I'm trying to make a convenient function template to retrieve different type of objects using tuple.

class Apple;
class Banana;
class Cherry;

typedef std::tuple<Apple, Banana, Cherry> Fruits;
Fruits fruits;

enum
{
  APPLE,
  BANANA,
  CHERRY
};


//When I try to use this, I get "Failed to specialize function template" error
template<std::size_t i>
decltype(std::get<i>(fruits)) getFruit()
{
   return std::get<i>(fruits)
}
ex) getFruit<APPLE>(); //error

I think the class types that you use in the tuple has to be defined and not just forward declared.

Indeed. As per this page http://msdn.microsoft.com/en-us/library/s8kw2xcc.aspx "... make sure you can instantiate every type."