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How to compare boost::variant in order to make it a key of std::map ? Seems that operator<() is not defined for boost::variant

Mat
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user222202
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2 Answers2

5

EDITED TO FIX ERROR APPLYING BOOST::APPLY_VISITOR

You can create a binary visitor for your variant, and then use boost::apply_visitor to create a comparator for your map:

class variant_less_than
    : public boost::static_visitor<bool>
{
public:

    template <typename T, typename U>
    bool operator()( const T & lhs, const U & rhs ) const
    {
            // compare different types
    }

    template <typename T>
    bool operator()( const T & lhs, const T & rhs ) const
    {
            // compare types that are the same
    }

};

You'll probably need to overload operator() for each possible pair of types, as apposed to using the templated operator(const T &, const U &). Then you'd need to declare your map like this:

class real_less_than
{
public:
  template<typename T>
  bool operator()(const T &lhs, const T &rhs)
  {
    return boost::apply_visitor(variant_less_than(), lhs, rhs);
  }
};

std::map<boost::variant<T, U, V>, ValueType, real_less_than> myMap;

Edit: for what it's worth, operator<() is defined for boost::variant however it's defined as:

bool operator<(const variant &rhs) const
{
  if(which() == rhs.which())
    // compare contents
  else
    return which() < rhs.which();
}

which I'm assuming is not what you want.

Niki Yoshiuchi
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  • oh, thank you. operator<() did not work for my variant as it is defined with boost::make_recursive_variant<>, not with regular boost::variant<> I will copy the code from regular boost::variant<> – user222202 Dec 03 '10 at 23:21
  • @user222202 - see my edit. You can't call boost::apply_visitor in the template list for map. You're going to need to create a function object that applies the visitor. – Niki Yoshiuchi Dec 03 '10 at 23:34
1

Perhaps you can pass a comparator to the map. Please see http://www.sgi.com/tech/stl/Map.html for an example on how to write a comparator.

kvs
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