Undefined stream insertion operator in boost::optional .

[amit.abhash@…]

I get undefined reference error while using boost::optional. Following code can be used to reproduce the error:

#include <iostream>
#include <boost/optional.hpp>

void print_optional(int type)
{
    boost::optional<int> value;
    if (type == 1) {
        value = boost::optional<int>(1000);
    }

    std::cout << value;
}

int main()
{
    print_optional(1);
}

test_opt.cpp:(.text+0x64): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& boost::operator<< <char, std::char_traits<char>, int>(std::basic_ostream<char, std::char_traits<char> >&, boost::optional<int> const&)'
collect2: error: ld returned 1 exit status

We are getting this because starting boost-1.56, stream insertion operator is declared in optional.hpp but not defined:

// Forward declaration to prevent operator safe-bool from being used.
template<class CharType, class CharTrait, class T>
std::basic_ostream<CharType, CharTrait>&
operator<<(std::basic_ostream<CharType, CharTrait>& out, optional<T> const& v);      
// There is no definition of this function.

Possible Solution

Adding following code fixed this issue for me:

namespace boost {
    template<class CharType, class CharTrait, class T>
    std::basic_ostream<CharType, CharTrait>& operator<<(std::basic_ostream<CharType, CharTrait>& out,
                                                        optional<T> const& v)
    {
        if (v) {
            out << *v;
        }

        return out;
    }
}

[rion4ik@…]

related bug ​https://svn.boost.org/trac/boost/ticket/2103

rolling back changes from #2103 fixes this one.

[viboes]

Why you don't include

 #include <boost/optional/optional_io.hpp>

if you want to print an optional?

[akrzemi1]

In order to stream out optional objects you have to include the IO header:

#include <boost/optional/optional_io.hpp>

I changed the implementation a bit, so that rather than a link-time error you get a compile time error about missing header. The change is now in develop branch.