How do I get Boost.LexicalCast to work?

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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);

and:

std::stringstream ss("640 480");
glm::ivec2 pt;
ss >> pt;
std::cout << pt << std::endl;

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>]’

EDIT: It seems the error only occurs if headers for Boost.PropertyTree are included.

2

There are 2 answers

0
Chris_F On BEST ANSWER

The answer was found by another, so I'll leave it here.

He mentioned something about argument dependent lookup, and suggested that the stream operators be placed in the glm::detail namepace like so:

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;
}

}} // glm::detail

After that, everything works fine.

6
Useless On

You posted this extraction operator:

template <class T>
std::istream& operator>>(std::istream& in, glm::detail::tvec2<T>& vec)

but the error implies it is trying to compile with this:

std::istream& std::operator>>(std::istream&&, _Tp&)
[with ... _Tp = glm::detail::tvec2<int>]

Are you sure the code you posted matches what you're compiling? (Focus on the first argument to the operator if you can't see it right away).


OK, this complete code works fine for me:

#include <sstream>
#include <iostream>

#include <boost/lexical_cast.hpp>

template <class T>
struct tvec2 { 
    tvec2() : x(), y() {}
    tvec2(tvec2 const &) = default;
    tvec2(T x_, T y_) : x(x_), y(y_) {}

    T x;
    T y;
};

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

template <class T>
std::istream& operator>>(std::istream& in, tvec2<T>& vec)
{
    // yuck, boost disables skipws on the input stream
    in >> vec.x;
    if (in.good() && in.ignore(256, ' ').good())
        in >> vec.y;
    return in;
}

void test_operators()
{
    tvec2<int> intvec(2,3);
    std::cout << "intvec = {" << intvec << "}\n";

    std::stringstream ss;
    ss << intvec;

    tvec2<int> dupvec;
    ss >> dupvec;
    std::cout << "dupvec = {" << dupvec << "}\n";
}

void test_lexical_cast()
{
    std::cout << "and now with lexical_cast ...\n";
    tvec2<int> dupvec = boost::lexical_cast<tvec2<int> >("2 3");
    std::cout << "dupvec = {" << dupvec << "}\n";
}

int main()
{
    test_operators();
    test_lexical_cast();
}