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Trying to Convert Simple Java ECS to C++

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Hi, I'm reading up on ECS on t-machine and other places, but now I'm trying to implement my own simple ECS. I'm stuck on trying to convert https://github.com/adamgit/Entity-System-RDBMS-Inspired-Java from Java to C++. In the EntityManager, he has a variable called componentStores which is defined as 

HashMap<Class, HashMap<Integer, ? extends Component>> componentStores;

However, I don't know the equivalent of this in C++.


So far, I've been able to create and generate entity ids, but I don't know how to actually add the component to the entity in a map. In the hashmap above, what types of data is it containing? Also, the second hashmap inside the hashmap, is the "? extends Component" supposed to be a pointer to a given Component? For example, I made a PositionComponent, would the hashmap contain a pointer to the PositionComponent? And, what is the point of the first data type "Class?" Sorry if this doesn't make sense, I don't fully understand what's happening, but the big question is how can you convert the hashmap into C++?


My entity header file looks like this:


#include <map>
#include <vector>
#include <cstdint>
#include <iostream>
#include <algorithm>

class EntityManager

        uint32_t create_entity();
        void kill_entity(uint32_t id);

        std::vector<uint32_t> entity_list;
        uint32_t assignable_entity_id;

        uint32_t generate_entity_id();


and this is my EntityManager source file

#include "EntityManager.h"

: assignable_entity_id(1)




uint32_t EntityManager::create_entity()
        uint32_t id = generate_entity_id();

        if (id < 1) {
                // throw error
                return 0;
        } else {
                return id;

uint32_t EntityManager::generate_entity_id()
        if (assignable_entity_id < UINT32_MAX) {
                std::cout << "Generated ID: " << assignable_entity_id << std::endl;
                return assignable_entity_id++;
        } else {
                for (unsigned int i = 1; i < UINT32_MAX; i++) {
                        std::vector<uint32_t>::iterator it = std::find(entity_list.begin(), entity_list.end(), i);
                        if (*it != i) {
                                std::cout << "Generated ID: " << i << std::endl;
                                return i;

                // throw error
                return 0;

void EntityManager::kill_entity(uint32_t id)
        std::vector<uint32_t>::iterator it = entity_list.begin();
        for (; it != entity_list.end(); it++) {
                if (*it == id) {
                        std::cout << "Killing Entity: " << *it << std::endl;

Any help would be greatly appreciated, thanks.

Edited by Krankles

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