You can get pretty far with variadic templates and some template/virtual techniques. With the following codes, you'll be able to do something like:
std::string select_string (bool cond, std::string a, std::string b) {
return cond ? a : b;
}
int main () {
Registry reg;
reg.set ("select_it", select_string);
reg.invoke ("select_it", "1 John Wayne"));
reg.invoke ("select_it", "0 John Wayne"));
}
output:
John
Wayne
Full implementation:
These codes are exemplary. You should optimize it to provide perfect forwarding less redundancy in parameter list expansion.
Headers and a test-function
#include <functional>
#include <string>
#include <sstream>
#include <istream>
#include <iostream>
#include <tuple>
std::string select_string (bool cond, std::string a, std::string b) {
return cond ? a : b;
}
This helps us parsing a string and putting results into a tuple:
//----------------------------------------------------------------------------------
template <typename Tuple, int Curr, int Max> struct init_args_helper;
template <typename Tuple, int Max>
struct init_args_helper<Tuple, Max, Max> {
void operator() (Tuple &, std::istream &) {}
};
template <typename Tuple, int Curr, int Max>
struct init_args_helper {
void operator() (Tuple &tup, std::istream &is) {
is >> std::get<Curr>(tup);
return init_args_helper<Tuple, Curr+1, Max>() (tup, is);
}
};
template <int Max, typename Tuple>
void init_args (Tuple &tup, std::istream &ss)
{
init_args_helper<Tuple, 0, Max>() (tup, ss);
}
This unfolds a function pointer and a tuple into a function call (by function-pointer):
//----------------------------------------------------------------------------------
template <int ParamIndex, int Max, typename Ret, typename ...Args>
struct unfold_helper;
template <int Max, typename Ret, typename ...Args>
struct unfold_helper<Max, Max, Ret, Args...> {
template <typename Tuple, typename ...Params>
Ret unfold (Ret (*fun) (Args...), Tuple tup, Params ...params)
{
return fun (params...);
}
};
template <int ParamIndex, int Max, typename Ret, typename ...Args>
struct unfold_helper {
template <typename Tuple, typename ...Params>
Ret unfold (Ret (*fun) (Args...), Tuple tup, Params ...params)
{
return unfold_helper<ParamIndex+1, Max, Ret, Args...> ().
unfold(fun, tup, params..., std::get<ParamIndex>(tup));
}
};
template <typename Ret, typename ...Args>
Ret unfold (Ret (*fun) (Args...), std::tuple<Args...> tup) {
return unfold_helper<0, sizeof...(Args), Ret, Args...> ().unfold(fun, tup);
}
This function puts it together:
//----------------------------------------------------------------------------------
template <typename Ret, typename ...Args>
Ret foo (Ret (*fun) (Args...), std::string mayhem) {
// Use a stringstream for trivial parsing.
std::istringstream ss;
ss.str (mayhem);
// Use a tuple to store our parameters somewhere.
// We could later get some more performance by combining the parsing
// and the calling.
std::tuple<Args...> params;
init_args<sizeof...(Args)> (params, ss);
// This demondstrates expanding the tuple to full parameter lists.
return unfold<Ret> (fun, params);
}
Here's our test:
int main () {
std::cout << foo (select_string, "0 John Wayne") << '\n';
std::cout << foo (select_string, "1 John Wayne") << '\n';
}
Warning: Code needs more verification upon parsing and should use std::function<> instead of naked function pointer
Based on above code, it is simple to write a function-registry:
class FunMeta {
public:
virtual ~FunMeta () {}
virtual boost::any call (std::string args) const = 0;
};
template <typename Ret, typename ...Args>
class ConcreteFunMeta : public FunMeta {
public:
ConcreteFunMeta (Ret (*fun) (Args...)) : fun(fun) {}
boost::any call (std::string args) const {
// Use a stringstream for trivial parsing.
std::istringstream ss;
ss.str (args);
// Use a tuple to store our parameters somewhere.
// We could later get some more performance by combining the parsing
// and the calling.
std::tuple<Args...> params;
init_args<sizeof...(Args)> (params, ss);
// This demondstrates expanding the tuple to full parameter lists.
return unfold<Ret> (fun, params);
}
private:
Ret (*fun) (Args...);
};
class Registry {
public:
template <typename Ret, typename ...Args>
void set (std::string name, Ret (*fun) (Args...)) {
funs[name].reset (new ConcreteFunMeta<Ret, Args...> (fun));
}
boost::any invoke (std::string name, std::string args) const {
const auto it = funs.find (name);
if (it == funs.end())
throw std::runtime_error ("meh");
return it->second->call (args);
}
private:
// You could use a multimap to support function overloading.
std::map<std::string, std::shared_ptr<FunMeta>> funs;
};
One could even think of supporting function overloading with this, using a multimap and dispatching decisions based on what content is on the passed arguments.
Here's how to use it:
int main () {
Registry reg;
reg.set ("select_it", select_string);
std::cout << boost::any_cast<std::string> (reg.invoke ("select_it", "0 John Wayne")) << '\n'
<< boost::any_cast<std::string> (reg.invoke ("select_it", "1 John Wayne")) << '\n';
}
