Hi.

I've done a search in the archive for this, but haven't found anything, so
therefore I post this.

Does anyone know why function template template parameters aren't allowed?
We have class template template parameters, but not function template
template parameters.

This came up in a discussion at the Boost list (but as language extensions
are off topic for Boost, I took it here), regarding enabling higher order
functional programming with function templates in C++. Specifically, there
was a question about how to iterate over a typelist, calling a function for
each type, and passing a parameter, using the Boost Metaprogramming Library
(MPL).

A solution to this currently involves making a function object, which passes
the call to the required function:

// Function to call for each type

template<class T>
void my_function(std::string &) { ... }

struct function
{
    function(std::string &s) : str(s) {}

    template <class T>
    void operator()(T) const
    {
        my_function<T>(str);
    }

 private:
     std::string &str;
};

mpl::for_each<my_list>(function(s));

Had function template template parameters existed, you might be able to
collapse this code to one line:

mpl::for_each<my_list, std::string &, function>(s);

It could be defined something like this:

template<class Sequence, class T, template<class> void Function(T)>
void for_each(T);

Is there any specific reason for not allowing this, or is it simply that one
didn't want to delay standardisation?


Regards,

Terje
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