Constructor arguments from tuple

Question

Suppose I have a template which is parametrized by a class type and a number of argument types. a set of arguments matching these types are stored in a tuple. How can one pa

Answer 1

You need some template meta-programming machinery to achieve that.

The easiest way to realize the argument dispatch is to exploit pack expansion on expressions which contain a packed compile-time sequence of integers. The template machinery is needed to build such a sequence (also see the remark at the end of this answer for more information on a proposal to standardize such a sequence).

Supposing to have a class (template) index_range that encapsulates a compile-time range of integers [M, N) and a class (template) index_list that encapsulates a compile-time list of integers, this is how you would use them:

template
struct foo
{
    tuple args;

    // Allows deducing an index list argument pack
    template
    T gen(index_list const&)
    {
        return T(get(args)...); // This is the core of the mechanism
    }

    T gen()
    {
        return gen(
            index_range<0, sizeof...(Args)>() // Builds an index list
            );
    }
};

And here is a possible implementation of index_range and index_list:

//===============================================================================
// META-FUNCTIONS FOR CREATING INDEX LISTS

// The structure that encapsulates index lists
template 
struct index_list
{
};

// Collects internal details for generating index ranges [MIN, MAX)
namespace detail
{
    // Declare primary template for index range builder
    template 
    struct range_builder;

    // Base step
    template 
    struct range_builder
    {
        typedef index_list type;
    };

    // Induction step
    template 
    struct range_builder : public range_builder
    {
    };
}

// Meta-function that returns a [MIN, MAX) index range
template
using index_range = typename detail::range_builder::type;

Also notice, that an interesting proposal by Jonathan Wakely exists to standardize an int_seq class template, which is something very similar to what I called index_list here.