Re: [Boost-users] composite_key with array member

Subject: Re: [Boost-users] composite_key with array member
From: Joaquin M Lopez Munoz (joaquin_at_[hidden])
Date: 2014-08-13 14:36:40

Uthpal Urubail <uthpal.urubail <at> altair.com> writes:

> 
> I am looking for a container where I have to use 5-10 keys to get the
> value. I am trying to write something like below.. Can some one help
> to solve this?
> 
> struct entry
> {
>     int a[10];
> Â  Â  int id;
> };
>
> [...] 
> 
> typedef multi_index_container<
> Â  Â  entry,
> Â  Â  indexed_by<
> Â  Â    ordered_non_unique<Â 
> Â  Â      composite_key<
> Â  Â        entry,
> Â  Â          member<entry,unsigned int,&entry::a> 
> [...]

Not sure if this is what you're looking for, but you can define
a global template function extracting the N-th component of
entry::a:

  template<int N>
  int get_a(const entry& e){return e.a[N];}

and then define composite_key on the ten elements of the array as follows:

  composite_key<
    entry,
    global_fun<const entry&,int,get_a<0>>,
    global_fun<const entry&,int,get_a<1>>,
    global_fun<const entry&,int,get_a<2>>,
    global_fun<const entry&,int,get_a<3>>,
    global_fun<const entry&,int,get_a<4>>,
    global_fun<const entry&,int,get_a<5>>,
    global_fun<const entry&,int,get_a<6>>,
    global_fun<const entry&,int,get_a<7>>,
    global_fun<const entry&,int,get_a<8>>,
    global_fun<const entry&,int,get_a<9>>
  >

Is there any particular reason why you're packing the ten keys
in an array 'a' rather than naming them like this?

  int a0,a1,a2,a3,a4,a5,a6,a7,a8,a9;

It'd make thing easier wrt to declaring the composite_key.

Joaquín M López Muñoz
Telefónica