Re: [Boost-users] modify_key() for a composite_key of a multi_index_container that contains a shared_ptr

Subject: Re: [Boost-users] modify_key() for a composite_key of a multi_index_container that contains a shared_ptr
From: Joaquin M Lopez Munoz (joaquin_at_[hidden])
Date: 2009-10-04 13:12:50

Hello Zsolt, 

Zsolt Vajna wrote:
> 
> I'd like to modify_key() for a composite_key of a multi_index_container
> that contains a shared_ptr.
> 
> This compiles fine:
> 
> struct entry
>     {
>         entry(int a, int b):m_x(a), m_y(b){}
>         int m_x;
>         int m_y;
>     };
>         
>     typedef multi_index_container<
>        entry,
>        indexed_by<
>          ordered_unique<
>            composite_key<
>              entry,
>              member<entry,int,&entry::m_x>,
>              member<entry,int,&entry::m_y>>>>> C; 
> 
> void updateX(C::iterator iter,int x, C& m_c)
> {
>    m_c.get<0>().modify(
>     iter, (&ll::_1)->*&entry::m_x=x); 
> }
> 
> My first problem is if I change modify to modify_key I get the following
> error:
> operator_lambda_func_base.hpp(237) : error C3892:
> 'boost::lambda::detail::select' : you cannot assign to a variable that is
> const

When you use modify_key, you modifier functor is passed
*the key* rather than the whole element as is the case
with modify. In this particular situation, the key is
a composite key; so the key object (defined privately
by composite_key) is some entity that somehow references
entry::m_x and entry::m_y. This object is not exposed
to the public and you're not supposed to try to use
it or modify it, hence the problems you're seeing.
In short, when using composite_key you cannot use
modify_key.

> My second problem is that instead of
> 
> typedef multi_index_container<
>        entry,
>        indexed_by<
>          ordered_unique<
>            composite_key<
>              entry,
>              member<entry,int,&entry::m_x>,
>              member<entry,int,&entry::m_y>>>>> C; 
> 
> i want to have a
> 
> typedef multi_index_container<
>        shared_ptr<entry>,
>        indexed_by<
>          ordered_unique<
>            composite_key<
>              entry,
>              member<entry,int,&entry::m_x>,
>              member<entry,int,&entry::m_y>>>>> C; 
> 
> Note the shared_ptr added.
> 
> In this later case I get another error:
> 
> boost\lambda\detail\member_ptr.hpp(512) : error C2440: 'newline' : cannot
> convert from 'const rt0 ' to 'entry *const '

As your elements are now shared_ptrs rather than plain
entries, you need an additional dereferencing operation:

void updateX(C::iterator iter,int x, C& m_c)
{
  m_c.get<0>().modify(
    iter, (&*ll::_1)->*&entry::m_x=x); 
}

(note the * before ll::_1). Lambda expressions can rapidly
become unreadable, you might want to consider writing
an adhoc functor for this.

HTH,

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo
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