How to write a `` operator for boost :: tuple?

Question

The code example below shows that boost :: tuple can be created implicitly from the first argument of a template. Because of this, I cannot write an operator <<, because it becomes ambiguous.

Also, I do not understand why it is ostringstream& << floatalso ambiguous. It has no implicit construction. Why does this also cause a mixed error?

#include <iostream>
#include <boost/tuple/tuple.hpp>
#include <sstream>
#include <string>

using namespace std;

class Myclass
{
};

typedef boost::tuple<int,float,Myclass> Mytuple;

ostringstream& operator<<(ostringstream& os_, Mytuple tuple_)
{
  float f = tuple_.get<1>();
  //os_ << (int)tuple_.get<0>(); // Error because int is implicitly converted into Mytuple. WHYY?
  //os_ << tuple_.get<1>();      // No Clue Why this is ambiguous.
  //os_ << tuple_.get<2>();      // Error because no matching operator. Fine.
  return os_;
}

int main()
{
  Mytuple t1;
  t1 = 3;      // Working because int is implicitly converted into Mytuple!! WHY?
  //t1 = 3.0f; // Error because no matching constructor. Fine.
  return 0;
}

Mesasge Error:

tupleTest2.C: 18: error: ISO C ++ says that they are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:

Answer 1

, . :

ostream& operator<<(ostream& os_, Mytuple tuple_)
{
    os_ << tuple_.get<0>(); // Error because int is implicitly converted into Mytuple. WHYY?
    os_ << tuple_.get<1>();      // No Clue Why this is ambiguous.
    //os_ << tuple_.get<2>();      // Error because no matching operator. Fine.
    return os_;
}

, ostringstream operator<< ostream, : ostringstream& operator<<(ostringstream& os_, Mytuple tuple_) .

ostream& operator<<(ostream& os, T t)

( T ++, . <

( ):

ostringstream& operator<<(ostringstream& os_, Mytuple tuple_)
{
    const int i = tuple_.get<0>();
    os_ << i; // error in this line
    return os_;
}

:

dfg.cpp: In function ‘std::ostringstream& operator<<(std::ostringstream&, Mytuple)’:
dfg.cpp:18: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
/usr/lib/gcc/i386-redhat-linux/4.3.0/../../../../include/c++/4.3.0/bits/ostream.tcc:111: note: candidate 1: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char, _Traits = std::char_traits<char>]
dfg.cpp:14: note: candidate 2: std::ostringstream& operator<<(std::ostringstream&, Mytuple)

: operator<<(ostream&,...) operator<<(ostringstream&,...). This also raises another question : why on earth do you need operator < < (ostringstream &,...) `?

Answer 2

 os << tuple_.get<0>();

, .

std::ostream << int

std::ostringstream << MyTuple

boost::tuple, . ( float , float int.)

(ostream basic_ostream<CharT, Traits>.

---

:. , first.

ostringstream& operator<<(ostringstream& os_, Mytuple tuple_)
{
  static_cast<std::ostream&>(os_) << tuple_.get<0>(); 
  static_cast<std::ostream&>(os_)  << tuple_.get<1>();      
  static_cast<std::ostream&>(os_)  << tuple_.get<2>();      // Error because no matching operator. Fine.
  return os_;
}

ostringstream - , .

MyTuple a, b;
ostringstream ss;
ss << a << ' ' << b;

:

1) ostringstream& operator<<(ostringstream& os_, Mytuple tuple_)

2) ostream& ostream::operator<<(char)

3) ostream& operator<<(ostream&&, boost::tuple<int,float,Myclass>

Answer 3

, ::std::ostream ::std::ostringstream, . ::std::ostringstream .

- . , .

So, I wrote operator <<for ::std::tuplein C ++ 0x, which works for any set whose members can be individually written with operator <<. This can probably be easily translated to work with a type of type Boost. There he is:

template < ::std::size_t fnum, typename tup_type>
void print_fields(::std::ostream &os, const tup_type &val)
{
   if (fnum < ::std::tuple_size<tup_type>::value) {
      ::std::cerr << "Fred " << fnum << '\n';
      os << ::std::get<fnum, tup_type>(val);
      if (::std::tuple_size<tup_type>::value > (fnum + 1)) {
         os << ", ";
      }
      print_fields<fnum + 1, tup_type>(os, val);
   }
}

template < ::std::size_t fnum, typename... Elements>
class field_printer;

template <typename... Elements>
class field_printer<0, Elements...> {
 public:
   typedef ::std::tuple<Elements...> tup_type;

   static void print_field(::std::ostream &os, const tup_type &val) {
   }
};

template < ::std::size_t fnum, typename... Elements>
class field_printer {
 public:
   typedef ::std::tuple<Elements...> tup_type;

   static void print_field(::std::ostream &os, const tup_type &val) {
      constexpr auto tupsize = ::std::tuple_size<tup_type>::value;
      os << ::std::get<tupsize - fnum, Elements...>(val);
      if (fnum > 1) {
         os << ", ";
      }
      field_printer<fnum - 1, Elements...>::print_field(os, val);
   }
};

template <class... Types>
::std::ostream &operator <<(::std::ostream &os, const ::std::tuple<Types...> &val)
{
   typedef ::std::tuple<Types...> tup_type;
   os << '(';
   field_printer< ::std::tuple_size<tup_type>::value, Types...>::print_field(os, val);
   return os << ')';
}

It prints a tuple like "(element1, element2, ...elementx)".