Re: [boost] [Preprocessor] How to get the size of a tuple usingvariadic macros

From: Paul Mensonides (pmenso57_at_[hidden])
Date: 2005-10-10 14:32:58

> -----Original Message-----
> From: boost-bounces_at_[hidden] 
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Yao Zhen

> I am aware that variadic macros are not in the current C++ 
> standard. However, it is declared that "the C++ 
> standardization committee is likely to adopt C99's 
> preprocessor extensions for the next version of the C++ 
> standard." So here is a question about it.

Take a look at Chaos; it supports variadics from the ground up.  I'll send you a
snapshot.

> I have though that with the help of variadic macros, it would 
> be nolonger necessary for us to povide size of preprocessor 
> tuples manually.

It isn't necessary, but it is sometimes more efficient.

> For instance, in fact it is easy to 
> implement the BOOST_PP_TUPLE_ELEM macro more clearly as:
> 
> #define TUPLE_ELEM(i, tuple) TUPLE_ELEM_ ## i tuple
> 
> #define TUPLE_ELEM_0(a0, ...) a0
> #define TUPLE_ELEM_1(a0, a1, ...) a1
> #define TUPLE_ELEM_2(a0, a1, a2, ...) a2 #define 
> TUPLE_ELEM_3(a0, a1, a2, a3, ...) a3

Except that this will fail if you try to extract the last element of a tuple.
You have to increase the input tuple's size to account for this:

#define TUPLE_ELEM(i, tuple) TUPLE_ELEM_II(i, TUPLE_ELEM_I tuple) \
#define TUPLE_ELEM_I(...) (__VA_ARGS__,,,,,,,,,,,,,,,,,,,,,,,,)
#define TUPLE_ELEM_II(i, tuple) TUPLE_ELEM_ ## i tuple

> However, I failed to think out a way to implement TUPLE_SIZE. 
> Is there any way to do this?

Yes.  Up to a certain size, this can be a constant time (and fast) operation.
For larger tuples, the size can be calculated with an algorithm.

Regards,
Paul Mensonides