Implicit cast doesnt work for BOOST_STRONG_TYPEDEF

Question

I have defined a boost::spirit::qi rule:

boost::spirit::qi::rule<Iterator, Identifier()> id;

where Identifier is defined by:

BOOST_STRONG_TYPEDEF(std::string, Identifier)

but when I use

BOOST_SPIRIT_DEBUG_NODE(id);

It fails to compile with following error:

boost_1_51_0/boost/spirit/home/support/attributes.hpp:1203: error: no match for 'operator<<' in 'out << val'

and it lists the overloaded operators of ostream.

Knowing that BOOST_STRONG_TYPEDEF defines a cast operator to the original type, shouldn't compiler implicitly cast from Identifier to std::string when using operator<<? or is there a restriction that prevents compiler to apply a cast operator of a type when it is trying to match the other operator (namely operator<< )?

When I define the following operator it compiles:

inline std::ostream& operator<<(std::ostream& os, const Identifier& id)
{
    return os << static_cast<std::string const&>(id);
}

I am using gcc4.4.2

Answer 1

This has nothing to do with boost, strong_typedef or spirit.

It has a lot to do with type deduction for template arguments. In short, when argument types are deduced, implicit conversions never take place [1]

Cf.:

#include <iostream>
#include <string>
#include <boost/strong_typedef.hpp>

BOOST_STRONG_TYPEDEF(double, X)
int main() { std::cout << X(); }

No problem! Replace double by std::string, and it doesn't work anymore. What's different?

The declaration of the streaming operator differs.

Contrast

ostream& ostream::operator<<(double);

To

template<typename _CharT, typename _Traits, typename _Alloc>
   inline basic_ostream<_CharT, _Traits>&
   operator<<(basic_ostream<_CharT, _Traits>&, basic_string<_CharT, _Traits, _Alloc> const&)

The fact that the operator overload is a function template disallows any implicit conversions.

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[1] I guess initializer_list may look like a bit of an exception here, what with widening/narrowing that it can do. Different subject, though