[Boost-bugs] [Boost C++ Libraries] #2128: Add the ability to implicitly traverse reference wrappers

Subject: [Boost-bugs] [Boost C++ Libraries] #2128: Add the ability to implicitly traverse reference wrappers
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2008-07-18 19:12:47

#2128: Add the ability to implicitly traverse reference wrappers
----------------------------------------------------+-----------------------
 Reporter:  mgoldshteyn_at_[hidden]                 |       Owner:  pdimov 
     Type:  Feature Requests                        |      Status:  new    
Milestone:  Boost 1.36.0                            |   Component:  bind   
  Version:  Boost 1.35.0                            |    Severity:  Problem
 Keywords:  implicitly traverse reference_wrappers  |  
----------------------------------------------------+-----------------------
 Please add the following to boost:

 namespace boost
 {

 template<class T> T* get_pointer( reference_wrapper<T> const & r )
 {
     return r.get_pointer();
 }

 }

 ----

 Code demonstrating the issue:

 #include <boost/bind.hpp>
 #include <boost/ref.hpp>
 #include <string>

 // Define a simple class for our example class Crayon {
 public:
   explicit Crayon(const std::string &color=std::string()):m_color(color)
 {}
   const std::string &GetColor() const {return m_color;}

 private:
   std::string m_color;
 };

 int main(int argc, char *argv[])
 {
   using namespace std;
   using namespace boost;

   Crayon cr("blue");
   boost::reference_wrapper<const Crayon> crefCr(cr);

   // This doesn't compile, however, even though the reference_wrapper
 class template has
   // an overloaded operator T& () const function
   std::string
 colorFromConstRef2(boost::bind(&Crayon::GetColor,_1)(crefCr));
 }

 ----

 Peter Dimov's reply:

 It doesn't because of the mem_fn behavior that bind inherits. The argument
 is not a reference to Crayon and so is assumed to be a smart pointer to
 Crayon. mem_fn then tries to invoke get_pointer() on it to obtain a
 Crayon*.
 You should be able to make it compile by adding a get_pointer overload for
 reference_wrapper:

 namespace boost
 {

 template<class T> T* get_pointer( reference_wrapper<T> const & r )
 {
     return r.get_pointer();
 }

 }

-- 
Ticket URL: <http://svn.boost.org/trac/boost/ticket/2128>
Boost C++ Libraries <http://www.boost.org/>
Boost provides free peer-reviewed portable C++ source libraries.