doc/refmanual/arg.html wouldn't need the preprocessor for its implementation. Instead, using something like the following grammar production: get-nth-expansion-element: expansion-pattern '...[' constant-expression ']' where: expansion-pattern is the "pattern of the expansion" described on p. 327 of: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n3000.pdf constant-expression is defined here: http://www.csci.csusb.edu/dick/c++std/cd2/gram.html#gram.expr then boost::mpl::arg would be simply implemented as: template< int n > struct arg { template< typename A...> struct apply { typedef A...[n] type; }; }; where: A is parsed as a expansion-pattern. n is parsed as a constant-expression. Could g++ provide this feature? How hard would it be to implement. I would think it would just involve defining a new token: ...[ and then, wherever a expansion-pattern is currently parsed, this would be followed by a test for whether the next token is: ... or: ...[ TIA. -regards, Larry
