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How do I get a reference to a "get"-function for a specific tuple instance?

My best try is given below but does not compile against g++4.5.1

#include <tuple>
#include <string>
typedef std::tuple<int,std::string> Tuple;
auto t=(std::string& (Tuple&))(std::get<1,Tuple>);
The compiler error is:
a.cc:5: error: invalid cast to function type ‘std::string&(Tuple&)’ 
a.cc:5: error: unable to deduce ‘auto’ from ‘<expression error>’
I would like to use the function reference as an input to some stl-algorithms. I am actually a bit surprised on how non-trivial this seems to be for me.
    template <std::size_t I, typename... Types>
    decltype(&std::get<I, Types...>) get_function(std::tuple<Types...>*)
        return &std::get<I, Types...>;
template <std::size_t I, typename Tuple>
decltype(detail::get_function<I>((Tuple*)nullptr)) get_function()
    return detail::get_function<I>((Tuple*)nullptr);
auto t = get_function<I, Tuple>();
Kind of ugly. Surely there's a better way, but that's all I have for now.
                I was hoping for a one-liner, but I see how this is supposed to work. gcc-4.5.1 complains about the nullptr, and about type-deduction after that. I will try to adapt this for my compiler later.
– mirk
                Oct 26, 2010 at 7:28
                @user Move it into a header, #include "get_function.hpp" is one line. :) What are the errors?
– GManNickG
                Oct 26, 2010 at 7:32
                I used "#define nullptr NULL;" as a workaround and got in  decltype(detail::get_function<I>((Tuple*)nullptr)) get_function()  q.cc:19: error: expected ‘)’ before ‘;’ token q.cc:19: error: expected unqualified-id before ‘)’ token
– mirk
                Oct 26, 2010 at 7:43
                @user: Why do you need to define nullptr? It should be defined already. Keep in mind my code is untested, I just wrote it from brain; the error could be in my part and not yours.
– GManNickG
                Oct 26, 2010 at 7:46
After a bit of experimenting, I've found that it is possible using lambda expressions:
#include <tuple>
#include <string>
typedef std::tuple<int, std::string> Tuple;
auto t = [] (Tuple& t) { return std::get<1> (t); };
If you need to use this a lot you could always write a macro:
#define REFERENCE_GET(N, Tuple) \
    [] (Tuple &t) { return std::get<N> (t); }
                I did not consider myself "ready" for lambda's yet, but it looks like there is no way around them in C++0x. The only drawback seems to be that the lambda is not usable as an input to the boost::transform_iterator due to a missing result_type, but that is a different story. Thank you.
– mirk
                Oct 25, 2010 at 18:47
The code below works for me. The trick is to use a function-pointer typedef and not a function reference typedef. 
I am impressed that the compiler does not need all template parameters, but can optionally work out missing ones through the overloaded function type.
#include <tuple>
#include <string>
typedef std::tuple<int,std::string> Tuple;
typedef std::string& (*PFun)(Tuple&);
PFun p1=(PFun)std::get<1U>;
PFun p2=(PFun)std::get<1U,int,std::string>;
In one line:
auto p3=(std::string& (*)(Tuple&))std::get<1U>;
Sorry to answer my own question. A night sleep can make a big difference.
                Ah, if you had been okay with re-specifying the tuples types, it is indeed much shorter. :) Most of the my code is extracting them for you.
– GManNickG
                Oct 26, 2010 at 17:21
                Indeed, I did not make it clear that the tuple types are not required.   However, the standard provides tuple_element for that. So replacing std::string with std::tuple_element<1,Tuple> in the code above would have handled this as well.
– mirk
                Oct 26, 2010 at 18:13
        
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