c++ - boost multi_index 示例 : error: invalid operands to binary expression

Question

只是尝试基本的 boost::multi_index 示例并接收和错误:

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/identity.hpp>
#include <boost/multi_index/member.hpp>

using namespace boost::multi_index;

struct employee
{
  int id;
  std::string name;

  employee(int id, const std::string &name) : id(id), name(name) {}

  bool operator<(const employee &e) const { return id < e.id; }
};

// define a multiply indexed set with indices by id and name
typedef multi_index_container<
    employee,
    indexed_by<
        // sort by employee::operator<
        ordered_unique<identity<employee> >,

        // sort by less<string> on name
        ordered_non_unique<member<employee, std::string, &employee::name> > > >
    employee_set;

void print_out_by_name(const employee_set &es)
{
  // get a view to index #1 (name)
  const employee_set::nth_index<1>::type &name_index = es.get<1>();
  // use name_index as a regular std::set
  std::copy(
      name_index.begin(), name_index.end(),
      std::ostream_iterator<employee>(std::cout));  // Looks like the problem is here.
}  

出现此错误:

/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/iterator:813:28: error: invalid operands to binary expression ('ostream_type' (aka
          'basic_ostream<char, std::__1::char_traits<char> >') and 'const employee')
                *__out_stream_ << __value_;

Answer 1

似乎需要一个实现:

std::ostream &operator<<(std::ostream &os, const employee &obj)
{
  os << obj.id << "/" << obj.name;
  return os;
}