Container for different functions? - c++

Question

I'm trying to implement a container class for different functions where I can hold function pointers and use it to call those functions later. I'll try to discribe my problem more accurate.
As example, I have 2 different test functions:
int func1(int a, int b) {
printf("func1 works! %i %i\n", a, b);
return 0;
}
void func2(double a, double b) {
printf("func2 works! %.2lf %.2lf\n", a, b);
}
and I also have array of variants, which holds function arguments:
std::vector<boost::variant<int, double>> args = {2.2, 3.3};
I've decided to use my own functor class derived from some base class ( I thought about using virtual methods):
class BaseFunc {
public:
BaseFunc() {}
~BaseFunc() {}
};
template <typename T>
class Func;
template <typename R, typename... Tn>
class Func<R(Tn...)> : public BaseFunc {
typedef R(*fptr_t)(Tn...);
fptr_t fptr;
public:
Func() : fptr(nullptr) {}
Func(fptr_t f) : fptr(f) {}
R operator()(Tn... args) {
return fptr(args...);
}
Func& operator=(fptr_t f) {
fptr = f;
return *this;
}
};
Also I've decided to store some information about function and its arguments:
struct TypeInfo {
int type_id; // for this example: 0 - int, 1 - double
template <class T>
void ObtainType() {
if (std::is_same<void, T>::value)
type_id = 0;
else if (std::is_same<int, T>::value)
type_id = 1;
else if (std::is_same<double, T>::value)
type_id = 2;
else
type_id = -1;
}
};
struct FunctionInfo {
public:
FunctionInfo() {}
FunctionInfo(BaseFunc *func, const TypeInfo& ret, std::vector<TypeInfo>& args) :
func_ptr(func), return_info(ret)
{
args_info.swap(args);
}
~FunctionInfo() {
delete func_ptr;
}
BaseFunc * func_ptr;
TypeInfo return_info;
std::vector<TypeInfo> args_info;
};
So now I can define a container class:
class Container {
private:
template <size_t n, typename... T>
void ObtainTypeImpl(size_t i, TypeInfo& t)
{
if (i == n)
t.ObtainType<std::tuple_element<n, std::tuple<T...>>::type>();
else if (n == sizeof...(T)-1)
throw std::out_of_range("Tuple element out of range.");
else
ObtainTypeImpl<(n < sizeof...(T)-1 ? n + 1 : 0), T...>(i, t);
}
template <typename... T>
void ObtainType(size_t i, TypeInfo& t)
{
return ObtainTypeImpl<0, T...>(i, t);
}
public:
template <class R, class ...Args>
void AddFunc(const std::string& str, R(*func)(Args...)) {
BaseFunc * func_ptr = new Func<R(Args...)>(func);
size_t arity = sizeof...(Args);
TypeInfo ret;
ret.ObtainType<R>();
std::vector<TypeInfo> args;
args.resize(arity);
for (size_t i = 0; i < arity; ++i)
{
ObtainType<Args...>(i, args[i]);
}
cont_[str] = FunctionInfo(func_ptr, ret, args);
}
void CallFunc(const std::string& func_name,
std::vector<boost::variant<int, double>>& args_vec) {
auto it = cont_.find(func_name);
if (it != cont_.end())
{
// ???????
// And here I stucked
}
}
private:
std::map<std::string, FunctionInfo> cont_;
};
And then I stucked.
Don't know how to get function type information from my struct :).
Don't know how to convert vector of variants to arguments list.
Maybe my path was wrong? Can you suggest any solution of this problem except script engine like Lua?

Answer 1

You may do something like:
class BaseFunc {
public:
virtual ~BaseFunc() = default;
virtual void Call(std::vector<boost::variant<int, double>>& args_vec) const = 0;
};
template <typename F> class Function;
template <typename R, typename... Args> class Function<R(Args...)> : public BaseFunc
{
public:
Function(R (*f)(Args...)) : f(f) {}
void Call(std::vector<boost::variant<int, double>>& args_vec) const override
{
Call(args_vec, std::index_sequence_for<Args...>());
}
private:
template <std::size_t ... Is>
void Call(
std::vector<boost::variant<int, double>>& args_vec,
std::index_sequence<Is...>) const
{
// Add additional check here if you want.
f(boost::get<Args>(args_vec.at(Is))...);
}
private:
R (*f)(Args...);
};
Live example