Re: [Boost-users] generate structs with all combinations of members from a given struct

31 Jan 2010

      On 01/30/10 13:35, Larry Evans wrote:
...
On 01/30/10 00:37, Hicham Mouline wrote:
...
Hello,
struct params {
  double field1;
  double field2;
  double field3;
  double fiedl4;
};
I would like to generate all the types that hold any combination of 
the members of params.
[snip]
Step1:
  Create cross-product of void and {field1,field2,...}.
  This will produce a sequence of pairs:
no_yes =
     ( (void,field1)
     , (void,field2)
     ...
     , (void,fieldn)
     )
Step2:
Create the crosss product of all rows in no_yes.
cross_no_yes =
    ( (void,void,..void)
    , (field1,void,...void)
    , (void,field2,void...,void)
    ...
    , (field1,field2,...fieldn)
    )
step3:
remove all the voids in all the rows of cross_no_yes.
Since the size<no_yes> == n and each element in that row is size 2,
the cross_no_yes size would be 2^n.  Then simply rm the all void row.
Not tested.
Now tested.  The attached produces:
/home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_4v/sandbox/variadic_templates/libs/mpl/sandbox/list_comprehension~subsets.exe 

absent_fields_seq=
( Absent=9000, Field=9001)
( Absent=9000, Field=9002)
( Absent=9000, Field=9003)
size<select_fields_seq>::value=8
select_fields_seq=
{ 9000, 9000, 9000}
{ 9000, 9000, 9003}
{ 9000, 9002, 9000}
{ 9000, 9002, 9003}
{ 9001, 9000, 9000}
{ 9001, 9000, 9003}
{ 9001, 9002, 9000}
{ 9001, 9002, 9003}

Compilation finished at Sat Jan 30 18:17:41

9000 indicates absence of a field.  900I, for I>0,
indicates presence of the I-th field.

HTH.

-regards,
Larry