How to use boost::fusion::transform on heterogeneous containers?

Question

Boost.org's example given for fusion::transform is as follows:

struct triple
{
    typedef int result_type;

    int operator()(int t) const
    {
        return t * 3;
    };
};
// ...
assert(transform(make_vector(1,2,3), triple()) == make_vector(3,6,9));

Yet I'm not "getting it." The vector in their example contains elements all of the same type, but a major point of using fusion is containers of heterogeneous types. What if they had used make_vector(1, 'a', "howdy") instead?

int operator()(int t)
would need to become
template<typename T> T& operator()(T& const t)

But how would I write the result_type? template<typename T> typedef T& result_type certainly isn't valid syntax, and it wouldn't make sense even if it was, because it's not tied to the function.

Answer 1

Usually, fusion::transform is used with a templated (or -as shown above- otherwise overloaded) function operator:

struct triple 
{ 
    template <typename Sig>
    struct result;

    template <typename This, typename T>
    struct result<This(T)>
    {
        typedef /*...figure out return type...*/ type;
    };

    template <typename T> 
    typename result<triple(T)>::type 
    operator()(T t) const 
    { 
        return 3*t;    // relies on existing operator*() for 'T'
    }
}; 

And, additional sources of information about Fusion are the examples and the test directory where you can find demonstrations of some of the techniques.

Regards Hartmut