Simple Pool Implementation (gcc 8, C++2a)

pool.cpp

#include <thread>
#include <mutex>
#include <tuple>
#include <list>
#include <memory>

namespace pool{

namespace internal {
    template<typename T>
    struct PoolObjectWrapper{
        /*PoolObjectWrapper(bool f, T& obj):
            free(f), object(obj)
        {}*/
        
        bool free = false;
        T object;
    };

    template<typename T>
    struct Deleter
    {
        Deleter() {}

        static void deleter(internal::PoolObjectWrapper<T>* t) {
            t->free = true;
        }

        void operator () (internal::PoolObjectWrapper<T>* t) {
            deleter(t);
        }
    };
}

template<typename T>
struct PooledObjectTrait{
    static T make(){ return T(); }
};

template<typename T, typename PtrType>
class PooledObject{
public:
    PooledObject(internal::PoolObjectWrapper<T>* object):
        _ptr(object)
    {}

    PooledObject(internal::PoolObjectWrapper<T>* object, void(*deleter)(internal::PoolObjectWrapper<T>*)):
        _ptr(object, deleter)
    {}

    inline T& operator* ()  {   return (*_ptr).object;             }
    inline T* operator->()  {   return _ptr->object; }
    inline operator bool() const     {   return _ptr;              }

private:
    PtrType _ptr;
};

template<typename T>
using UniquePooledObject = PooledObject<T, std::unique_ptr<internal::PoolObjectWrapper<T>, internal::Deleter<T>>>;

template<typename T>
using SharedPooledObject = PooledObject<T, std::shared_ptr<internal::PoolObjectWrapper<T>>>;

template<typename T>
class DynamicPool{
    public:
        DynamicPool(std::size_t initial_size){
            //_pool.reserve(initial_size);

            for(std::size_t i = 0; i < initial_size; ++i){
                _pool.push_back({true, PooledObjectTrait<T>::make()});
            }
        }

        UniquePooledObject<T> get_unique(){
            return UniquePooledObject<T>(get_free_or_make());
        }

        SharedPooledObject<T> get_shared(){
            void(*deleter)(internal::PoolObjectWrapper<T>*) = &internal::Deleter<T>::deleter;
            return SharedPooledObject<T>(get_free_or_make(), deleter);
        }

        std::size_t size() const  { return _pool.size(); }

    private:
        internal::PoolObjectWrapper<T>* get_free_or_make(){
            std::lock_guard<std::mutex> lock(_mutex);
            
            for(auto& elem: _pool){
                if (elem.free){
                    elem.free = false;
                    return &elem;
                }
            }
            
            _pool.push_back({false, PooledObjectTrait<T>::make()});
            return &_pool.back();
        }

    private:
        std::list<internal::PoolObjectWrapper<T>> _pool;
        std::mutex _mutex;
};

}

#include <iostream>

int main(){

    pool::DynamicPool<int> pool(20);

    {
        auto a = pool.get_unique();
        auto b = pool.get_shared();
        (*a) = 10;
        (*b) = 20;
    }

    {
        auto a = pool.get_unique();
        auto b = pool.get_shared();

        std::cout << (*a)  << std::endl;
        std::cout << (*b)  << std::endl;
    }
    
    auto a = pool.get_shared();
    std::cout << (*a)  << std::endl;
    
    
}