Re: Help with constness and output operator<<

The problem is that the "const" method is not called in the output
operator <<.

--- [code] ---
#include <cstddef>
#include <iostream>
#include <stdexcept>

template <typename T>
struct foobar
{
     typedef T value_type;
     typedef T& reference;
     typedef T const& const_reference;

     foobar(std::size_t n)
         : v_(new value_type[n/2])
     {
     }

     ~foobar() { delete[] v_; }

     reference operator()(std::size_t i)
     {
         if (!(i % 2))
         {
             return v_[i];
         }

         throw std::runtime_error("Not assignable");
     }

     const_reference operator()(std::size_t i) const
     {
         if (!(i % 2))
         {
             return v_[i];
         }

         return this->zero_;
     }

     private: value_type* v_;
     private: static const value_type zero_;

};
template <typename T>
const T foobar<T>::zero_ = 0;

int main()
{
     const std::size_t N = 5;
     foobar<int> foo(N);

     std::cout << "Populating..." << std::endl;;
     for (std::size_t i=0; i < N; i += 2)
     {
         foo(i) = i+1;
     }
     std::cout << "Querying..." << std::endl;;
     for (std::size_t i=0; i < N; ++i)
     {
         std::cout << "foo(" << i << ") ==> " << std::flush;
         std::cout << foo(i) << std::endl; // DON'T WORK