Overriding a template function with polymorphic

Question

If i have

template<class T>
TalkyBuffer& operator<<(T const &object) { // Template
...
}
TalkyBuffer& operator<<(TalkySerialisable const &object); // Override

and class

class A : public TalkySerialisable {
...}

Then if I do

TalkyBuffer b;
A test;
b << test;

Then gcc calls the Template function, not the Override function.

However, if I specifically define an override

TalkyBuffer& operator<<(A const &object); // Override without polymorphism

Then gcc selects this one. Is there a practical way to override a template function with an abstract class?

I read this, but it does not shed light on what happens when you throw polymorphism into a mix: http://www.gotw.ca/publications/mill17.htm Also I could not find a solution here, but maybe I I use the wrong terms.

Answer 1

TalkyBuffer& operator<<(TalkySerialisable const &object); . tmeplated.

b << test;, , A. , , . .

( A TalkySerialisable) , , .

Answer 2

, function, .

struct specialized {};
struct generic {};

template <class T>
TalkyBuffer& serialize(TalkyBuffer& buffer, T const& object, generic) {
  ...
}

generic dispatch(...) {} // always picked up last in overload resolution

template <class T>
TalkyBuffer& TalkyBuffer::operator<<(T const& object) { // Template
  return serialize(*this, object, dispatch(object));
}

:

TalkyBuffer& serialize(TalkyBuffer& buffer,
                       TalkySerialisable const& object,
                       specialized);

specialized dispatch(TalkySerialisable const&) {}    

:

class A: public TalkySerialisable {};

, ?

• TalkyBuffer::operator<<(T const&)
• serialize dispatch
• dispatch, dispatch(TalkySerializable const&) , dispath(...), specialized
• serialize ( specialized generic),

Answer 3

Boost.enable_if:

#include <boost/utility/enable_if.hpp>
#include <boost/type_traits/is_base_of.hpp>

template<typename T>
typename boost::disable_if<
    boost::is_base_of<TalkySerializable, T>,
    TalkyBuffer &
>::type operator<<(T const & object) { // Template for non TalkySerializable
...
}

template <typename T>
typename boost::enable_if<
    boost::is_base_of<TalkySerializable, T>,
    TalkyBuffer &
>::type operator<<(T const & object); // Template overload for TalkySerializable

...

TalkyBuffer b;
A test;
b << test; // calls operator<< <A>(A const &), which instantiates 
           // the overload for TalkySerializable
b << 41; // calls operator<< <int>(int const &), which corresponds to
         // the "default" overload

, , : .

---

@Matthieu, : , , , .

, , Boost.MPL.

// TalkyBuffer.hpp

#include <iostream>
#include <boost/utility/enable_if.hpp>
#include <boost/mpl/has_xxx.hpp>

// defines a metafunction has_talky_buffer_tag<T> that allows us to know at
// compile-time if T has a member type named talky_buffer_tag
BOOST_MPL_HAS_XXX_TRAIT_DEF(talky_buffer_tag)

// tag for the default case
struct default_talky_buffer_tag {};

// trait class for obtaining the tag of a type
template <typename T, typename Enable = void >
struct talky_buffer_trait
{
    typedef default_talky_buffer_tag type;
};

// specialization for types that provide a nested typedef
template <typename T>
struct talky_buffer_trait<T, 
    typename boost::enable_if<has_talky_buffer_tag<T> >::type>
{
    typedef typename T::talky_buffer_tag type;
};


struct TalkyBuffer 
{
    // Insertion operator, which calls an implementation function that can
    // be overloaded depending on the tag
    template<typename T>
    TalkyBuffer & operator<<(T const & object) 
    {
        typename talky_buffer_trait<T>::type tag;
        return insertionOperatorImpl(*this, object, tag);
    }
};

// default implementation
template <typename T>
TalkyBuffer & insertionOperatorImpl(TalkyBuffer & buf, T const & object,
    default_talky_buffer_tag)
{
    std::cout << "default";
    return buf;
}


//-------
// TalkySerializable.hpp

struct TalkySerializable 
{ 
    struct tag {}; 
    typedef tag talky_buffer_tag; 
};

// A inherits from the nested typedef
struct A : public TalkySerializable {};

// implementation for TalkySerializable objects
template <typename Serializable>
TalkyBuffer & insertionOperatorImpl(TalkyBuffer & buf, Serializable const & object,
    TalkySerializable::tag)
{
    std::cout << "specialized";
    return buf;
}


//-------
int main()
{
    TalkyBuffer b;
    A test;
    b << test; // outputs "specialized"
    b << 41;   // outputs "default"
}

T, , (TypeSerializable::tag ), T ( typedef, , : template <> talky_buffer_trait<T> { typedef new_tag type };) , , (insertionOperatorImpl ).