安全C++代码pool

转自：编写安全C++代码杂谈

class noncopyable
{
 public:
  noncopyable(const noncopyable&) = delete;
  void operator=(const noncopyable&) = delete;

 protected:
  noncopyable() = default;
  ~noncopyable() = default;
};

struct connection {
    std::string id;
    //...other fields
};

template<typename _Tp , size_t N>
class object_pool final : public noncopyable {
public:
    using DeleterType = std::function<void(_Tp*)>;
    using value_u_ptr = std::unique_ptr<_Tp, DeleterType>;

    static object_pool& instance() {
        static object_pool<_Tp, N> pool;
        return pool;
    }

    value_u_ptr acquire() {
        std::lock_guard<std::mutex> lock(mtx_);
        if (empty()) {
            return nullptr;
        }

        value_u_ptr ptr(pool_.front().release(), [this](_Tp* t) {
            pool_.push_back(std::unique_ptr<_Tp>(t));
        });

        pool_.pop_front();
        return ptr;
    }

    bool empty() const {
        return pool_.empty();
    }

    size_t size() const {
        return pool_.size();
    }
private:
    object_pool() {
        static_assert(N < 500, "init size is out of range");
        for (size_t i = 0; i < N; i++) {
            pool_.push_back(std::unique_ptr<_Tp>(new _Tp));
        }
    }

private:
    std::deque<std::unique_ptr<_Tp>> pool_;
    std::mutex mtx_;
};