Non-type template parameter... that's a template! (C++)

Question

I'm basically looking to generate a wrapper for a generic C function without having to manually specify the types. So I have a callback with a fixed prototype but I'm going to need to do some special code in the wrapper based on the type of the wrapped function... So basically I'm thinking about using a static method in a class template to wrap my function to a conforming interface e.g.:

// this is what we want the wrapped function to look like
typedef void (*callback)(int); 
void foobar( float x ); // wrappee

// doesn't compile
template< T (*f)(S) > // non-type template param, it's a function ptr
struct Wrapper
{
  static void wrapped(int x)
  {
     // do a bunch of other stuff here
     f(static_cast<S>(x)); // call wrapped function, ignore result

  }
}

And then I'd like to do something like:

AddCallback( Wrapper<foobar>::wrapped );

However, the problem is that I can't just go ahead and use a "S" in the parameter of the function in the Wrapper template, I have to first list it as a parameter:

template< class T, class S, T (*f)(S) >
struct Wrapper
// ...

But this means it's a lot more painful to use (Wrapper<void,float,foobar>::wrapped), ideally I'd like to just pass in the function pointer there and have it work out the types of the parameters (and return types) automatically. To be clear, inside the wrapped function I'm going to need to refer to the types of the function pointer (so I do need some equivalent of S or T).

Is there a way of doing this?

Answer 1

If you use a function that returns the "wrapped" rather than referring to it directly, the compiler will attempt to automatically match the template parameters for the function call.

edit: What about this?

int foobar( float x ); // wrappee

template <typename T, typename S>
struct Wrapper {
    typedef T (*F)(S);
    F f;

    Wrapper(F f) : f(f) { }

    void wrapped(S x) {
        // do a bunch of other stuff here
        f(x); // call wrapped function, ignore result
    }
};

template <typename T, typename S>
Wrapper<T,S> getWrapper(T (*f)(S)) {
    return Wrapper<T,S>(f);
}

...
getWrapper(foobar).wrapped(7);

Answer 2

EDIT: completely new answer

OK, I've completely re-thought the question and believe that I get what you want. I've actually done this before :-P.

Here's the idea, I have a Base class which overloads operator(), then I have a subclass for each "arity" of functions. Finally I have a factory function which will return one of these things. The code is big (and probably a little overkill) but works nicely. Much of the library_function overloads are to support different syntaxes, mostly unnecessary. It also supports boost::bind functions, member functions, etc, very much more than you probably need.

http://pastebin.com/m35af190

Example, usage:

// map of library functions which will return an int.
std::map<std::string, LibraryFunction<int> > functions;

// function to register stuff in the map
void registerFunction(const std::string &name, LibraryFunction<int> func) {
    functions.insert(std::make_pair(name, func));
}

later you can do this:

// the this param is so the function has access to the scripting engine and can pop off the parameters, you can easily chop it out

// register 2 functions, one with no params, one with 1 param
registerFunction("my_function", library_function1(*this, call_my_function));
registerFunction("my_function2", library_function0(*this, call_my_function2));

functions["my_function"]();
functions["my_function2"]();

Answer 3

I'd look at boost. At first reading of your question, it seems to me than <boost/function_types/parameter_types.hpp> provides what your need.

Answer 4

One thing you might wish to consider is using LLVM or similar to generate an appropriate trampoline function at runtime. Or here's a static solution:

#include <iostream>

void f(float f) { std::cout << f << std::endl; }

template<typename T, typename S> struct static_function_adapter {
        template<T(*f)(S)> struct adapt_container {
                static void callback(int v) {
                        f(static_cast<S>(v));
                }
        };

        template<T(*f)(S)> adapt_container<f> adapt() const {
                return adapt_container<f>();
        }
};

template<typename T, typename S> struct static_function_adapter<T, S> get_adapter(T (*)(S)) {
        return static_function_adapter<T, S>();
}

#define ADAPTED_FUNCTION(f) (&get_adapter(f).adapt<f>().callback)

int main() {
        void (*adapted)(int) = ADAPTED_FUNCTION(f);
        adapted(42);
        return 0;
}

The get_adapter function allows us to infer the argument and return type; adapt() then converts this into a type parameterized on the actual function, and finally we get a static function in callback.