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

3 Apr 2012

      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'? Anyway, this (below) compiles fine with me with
g++ 4.6.1 and MSVC 10:

#include 
#include 
#include 
#include <iostream>

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 main()
{
    using namespace boost::fusion;

    int i = 999;
    std::cout << invoke(mutable_func(), tuple(i, 1)) << std::endl;
    return 0;
}

Prints 1000. What am I missing?

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://boost-spirit.com