Variadic argument subset - c++

Question

I am trying to obtain a subset of the variadic arguments of current class wrapper to instantiate a new one
Currently I have this:
// Reference: https://stackoverflow.com/questions/27941661/generating-one-class-member-per-variadic-template-argument
// Template specialization
template<typename T, typename... Next> class VariadicClass;
// Base case extension
template <typename T>
class VariadicClass<T> {
private:
T value_;
protected:
void SetField(T & value) {
value_ = value;
}
T & GetField() {
return value_;
}
};
// Inductive case
template <typename T, typename ... Next>
class VariadicClass : public VariadicClass<T>, public VariadicClass<Next...> {
public:
// Copy the values into the variadic class
template <typename F>
void Set(F f) {
this->VariadicClass<F>::SetField(f);
}
// Retrieve by reference
template <typename F>
F & Get() {
return this->VariadicClass<F>::GetField();
}
};
And what I want to achieve is something along the following:
[C]: A subset of Args...
VariadicClass<[C]> * Filter(VariadicClass<Args...> input) {
return new VariadicClass<[C]>(GetSubsetFrom(input, [C]));
}
VariadicClass<int, bool, char> class1;
VariadicClass<int, bool> * variadic = Filter(class1);
You can assume that each type is only once in the variadic class and that I will always ask for a subset of the current variadic types. I don't know if this is currently possible in C++ 11?
Thank you for your help.

Answer 1

It seems to me that you're trying to reinvent the wheel (where "wheel", in this case, is std::tuple).
Anyway, what you ask seems simple to me
template <typename ... As1, typename ... As2>
VariadicClass<As1...> * Filter(VariadicClass<As2...> in)
{
using unused = int[];
auto ret = new VariadicClass<As1...>();
(void)unused { 0, (ret->template Set<As1>(in.template Get<As1>()), 0)... };
return ret;
}
The problem I see is that the As1... types (the types of the returned VariadicClass) aren't deducible by the returned value, so you can't write
VariadicClass<int, bool> * variadic = Filter(class1);
You have to explicit the As1... types calling Filter(), so
VariadicClass<int, bool> * variadic = Filter<int, bool>(class1);
or, maybe better,
auto variadic = Filter<int, bool>(class1);
The following is a full compiling example
#include <iostream>
template <typename, typename...>
class VariadicClass;
template <typename T>
class VariadicClass<T>
{
private:
T value_;
protected:
void SetField (T & value)
{ value_ = value; }
T & GetField ()
{ return value_; }
};
template <typename T, typename ... Next>
class VariadicClass : public VariadicClass<T>, public VariadicClass<Next...>
{
public:
template <typename F>
void Set (F f)
{ this->VariadicClass<F>::SetField(f); }
template <typename F>
F & Get()
{ return this->VariadicClass<F>::GetField(); }
};
template <typename ... As1, typename ... As2>
VariadicClass<As1...> * Filter(VariadicClass<As2...> in)
{
using unused = int[];
auto ret = new VariadicClass<As1...>();
(void)unused { 0, (ret->template Set<As1>(in.template Get<As1>()), 0)... };
return ret;
}
int main()
{
VariadicClass<int, bool, char> c1;
c1.Set<int>(42);
c1.Set<bool>(true);
c1.Set<char>('Z');
auto pC2 = Filter<int, bool>(c1);
std::cout << pC2->Get<int>() << std::endl;
std::cout << pC2->Get<bool>() << std::endl;
delete pC2;
}
Off Topic Unrequested Suggestion: you're using C++11 so... try to avoid the direct use of pointer and try to use smart pointers (std::unique_ptr, std::shared_ptr, etc.) instead.

Answer 2

First of all I think you shouldn't write your own variadic class as we already have std::tuplein place.
I wonder that you sit on c++11because it is quite old. Even c++14is outdated but if you can switch, the solution is very simple:
template < typename DATA, typename FILTER, std::size_t... Is>
auto Subset_Impl( const DATA& data, FILTER& filter, std::index_sequence<Is...> )
{
filter = { std::get< typename std::remove_reference<decltype( std::get< Is >( filter ))>::type>( data )... };
}
template < typename DATA, typename FILTER, typename IDC = std::make_index_sequence<std::tuple_size<FILTER>::value >>
auto Subset( const DATA& data, FILTER& filter )
{
return Subset_Impl( data, filter, IDC{} );
}
int main()
{
std::tuple< int, float, std::string, char > data { 1, 2.2, "Hallo", 'c' };
std::tuple< float, char > filter;
Subset( data, filter );
std::cout << std::get<0>( filter ) << " " << std::get<1>( filter ) << std::endl;
}
If you really want sit on outdated standards, you can easily implement the missing parts from the standard library your self. One related question is answered here: get part of std::tuple
How the helper templates are defined can also be seen on: https://en.cppreference.com/w/cpp/utility/integer_sequence