[boost] [range] adaptors vs. rvalue to lvalue& binding

Subject: [boost] [range] adaptors vs. rvalue to lvalue& binding
From: Arno Schödl (aschoedl_at_[hidden])
Date: 2012-03-22 02:40:02

Hello,

ideally I would like to use range-adapted lvalue ranges like any other lvalue ranges. Writing it down naively, as below, does not compile because LoadRefRange takes (and should take?) the range it modifies by lvalue&, while adaptor expressions are rvalues.

template<typename Range>
void modifies_range( Range& rng );

A a[5];

main() {
      // modifies all elements of a
        modifies_range ( a );
       
      // modifies all elements (except for one) of a
        // adaptor is rvalue, does not compile:
      modifies_range (
                boost::range::join(
                        boost::adaptors::slice( a, 0, 1 ),
                        boost::adaptors::slice( a, 2, countof(a) )
                )
      );

        // compiles, but ugly:
        auto rng=boost::range::join(
                boost::adaptors::slice( a, 0, 1 ),
                boost::adaptors::slice( a, 2, countof(a) )
        );
        modifies_range(rng);
}

Any thoughts? Should adaptors receiving lvalues offer an 

        operator adaptor&() &&; // overload on rvalue-ness

so they are passing through the lvalue or their argument? A suitable baseclass a la boost::additive could do that.

Regards,

Arno

--
Dr. Arno Schödl | aschoedl_at_[hidden]
Technical Director
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