In C++, can you have a function that modifies a tuple of variable length?

Question

In C++0x I would like to write a function like this:

template <typename... Types>
void fun(typename std::tuple<Types...> my_tuple) {
    //Put things into the tuple
}

I first tried to use a for loop on int i and then do:

get<i>(my_tuple);

And then store some value in the result. However, get only works on constexpr.

If I could get the variables out of the tuple and pass them to a variadic templated function I could recurse through the arguments very easily, but I have no idea how to get the variables out of the tuple without get. Any ideas on how to do that? Or does anyone have another way of modifying this tuple?

Answer 1

Since the "i" in

get<i>(tup)

needs to be a compile-time constant, template instantiation is used to "iterate" (actually recurse) through the values. Boost tuples have the "length" and "element" meta-functions that can be helpful here -- I assume C++0x has these too.

Answer 2

Boost.Fusion is worth a look. It can 'iterate' over std::pair, boost::tuple, some other containers and its own tuple types, although I don't think it supports std::tuple yet.

Answer 3

AFAICT, C++ tuples basically need to be handled with recursion; there don't seem to be any real ways of packing/unpacking tuples except using the typesystem's only variadic template handling.

Answer 4

Take a look at section 6.1.3.4 of TR1, http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf

get is defined for both const and non-const qualified tuples and returns the appropriate reference type. If you change your function declaration to the following:

template 
void fun(typename std::tuple& my_tuple) {
    //Put things into the tuple
}

Then the argument to your function is a non-const tuple and get will allow you to make the necessary assignments once you've written the iteration using the information provided in previous responses.

Answer 5

Have a look at my answer here for an example of template recursion to unwind tuple arguments to a function call.

How do I expand a tuple into variadic template function's arguments?