casting a function back to the original signature using variadic args

Question

I've found this interesting code here on stackoverflow from: Using a STL map of function pointers

template<typename T,typename... Args>
    T searchAndCall(std::string s1, Args&&... args){

// ....
        // auto typeCastedFun = reinterpret_cast<T(*)(Args ...)>(mapVal.first);
        auto typeCastedFun = (T(*)(Args ...))(mapVal.first);

        //compare the types is equal or not
        assert(mapVal.second == std::type_index(typeid(typeCastedFun)));
        return typeCastedFun(std::forward<Args>(args)...);
    }
};

Basically, mapVal is a map of function pointers casted to void(*)(void) that will be casted back to their original type with this function. What I would like to do know is how typeCastedFun will be deduced when you don't specify the template parameters.

For instance, let's suppose that you had:

int f(const MyClass& a, MyClass b) {...}

... if you have:

MyClass first, second;
searchAndCall<int>(first, second);

What Args... parameter will be deduced? if I recall correctly, using the function casted back to a function with a different signature compared to the original one, should yield undefined behavior. Is there any other alternative?

What I would like to do is a way to store the type of the function somewhere and use this information to do the correct cast. Everything in the most efficient way.

Thanks

[edit1] More specifically, I'm trying to build a kind of generic function dispatcher, able to call functions (templated with an enum class value) with different signatures using a lookup table for efficiency reasons. No boost::any as it internally uses a new

[edit2] Use of macros is not allowed

Answer 1

The key problem is that by taking the calling argument types directly, and attempting to cast the function pointer, you are losing all implicit conversions.

Your function signature has to match exactly, or you will get UB if you try to call it. And there is generally no way to get the signature from the args without manually specifying it at the call site.

One workaround to try would be to add a wrapper lambda which takes standardized args with pre-specified implicit coversions applied, e.g. T -> const T&, and possibly numeric types -> double.

Then, when you look up the function, you can cast it to use these standardized args, and the calling args will be implicitly converted.

This would rule out functions taking rvalue refs and non-const references, but I don't thing this is unreasonable for a function that you don't know the signature of, unless you want to disregard const-correctness completely.

Also, other implicit conversions wouldn't happen, e.g. Derived& -> Base&, or char* -> std::string, and I don't think there would be an easy way to make that happen without creating extra limitations.

Overall, it's definitely a tricky thing to do in c++, and anything you try will be hacky. This way should be decent enough. The performance overhead of one extra function call (which can be inlined), and possibly some extraneous argument conversions will be overshadowed by the unavoidable RTTI checking.

Here is a sample implementation (also here on ideone):

#include <unordered_map>
#include <typeinfo>
#include <typeindex>
#include <string>
#include <type_traits>
#include <iostream>
#include <assert.h>
#include <cxxabi.h>
#include <sstream>

#include <stdexcept>

template <typename Func, Func f>
struct store_func_helper;

// unix-specific
std::string demangle(const std::string& val) {
  int     status;
  char   *realname;

  std::string strname = realname = abi::__cxa_demangle(val.c_str(), 0, 0, &status);
  free(realname);   
  return strname;
}

// args will be implicitly converted to arg<T>::type before calling function
// default: convert to const Arg&
template <typename Arg, typename snifae=void>
struct arg {
  using type = const Arg&;
};

// numeric types: convert to double.
template <typename Arg>
struct arg <Arg, typename std::enable_if<std::is_arithmetic<Arg>::value, void>::type> {
    using type = double;
};

// set more special arg types here.


// Functions stored in the map are first wrapped in a lambda with this signature.
template <typename Ret, typename... Arg>
using func_type = Ret(*)(typename arg<Arg>::type...);

class func_map {
  template <typename Func, Func f>
  friend class store_func_helper;

public:
template <typename Func, Func f>
void store(const std::string& name){
  store_func_helper<Func, f>::call(this, name );
}

template<typename Ret, typename... Args>
Ret call(std::string func, Args... args){
  using new_func_type = func_type<Ret, Args...>;
  auto& mapVal = m_func_map.at(func);  

  if (mapVal.second != std::type_index(typeid(new_func_type))){
    std::ostringstream ss;
    ss << "Error calling function " << func << ", function type: " 
      << demangle(mapVal.second.name()) 
      << ", attempted to call with " << demangle(typeid(new_func_type).name());
    throw std::runtime_error(ss.str());
  }
  auto typeCastedFun = (new_func_type)(mapVal.first);

  //args will be implicitly converted to match standardized args
  return typeCastedFun(std::forward<Args>(args)...);
};



private:
std::unordered_map<std::string, std::pair<void(*)(),std::type_index> > m_func_map;
};

#define FUNC_MAP_STORE(map, func) (map).store<decltype(&func),&func>(#func);

  template <typename Ret, typename... Args, Ret(*f)(Args...)>
  struct store_func_helper<Ret(*)(Args...), f> {
    static void call (func_map* map, const std::string& name) {
      using new_func_type = func_type<Ret, Args...>;

      // add a wrapper function, which takes standardized args.
      new_func_type lambda = [](typename arg<Args>::type... args) -> Ret {
        return (*f)(args...);
      };
      map->m_func_map.insert(std::make_pair(
        name, 
        std::make_pair((void(*)()) lambda, std::type_index(typeid(lambda)))
        ));
    }
  };

//examples

long add (int i, long j){
  return i + j;
}

int total_size(std::string arg1, const std::string& arg2) {
    return arg1.size() + arg2.size();
}

int  main() {
  func_map map;
  FUNC_MAP_STORE(map, total_size);
  FUNC_MAP_STORE(map, add);

  std::string arg1="hello", arg2="world";
  std::cout << "total_size: "  << map.call<int>("total_size", arg1, arg2) << std::endl;
  std::cout << "add: " << map.call<long>("add", 3, 4) << std::endl;
}