让我坚定使用using而不是typedef进行类型声明的原因，除了因为它是新标准以外，更重要的就是using的模板声明能力。

首先考虑如下一个类中的各个回调函数的形式声明。可以看到，声明中多次使用了冗余的std::function。虽然使用的是using，但并没有体现出using的优势。

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class TcpConnectionPtr;

class TcpConnection : // noncopyable
  public std::enable_shared_from_this<TcpConnection>
{
  // kDisconnecting - LIKE POLLRDHUP, the write endian is closed
  enum StateEnum {kConnecting, kConnected, kDisconnecting, kDisconnected};
  using ConnectionCallback = std::function<void (const TcpConnectionPtr &)>;
  // using MessageCallback = std::function<void (const TcpConnectionPtr &, const std::string &)>;
  using MessageCallback = std::function<void (const TcpConnectionPtr &, muduo::net::Buffer *, muduo::Timestamp)>;
  using CloseCallback = std::function<void (const TcpConnectionPtr &)>;
  using WriteCompleteCallback = std::function<void (const TcpConnectionPtr &)>;
  using HighWaterCallback = std::function<void (const TcpConnectionPtr &)>;
// ...
}

下面是改进版本。是不是简洁多了？^-^核心原因就是using的泛化能力，减少了重复代码的编写。

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class TcpConnectionPtr;

class TcpConnection : // noncopyable
  public std::enable_shared_from_this<TcpConnection>
{
  // kDisconnecting - LIKE POLLRDHUP, the write endian is closed
  enum StateEnum {kConnecting, kConnected, kDisconnecting, kDisconnected};
  // using ConnectionCallback = std::function<void (const TcpConnectionPtr &)>;
  // // using MessageCallback = std::function<void (const TcpConnectionPtr &, const std::string &)>;
  // using MessageCallback = std::function<void (const TcpConnectionPtr &, muduo::net::Buffer *, muduo::Timestamp)>;
  // using CloseCallback = std::function<void (const TcpConnectionPtr &)>;
  // using WriteCompleteCallback = std::function<void (const TcpConnectionPtr &)>;
  // using HighWaterCallback = std::function<void (const TcpConnectionPtr &)>;
  template<typename ...T>
    using Callback = std::function<void(const TcpConnectionPtr &, T...)>;
  using ConnectionCallback = Callback<>;
  using MessageCallback = Callback<muduo::net::Buffer *, muduo::Timestamp>;
  using CloseCallback = Callback<>;
  using WriteCompleteCallback = Callback<>;
  using HighWaterCallback = Callback<>;