boost::lexical_cast with boost::units::quantity does not compile any more

Question

I have a problem with code that parses values taken via boost_program_options to boost::units quantities. It used to work just fine, and now, on a new setup, no longer does. The obscure error message hints at a problem with boost::lexical_cast, which seems to be used by program_options.

A minimal example (in which I got rid of the all program options stuff) is

#include <iostream>
#include <boost/units/io.hpp>
#include <boost/units/systems/si.hpp>
#include <boost/units/systems/si/io.hpp>
#include <boost/lexical_cast.hpp>
#include <sstream>

using namespace boost::units;

std::istream& operator>>(std::istream& in, quantity<si::current>& c)
{
    c = quantity<si::current>(0.7 * si::ampere);
    return in;
}

int main()
{
    quantity<si::current> c = boost::lexical_cast<quantity<si::current> >("3.0A");
    std::cout << c << std::endl;
}

This compiles without errors on my old setup (gcc 4.7.2, boost 1.49), but on gcc 4.9.2 with boost 1.55 no longer compiles (by the way, please ignore the fact that even if it compiles, it throws an Exception in either case, because nothing is actually read from the stream; I did not want to obfuscate this post by including the whole parsing logic). Instead, I get the error message

In file included from /usr/include/boost/serialization/tracking.hpp:20:0,
                 from /usr/include/boost/serialization/nvp.hpp:32,
                 from /usr/include/boost/units/io.hpp:27,
                 from bla3.cpp:2:
/usr/include/boost/lexical_cast.hpp: In instantiation of ‘struct boost::detail::deduce_target_char_impl<boost::detail::deduce_character_type_later<boost::units::quantity<boost::units::unit<boost::units::list<boost::units::dim<boost::units::current_base_dimension, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::homogeneous_system<boost::units::list<boost::units::si::meter_base_unit, boost::units::list<boost::units::scaled_base_unit<boost::units::cgs::gram_base_unit, boost::units::scale<10l, boost::units::static_rational<3l> > >, boost::units::list<boost::units::si::second_base_unit, boost::units::list<boost::units::si::ampere_base_unit, boost::units::list<boost::units::si::kelvin_base_unit, boost::units::list<boost::units::si::mole_base_unit, boost::units::list<boost::units::si::candela_base_unit, boost::units::list<boost::units::angle::radian_base_unit, boost::units::list<boost::units::angle::steradian_base_unit, boost::units::dimensionless_type> > > > > > > > > > > > > >’:
/usr/include/boost/lexical_cast.hpp:415:89:   required from ‘struct boost::detail::deduce_target_char<boost::units::quantity<boost::units::unit<boost::units::list<boost::units::dim<boost::units::current_base_dimension, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::homogeneous_system<boost::units::list<boost::units::si::meter_base_unit, boost::units::list<boost::units::scaled_base_unit<boost::units::cgs::gram_base_unit, boost::units::scale<10l, boost::units::static_rational<3l> > >, boost::units::list<boost::units::si::second_base_unit, boost::units::list<boost::units::si::ampere_base_unit, boost::units::list<boost::units::si::kelvin_base_unit, boost::units::list<boost::units::si::mole_base_unit, boost::units::list<boost::units::si::candela_base_unit, boost::units::list<boost::units::angle::radian_base_unit, boost::units::list<boost::units::angle::steradian_base_unit, boost::units::dimensionless_type> > > > > > > > > > > > >’
/usr/include/boost/lexical_cast.hpp:674:92:   required from ‘struct boost::detail::lexical_cast_stream_traits<const char*, boost::units::quantity<boost::units::unit<boost::units::list<boost::units::dim<boost::units::current_base_dimension, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::homogeneous_system<boost::units::list<boost::units::si::meter_base_unit, boost::units::list<boost::units::scaled_base_unit<boost::units::cgs::gram_base_unit, boost::units::scale<10l, boost::units::static_rational<3l> > >, boost::units::list<boost::units::si::second_base_unit, boost::units::list<boost::units::si::ampere_base_unit, boost::units::list<boost::units::si::kelvin_base_unit, boost::units::list<boost::units::si::mole_base_unit, boost::units::list<boost::units::si::candela_base_unit, boost::units::list<boost::units::angle::radian_base_unit, boost::units::list<boost::units::angle::steradian_base_unit, boost::units::dimensionless_type> > > > > > > > > > > > >’
/usr/include/boost/lexical_cast.hpp:2363:19:   required from ‘static Target boost::detail::lexical_cast_do_cast<Target, Source>::lexical_cast_impl(const Source&) [with Target = boost::units::quantity<boost::units::unit<boost::units::list<boost::units::dim<boost::units::current_base_dimension, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::homogeneous_system<boost::units::list<boost::units::si::meter_base_unit, boost::units::list<boost::units::scaled_base_unit<boost::units::cgs::gram_base_unit, boost::units::scale<10l, boost::units::static_rational<3l> > >, boost::units::list<boost::units::si::second_base_unit, boost::units::list<boost::units::si::ampere_base_unit, boost::units::list<boost::units::si::kelvin_base_unit, boost::units::list<boost::units::si::mole_base_unit, boost::units::list<boost::units::si::candela_base_unit, boost::units::list<boost::units::angle::radian_base_unit, boost::units::list<boost::units::angle::steradian_base_unit, boost::units::dimensionless_type> > > > > > > > > > > >; Source = const char*]’
/usr/include/boost/lexical_cast.hpp:2543:50:   required from ‘Target boost::lexical_cast(const Source&) [with Target = boost::units::quantity<boost::units::unit<boost::units::list<boost::units::dim<boost::units::current_base_dimension, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::homogeneous_system<boost::units::list<boost::units::si::meter_base_unit, boost::units::list<boost::units::scaled_base_unit<boost::units::cgs::gram_base_unit, boost::units::scale<10l, boost::units::static_rational<3l> > >, boost::units::list<boost::units::si::second_base_unit, boost::units::list<boost::units::si::ampere_base_unit, boost::units::list<boost::units::si::kelvin_base_unit, boost::units::list<boost::units::si::mole_base_unit, boost::units::list<boost::units::si::candela_base_unit, boost::units::list<boost::units::angle::radian_base_unit, boost::units::list<boost::units::angle::steradian_base_unit, boost::units::dimensionless_type> > > > > > > > > > > >; Source = char [5]]’
bla3.cpp:18:81:   required from here
/usr/include/boost/lexical_cast.hpp:388:13: error: invalid application of ‘sizeof’ to incomplete type ‘boost::STATIC_ASSERTION_FAILURE<false>’
             BOOST_STATIC_ASSERT_MSG((result_t::value || boost::has_right_shift<std::basic_istream<wchar_t>, T >::value), 
             ^

If I use a different custom type (not a boost::units::quantity) in the lexical cast and overload the >> operator for it, or if I directly try to read a quantity from a stringstream, all works fine.

Can someone tell me what I am doing wrong here?

I can't even compile a trivial `main()` containing only this: `boost::units::quantity c;`. Can you? — John Zwinck, May 10 '16 at 10:32
@JohnZwinck : Yes, putting just that in main compiles without problem for both setups. What error message do you get? — fedro, May 10 '16 at 10:53
Ah, I had to compile with `-Wno-unused-local-typedef` to avoid spammy stuff about unused typedefs in Boost. — John Zwinck, May 10 '16 at 10:55

score 1 · Accepted Answer · answered May 10 '16 at 12:30

1

An overloaded operator, like a free function, needs to be in an associated namespace to be located by argument-dependent lookup.

In this case you would need to open either the std or boost::units namespaces, since the arguments to operator>> are in namespace std and boost::units respectively:

namespace boost { namespace units {

std::istream& operator>>(std::istream& in, quantity<si::current>& c)
{
    c = quantity<si::current>(0.7 * si::ampere);
    return in;
}

}}

This should indicate to you that this is not entirely safe, as your code would collide with any stream in operator supplied by Boost.Units in a future version, or by another 3rd-party user. You should consider writing your own wrapper for quantity<si::current> and supplying a stream in operator there.

answered May 10 '16 at 12:30

ecatmur

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Thanks so much, that did indeed do the trick! I wonder how it worked before, though. Was gcc 4.7.2 more lenient than it should have been? Also, even with gcc 4.9.2, the lookup of the overloaded operator does work if I don't use lexical_cast but just try to read a `quantity` from a `std::stringstream`... – fedro May 10 '16 at 12:43
@fedro it's complicated; it's a difference in boost versions, as far as I can tell (gcc 4.7.2 is a bit outdated, but it got this sort of thing right). The difference is that since boost 1.53, Boost.LexicalCast makes sure to only consider stream operators in the associated namespaces; this is more robust because it is guaranteed that they will be located by argument-dependent lookup. Non-argument dependent lookup can lead to ODR violation (http://en.cppreference.com/w/cpp/language/dependent_name#Lookup_rules) – ecatmur May 10 '16 at 13:59

boost::lexical_cast with boost::units::quantity does not compile any more

1 Answers1