输出语句的C++顺序不一致[重复]

#include <iostream>
#include <memory>
#include <cstddef>

template<typename T>
class Queue
{
    std::unique_ptr<T[]> q_ptr;
    int front_idx = -1;
    int back_idx = -1;
    int capacity = 0;
public:
    Queue(std::size_t space)
    {
        q_ptr = std::unique_ptr<T[]>(new T[space]);
        capacity = space;
    }
    void push(T value);
    void pop();
    T front() const;
    T back() const;
    std::size_t size() const;
};

template<typename T>
void Queue<T>::push(T value)
{
    if(front_idx == -1)
    {
        front_idx++;
    }
    if(back_idx - front_idx + 1 == capacity)
    {
        std::cerr << "Queue full\n";
        return;
    }
    q_ptr[++back_idx] = value;
}

template<typename T>
void Queue<T>::pop()
{
    if(front_idx == -1)
    {
        std::cerr << "Empty queue\n";
        return;
    }
    q_ptr[front_idx++] = T{};
}

template<typename T>
T Queue<T>::front() const
{
    return q_ptr[front_idx];
}

template<typename T>
T Queue<T>::back() const
{
    return q_ptr[back_idx];
}

template<typename T>
std::size_t Queue<T>::size() const
{
    if(front_idx == -1)
    {
        return 0;
    }
    else
    {
        return back_idx - front_idx + 1;
    }
}

int main()
{
    Queue<int> q1{20};
    std::cout << "Queue size initial: " << q1.size() << "\n";
    q1.pop();
    for(int i = 0; i < 23; i++)
    {
        q1.push(i);
    }
    std::cout << "Queue size: " << q1.size() << "\n";
    std::cout << "Queue front: " << q1.front() << "\n";
    std::cout << "Queue back: " << q1.back() << "\n";
    q1.pop();
    std::cout << "Queue size: " << q1.size() << "\n";
    std::cout << "Queue front: " << q1.front() << "\n";
    q1.pop();
    std::cout << "Queue size: " << q1.size() << "\n";
    std::cout << "Queue front: " << q1.front() << "\n";
    q1.push(12);
    std::cout << "Queue size: " << q1.size() << "\n";
    std::cout << "Queue back: " << q1.back() << "\n";
}

我的问题主要在于以下陈述:

std::cout << "Queue size initial: " << q1.size() << "\n";

pop() 和 for

Empty queue
Queue full
Queue full
Queue full
Queue size initial: 0
Queue size: 20
Queue front: 0
Queue back: 19
Queue size: 19
Queue front: 1
Queue size: 18
Queue front: 2
Queue size: 19
Queue back: 12

Eclipse Cpp Photon IDE 具有 minGW GCC . 请提供指导。