Lock Free Queue

#include<atomic>
#include<utility>
#include<stdexcept>
#include<thread>
#include<iostream>

#include<atomic>
#include<utility>
#include<stdexcept>
#include<thread>
#include<iostream>
#include<vector>

template <typename T, size_t max_size = 20>
class lf_queue{
    T queue[max_size];
    std::atomic<size_t> _size;
    const size_t _maxs;
    size_t _read_pos;
    size_t _write_pos;
public:
    //constructor
    lf_queue()
        : _maxs(max_size), _size(0), _read_pos(0), _write_pos(0)
    {}

    void push( const T& item ) {
       if (_size.load() == _maxs) {
           std::cout<<"queue is full" <<std::endl;
           //throw exception
        }
       queue[_write_pos]= item;
       _write_pos = (_write_pos+1) % _maxs;
       _size.fetch_add(1);
    }

    T& front() {
        if(_size.load()>0) {
           return  queue[_read_pos];
        }
    }

    void pop() {
        if(_size.load()>0) {
           _read_pos = (_read_pos + 1) % _maxs;
           _size.fetch_sub(1);
        }
    }

    size_t size() {
        return _size.load();
    }
    
    bool full() {
        return _size.load() == _maxs;
    }
    
    bool empty() {
        return _size.load() == 0;    
    }

};
#endif

//main
int main(){
    const int sz = 100;
    lf_queue<int,sz> q;
    std::vector<int> res;
    auto reader = [&res,&q](){
            for(int i=0; i<sz;){
                if( !q.empty() ){
                    auto k = q.front();
                    q.pop();
                    res.push_back(k);
                    std::cout<< "front :" << k << std::endl;
                    ++i;
                }
            }
        };
    auto writer = [&q](){
            for(int i=0; i< sz;) {
                if( !q.full() ) {
                    q.push(i);
                    std::cout<< "push: " << i << std::endl;
                    ++i;
                }
            }
        };

    auto t1 = std::thread(writer);
    auto t2 = std::thread(reader);
    t1.join();
    t2.join();
    std::cout<<"queue size " << q.size()<<std::endl;
    if( !q.empty() ) {
        std::cout<<"queue front: " << q.front()<< std::endl;   
    }      
    
    std::cout<<"All elements:"<<std::endl;
    for(auto& it : res) {
        std::cout<< it <<std::endl;    
    }
}