I was working on creating an entity-component system on my own, and in doing this I have gotten help from a lot of YouTube videos, C++ references, recommended books, etc. While I was implementing a functionality that returns IDs for specific components(which are implements as structs), I started wondering:

1. How does this double-static thing work?

2. Would there be a better, simpler way to do this?

For question #1, I am certain that without using static twice, I cannot get the function I want, based upon the experimental code below:

#include <iostream>
#include <bitset>
#include <vector>

struct Physics_c {};
struct Render_c {};
struct Collision_c {};

using componentID = std::size_t;

const inline componentID getID() noexcept {
    static componentID lastID = 0;
    return lastID++;
}

template <typename T>
const inline componentID gettypeID() noexcept{
    std::cout << "function called for " << typeid(T).name() << "n";
    componentID typeID = getID();
    return typeID;
}

template <typename T>
const inline componentID getCompID() noexcept{
    std::cout << "function called for " << typeid(T).name() << "n";
    static componentID typeID = getID();
    return typeID;
}

int main() {
    std::cout << gettypeID<Physics_c>() << "n";
    std::cout << gettypeID<Physics_c>() << "n";
    std::cout << gettypeID<Physics_c>() << "n";
    std::cout << gettypeID<Render_c>() << "n";
    std::cout << gettypeID<Render_c>() << "n";
    std::cout << gettypeID<Render_c>() << "n";
    std::cout << gettypeID<Collision_c>() << "n";
    std::cout << gettypeID<Collision_c>() << "n";
    std::cout << gettypeID<Collision_c>() << "n";

    std::cout << getCompID<Physics_c>() << "n";
    std::cout << getCompID<Physics_c>() << "n";
    std::cout << getCompID<Physics_c>() << "n";
    std::cout << getCompID<Render_c>() << "n";
    std::cout << getCompID<Render_c>() << "n";
    std::cout << getCompID<Render_c>() << "n";
    std::cout << getCompID<Collision_c>() << "n";
    std::cout << getCompID<Collision_c>() << "n";
    std::cout << getCompID<Collision_c>() << "n";

    return 0;
}

I don’t get why the value for the static side of typeID doesn’t change, even if both gettypeID and getCompID get called for every function call. I thought I knew the basics about the static keyword, but this thing has been killing me…

For question #2, I tried to create a vector that contains each typeID, but I think it isn’t any simpler than the previous code, and it is a waste of memory to make another container just to get a proper typeID. Furthermore, I have to manually input all the possible component types to the component ID vector just for the sake of getting a number for each component. Not to mention the code itself is uglier than before…

#include <typeindex>
#include <typeinfo>
#include <vector>
#include <iostream>

struct Physics_c{};
struct Render_c{};

static std::vector<std::type_index> componentVec {};

template <typename T>
void createCompVec () {
    componentVec.push_back(typeid(T));
}
int main() {
    createCompVec<Physics_c>();
    createCompVec<Render_c>();
    for (auto p : componentVec) {
        std::cout << p.name() << "n";
    }
    return 0;
}

Would there be a more efficient way to achieve what I want to do?