How can I unpack variadic template, so as to initialize respective members?

Question

I am new to variadic templates and packed arguments and all. I want to have a "entity component system" in my program, and while trying to add component to the entity, I came to realize that my attempt has a major flaw.

Anyways here is my failed attempt.

struct A{
    float x;
    float y;
    
    A(float _x, float _y):x(_x), y(_y){}
};

struct B{
    float x;
    float y;
    float z;
    
    B(float _x, float _y, float _z):x(_x), y(_y), z(_z){}
};

struct Entity{
    A* a = NULL;
    B* b = NULL;
    
    Entity(){}
    
    template<typename T, typename... args>
    void Add(args... _args){
        if(typeid(T) == typeid(A))
            a = new A(_args...);
        else if(typeid(T) == typeid(B))
            b = new B(_args...);
        else
            throw("Invalid component added");
    }
};

And implementation looks like this..

Entity ent;
ent.Add<A>(12.24f, 123.246f);
ent.Add<B>(1.f, 1.2f, 1.23f);

I want the implementation to work somehow.. what has to be changed for it??

Answer 1

You can use the following (all in compile time):

1. std::is_same to check if 2 types are the same (requires #include <type_traits>).
2. if constexpr to branch based on it (c++17 onwards).
3. static_assert if you want to get a compilation error if Add is instatiated with types other than A or B (see note about it below).

Your modified code:

#include <type_traits>

struct A {
    float x;
    float y;

    A(float _x, float _y) :x(_x), y(_y) {}
};

struct B {
    float x;
    float y;
    float z;

    B(float _x, float _y, float _z) :x(_x), y(_y), z(_z) {}
};

struct Entity {
    A* a = nullptr;
    B* b = nullptr;

    Entity() {}

    template<typename T, typename... args>
    void Add(args... _args) {
        if constexpr (std::is_same_v<T, A>)
            a = new A(_args...);
        else if constexpr (std::is_same_v<T, B>)
            b = new B(_args...);
        else
            //static_assert(!sizeof(T), "T must be A or B");  // Enable this line to get a compilation error in this case
            throw("Invalid component added");
    }
};

int main() {
    Entity ent;
    ent.Add<A>(12.24f, 123.246f);
    ent.Add<B>(1.f, 1.2f, 1.23f);
}

Notes:

1. I used the standard nullptr which is recommended instead of NULL.
2. Using a simple static_assert(false, ...) is not guaranteed to work for the reason mentioned in the article in @RemyLebeau's comment (How can I create a type-dependent expression that is always false?).
   One of the solution suggested in that article is to use static_assert(!sizeof(T), ...) assuming sizeof is never 0. The article also suggests a slightly more complicated solution that avoids this assumption.

Live demo - Godbolt