Re: [Boost-users] [fusion] iterating over a sequence with non-const references

3 Apr 2012

      ----- Original Message -----
...
From: Joel de Guzman 
To: boost-users@lists.boost.org
Cc: 
Sent: Monday, April 2, 2012 11:26 PM
Subject: Re: [Boost-users] [fusion] iterating over a sequence with non-const references
On 4/3/2012 2:45 AM, paul Fultz wrote:
...
...
push_back is designed to be non-mutating. That is why. This is
 true of all the views returned from 'algos', by design. What
 you can do is to make a container from the view through, e.g.
 as_vector, as_list. It is *not* true that all sequences return
 const refs to its elements. Containers definitely return references,
 otherwise get<N>(c)= expr will not be possible.
Also, front and back return non-const reference, but thats not the point.
 During iteration, they return const reference. Thats why this is not 
 possible:
struct mutable_func
 {
     template<class F>
     struct result;
     template
     struct result
     {
         typedef T type;
     };
     template
     T operator()(T && x, U y) const
     {
         return x+=y;
     }
 };
int i = 0;
 invoke(foo, tuple(i, 1)); //doesn't compile
here is the output:
What is 'foo'? 
mutable_func foo = {};
I forgot that line.
...
Anyway, this (below) compiles fine with me with
g++ 4.6.1 and MSVC 10:
Yea, sorry about that. It seems it was still calling my own invoke and there
was a mistake in that. If you call invoke like this:

std::cout << invoke(mutable_func(), forward_as_tuple(i, 1)) << std::endl;

There are two errors:

invoke.hpp:209:24: error: non-const lvalue reference to type 'int' cannot bind to a temporary of type 'int'
                return f( BOOST_PP_ENUM(N,M,~) );
                         ^~~~~~~~~~~~~~~~~~~~~~~~~

Which I assume is because invoke takes the sequence as a const-reference
because it is a temporary. But then why does the other example work(its also
a temporary)?

The second error is:

at_impl.hpp:45:28: error: rvalue reference to type 'int' cannot bind to lvalue of type 'int'
                    return std::get(seq);
                              ^~~~~~~~~~~~~~~~~~~~~~~

Which I believe is because of the poor implemetation of tuples in gcc 4.6.
I am compiling using clang also. But I do get the same errors in gcc as well.

Thanks,
Paul Fultz II