Using SWIG with C++ template functions

Question

I have a few template functions for map in c++ and I want to use them in ruby.

template <class Collection>
const typename Collection::value_type::second_type& FindWithDefault(
    const Collection& collection,
    const typename Collection::value_type::first_type& key,
    const typename Collection::value_type::second_type& value) {
  typename Collection::const_iterator it = collection.find(key);
  if (it == collection.end()) {
    return value;
  }
  return it->second;
}

In c++ it works like this

int main(int argc, char const *argv[])
{
    typedef std::map<std::string, std::string> Map;
    Map m;
    std::string d= FindWithDefault(m, "foo", "");
    std::cout << d << "\n";
    //    
    m["foo"] = "bar";
    d=  FindWithDefault(m, "foo", "");
    std::cout << d << "\n";
    // bar
}

Its easy to make use of map and set in ruby by looking at the answer of the question I have answered before(take a look here).

As for this question My interface file has this

%include <std_map.i>
namespace std {
   %template(IntMap) map<int,int>;
}
template <typename Collection>
const typename Collection::value_type::second_type& FindWithDefault(
    const Collection& collection,
    const typename Collection::value_type::first_type& key,
    const typename Collection::value_type::second_type& value) ;
%template(d) FindWithDefault<std::map<int,int> >;

My c++ header file has the complete function mentioned above. Now when I try to check this in Irb, the map works fine, but I get an error on using the template function

2.2.1 :001 > a = Example::IntMap.new
 => std::map<int,int,std::less< int >,std::allocator< std::pair< int const,int > > > {} 
2.2.1 :002 > a[1]=2
 => 2 
2.2.1 :003 > a[5]=3
 => 3 
2.2.1 :004 > a[4]=11
 => 11 
2.2.1 :005 > a
 => std::map<int,int,std::less< int >,std::allocator< std::pair< int const,int > > > {1=>2,4=>11,5=>3} 
2.2.1 :006 > b=Example::d(a,2,2)
ArgumentError: Wrong arguments for overloaded method 'd'.
Possible C/C++ prototypes are:
    std::map< int,int,std::less< int >,std::allocator< std::pair< int const,int > > >::value_type::second_type const & d(int a, int b)
    std::map< int,int,std::less< int >,std::allocator< std::pair< int const,int > > >::value_type::second_type const & d(std::map< int,int,std::less< int >,std::allocator< std::pair< int const,int > > > const &collection, std::map< int,int,std::less< int >,std::allocator< std::pair< int const,int > > >::value_type::first_type const &key, std::map< int,int,std::less< int >,std::allocator< std::pair< int const,int > > >::value_type::second_type const &value)

    from (irb):6:in `d'
    from (irb):6
    from /usr/share/rvm/rubies/ruby-2.2.1/bin/irb:11:in `<main>'

How can I make this work ?

Answer 1

It seems SWIG has trouble determining the type of Collection::value_type::first_type and Collection::value_type::second_type. I am not exactly sure why.

A possible solution / workaround that happens to work in this case is to replace Collection::value_type::first_type with Collection::key_type and Collection::value_type::second_type with Collection::mapped_type. It is sufficient to do this in the SWIG interface file, i.e., leave your C++ code unchanged but put this in the interface file instead:

%include <std_map.i>
namespace std {
   %template(IntMap) map<int,int>;
}
template <typename Collection>
const typename Collection::mapped_type& FindWithDefault(
    const Collection& collection,
    const typename Collection::key_type& key,
    const typename Collection::mapped_type& value) ;
%template(d) FindWithDefault<std::map<int,int> >;