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

Subject: Re: [Boost-users] generate structs with all combinations of members from a given struct
From: Larry Evans (cppljevans_at_[hidden])
Date: 2010-02-06 14:26:19

On 02/06/10 12:06, Hicham Mouline wrote:
[snip]
> Larry,
> 
> I have finally reached a Boost.PP based solution which I describe below. I
> will then go back to your fusion-based solution.
> My point is to compare compile-time, memory requirement of the compiler and
> ease of use of the generated types.
> 
> Here is my solution (the attached files compiles and preprocesses at least
> under g++4.4 with:   g++ -E -c structcombos.cpp).
> 
> With
> 
>    struct S {
>       int m1;
>       double m2;
>       double m3;
>    };
> 
> the user is required to define the macro
> 
> #define struct_members_seq ((int,m1))((double,m2))((double,m3))
> 
> then calls the macro
> 
> STRUCT_MEMBERS_COMBINATIONS_SEQ(struct_members_seq)
> 
> This expands to 
> struct params_m3 { double m3; }; struct params_m2 { double m2; }; struct
> params_m2_m3 { double m2; double m3; }; struct params_m1 { int m1; }; struct
> params_m1_m3 { int m1; double m3; }; struct params_m1_m2 { int m1; double
> m2; }; struct params_m1_m2_m3 { int m1; double m2; double m3; };
> 

My *intuition* as well as posts from others (e.g. Joel Guzman and
Eric Niebler) and a proto webpage:

http://www.boost.org/doc/libs/1_41_0/doc/html/proto/appendices.html#boost_proto.appendices.rationale

suggests this PP method would be faster than the fusion-based solution.

However, I've been wondering how you would use this.  I suspect it's
related to spirit's permutation:

   m1_tok ^ m2_tok ^ m3_tok

where, using the notation from:

http://www.boost.org/doc/libs/1_41_0/libs/spirit/doc/html/spirit/qi/reference/operator/permutation.html

   m1_tok:int, m2_tok:double, m3_tok:double
     --> (m1_tok^m2_tok^m3_tok):params_subset

where:
       typedef
     tuple
     < optional<int>   //corresponding to S::m1
     , optional<double>//corresponding to S::m2
     , optional<double>//corresponding to S::m3
     >
   params_subset;

and maybe you were planning on somehow converting the
params_subset attribute to one of the structures generated
by TRUCT_MEMBERS_COMBINATIONS_SEQ(struct_members_seq).
Is that your aim?

> 
> Questions:

Sorry, I don't know enough to answer any of these questions :(.