c++ - fusion::nview 和 const 令人头疼

Question

我已经被困在代码中一段时间​​了。我可以解决这个问题，但如果我什至无法编译代码，我会感到非常糟糕。代码中描述了这些问题。

namespace fusion = boost::fusion;


  template <int N, typename T>
  struct viewTraits
  {

    typedef typename T::value_type value_type;

    typedef typename fusion::result_of::as_nview<value_type, N>::type view_type;

    typedef std::vector<view_type> result_type;
  };


  int main ()
  {

    typedef boost::fusion::vector<int, double> VecType;

    typedef std::vector<VecType> Matrix;

    typedef viewTraits<0, Matrix>::result_type R;

    Matrix m (20);
    for (int i = 0; i < 20; ++i)
    {
      m[i] = VecType (i, i+0.1*i);
    }
    R r;

    /*
    * the following can compile, but not what I want
    */
    BOOST_FOREACH (Matrix::value_type v, m)
    {
      r.push_back (fusion::as_nview <0>(v) );
    }

    /*
    * the following cannot compile???, why???
    */
    BOOST_FOREACH (Matrix::value_type const &v, m)
    {
      r.push_back (fusion::as_nview <0>(v) );
    }
  }

请任何人都可以指出我错过了什么。

Answer 1

BOOST_FOREACH expects the loop variable as first parameter so that can't be const (and it is picky about commas since it is a macro). Take a look at the documented pitfalls.