Wrapping up factory methods - c++

Question

I have 2 structs whose constructors accept different numbers and types of parameters passed in.
For example,
struct A:Base
{
A(int i, char c){}
};
struct B:Base
{
B(char c){}
};
And I also have a factory method to create these 2 structs's objects,
struct F
{
Base* Do(int i)
{
if (i==0)
{
return new A(i,'c');
}
else
{
return new B('c');
}
}
};
Now that I try to improve my Factory F like this
struct F
{
template<int i, class X>
Base* Do();
template<class X>
Base* Do<0>()
{
return new X(i, 'c');
}
template<class X>
Base* Do<1>()
{
return new X('c');
}
};
The only thing I am not interested in this method is that I have to write a lot of specialized templates in case I have plenty of structs A,B,C,... but I only love templates and I don't like return new X(...).
So my questions are:
How can I minimize the long code but still use templates ?
How can I stop returning a pointer to the base class ? Dynamic allocation sux, I prefer slow and tender classic allocators on the stack for my factory.
I like to use variadic templates for the arguments of my structs' ctors.

Answer 1

std::any and variadic templates will be your friend. Yuu can store anything in std::any. The any_cast will be a bit of a problem.
See the following working code:
#include <iostream>
#include <map>
#include <utility>
#include <any>
// Some demo classes ----------------------------------------------------------------------------------
struct Base {
Base(int d) : data(d) {};
virtual ~Base() { std::cout << "Destructor Base\n"; }
virtual void print() { std::cout << "Print Base\n"; }
int data{};
};
struct Child1 : public Base {
Child1(int d, std::string s) : Base(d) { std::cout << "Constructor Child1 " << d << " " << s << "\n"; }
virtual ~Child1() { std::cout << "Destructor Child1\n"; }
virtual void print() { std::cout << "Print Child1: " << data << "\n"; }
};
struct Child2 : public Base {
Child2(int d, char c, long l) : Base(d) { std::cout << "Constructor Child2 " << d << " " << c << " " << l << "\n"; }
virtual ~Child2() { std::cout << "Destructor Child2\n"; }
virtual void print() { std::cout << "Print Child2: " << data << "\n"; }
};
struct Child3 : public Base {
Child3(int d, long l, char c, std::string s) : Base(d) { std::cout << "Constructor Child3 " << d << " " << l << " " << c << " " << s << "\n"; }
virtual ~Child3() { std::cout << "Destructor Child3\n"; }
virtual void print() { std::cout << "Print Child3: " << data << "\n"; }
};
using UPTRB = std::unique_ptr<Base>;
template <class Child, typename ...Args>
UPTRB createClass(Args...args) { return std::make_unique<Child>(args...); }
// The Factory ----------------------------------------------------------------------------------------
template <class Key, class Object>
class Factory
{
std::map<Key, std::any> selector;
public:
Factory() : selector() {}
Factory(std::initializer_list<std::pair<const Key, std::any>> il) : selector(il) {}
template<typename Function>
void add(Key key, Function&& someFunction) { selector[key] = std::any(someFunction); };
template <typename ... Args>
Object create(Key key, Args ... args) {
if (selector.find(key) != selector.end()) {
return std::any_cast<std::add_pointer_t<Object(Args ...)>>(selector[key])(args...);
}
else return nullptr;
}
};
int main()
{
Factory<int, UPTRB> factory{
{1, createClass<Child1, int, std::string>},
{2, createClass<Child2, int, char, long>}
};
factory.add(3, createClass<Child3, int, long, char, std::string>);
// Some test values
std::string s1(" Hello1 "); std::string s3(" Hello3 ");
int i = 1; const int ci = 1; int& ri = i; const int& cri = i; int&& rri = 1;
UPTRB b1 = factory.create(1, 1, s1);
UPTRB b2 = factory.create(2, 2, '2', 2L);
UPTRB b3 = factory.create(3, 3, 3L, '3', s3);
b1->print();
b2->print();
b3->print();
b1 = factory.create(2, 4, '4', 4L);
b1->print();
return 0;
}