Boost Lexical Cast

[Chris_F]

I'm having issues with boost::lexical_cast. I am trying to use it on a class from the GLM (OpenGL Mathematics) library.

To allow for lexical casting, I have implemented operator<< functions for the class in question:

template <class T>
std::ostream& operator<<(std::ostream& out, const glm::detail::tvec2<T>& vec)
{
	out << vec.x << " " << vec.y;
	return out;
}

template <class T>
std::istream& operator>>(std::istream& in, glm::detail::tvec2<T>& vec)
{
	in >> vec.x;
	in >> vec.y;
	return in;
}

I tested the operators like so:

std::cout <<  glm::ivec2(1, 1) << glm::vec2(1.0f, 1.0f);

This works just fine, however if I try this:

glm::ivec2 pt = boost::lexical_cast<glm::ivec2>("1 1");

I get the following error:

/usr/include/boost/lexical_cast.hpp: In member function ‘bool boost::detail::lexical_stream_limited_src<CharT, Base, Traits>::operator>>(InputStreamable&) [with InputStreamable = glm::detail::tvec2<int>, CharT = char, Base = std::basic_streambuf<char>, Traits = std::char_traits<char>]’:
/usr/include/boost/lexical_cast.hpp:1151:13:   instantiated from ‘Target boost::detail::lexical_cast(typename boost::call_traits<B>::param_type, CharT*, std::size_t) [with Target = glm::detail::tvec2<int>, Source = const char*, bool Unlimited = false, CharT = char, typename boost::call_traits<B>::param_type = const char* const, std::size_t = long unsigned int]’
/usr/include/boost/lexical_cast.hpp:1174:77:   instantiated from ‘Target boost::lexical_cast(const Source&) [with Target = glm::detail::tvec2<int>, Source = char [8]]’
test2.cpp:41:59:   instantiated from here
/usr/include/boost/lexical_cast.hpp:785:29: error: cannot bind ‘std::basic_istream<char>’ lvalue to ‘std::basic_istream<char>&&’
/usr/include/c++/4.6/istream:852:5: error:   initializing argument 1 of ‘std::basic_istream<_CharT, _Traits>& std::operator>>(std::basic_istream<_CharT, _Traits>&&, _Tp&) [with _CharT = char, _Traits = std::char_traits<char>, _Tp = glm::detail::tvec2<int>]’

[Chris_F]

Here is a minimal example of what replicates the error:

#include <glm/glm.hpp>
#include <boost/lexical_cast.hpp>
#include <boost/property_tree/ptree.hpp>

template <class T>
std::ostream& operator<<(std::ostream& out, const glm::detail::tvec2<T>& vec)
{
  out << vec.x << " " << vec.y;
  return out;
}

template <class T>
std::istream& operator>>(std::istream& in, glm::detail::tvec2<T>& vec)
{
    in >> vec.x;
    if (in.good() && in.ignore(256, ' ').good())
        in >> vec.y;
    return in;
}

int main()
{
    glm::ivec2 pt = boost::lexical_cast<glm::ivec2>("640 480");
}

If I remove the Boost.PropertyTree header file, it will compile. I posted this on some other sites as well, and so far nobody has any clue.

[laserlight]

I can confirm the error with Boost 1.47.0 and GLM 0.9.2.7 when compiling on g++ 4.4.5 using your test program. A "manual" version that uses a stringstream directly works even with the Boost.PropertyTree header file included, so my guess is that this is a bug with boost::lexical_cast.

[laserlight]

Oh, on a hunch, from a past discussion on the Boost developers list concerning an input manipulator I once proposed, I suspected that argument dependent lookup was working against you, and a quick check seems to show that I am right. Try:

#include <glm/glm.hpp>
#include <boost/lexical_cast.hpp>
#include <boost/property_tree/ptree.hpp>

namespace glm
{
    namespace detail
    {
        template <class T>
        std::ostream& operator<<(std::ostream& out, const glm::detail::tvec2<T>& vec)
        {
          out << vec.x << " " << vec.y;
          return out;
        }

        template <class T>
        std::istream& operator>>(std::istream& in, glm::detail::tvec2<T>& vec)
        {
            in >> vec.x;
            if (in.good() && in.ignore(256, ' ').good())
                in >> vec.y;
            return in;
        }
    }
}

int main()
{
    glm::ivec2 pt = boost::lexical_cast<glm::ivec2>("640 480");
}

As such, it is not a bug with lexical_cast. My bad.

[Chris_F]

I have no idea (yet) what argument dependent lookup is, but that sure seems to be the problem and solution.

Thank you very much for coming up with that. I could have been tearing my hair out for days.

[laserlight]

I have no idea (yet) what argument dependent lookup is, but that sure seems to be the problem and solution.

Basically, as the call of operator>> within lexical_cast is unqualified, the name lookup also considers namespaces that the types of the arguments are from. As such, to overload operator>> for glm::detail::tvec2<T> to read from input streams, it is best to do so within the glm::detail namespace. Unfortunately, that namespace is reserved, by convention, to GLM developers working on GLM implementation detail.