相关但不重复：请参阅此答案的底部，在单击此问题下方的“关闭”按钮之前，我会解决您可能想要声明的重复项。

自动生成的ROS（机器人操作系统）消息 C++ 头文件包含如下类型定义：

typedef  ::std_msgs::Header_<std::allocator<void> > Header;

std::allocator<void>这里是什么意思？为什么是模板类型void？这是什么意思？什么时候使用？

这是文档std::allocator<>：

1. https://www.cplusplus.com/reference/memory/allocator/
2. https://en.cppreference.com/w/cpp/memory/allocator

这是要查看的自动生成文件示例：http : //docs.ros.org/en/electric/api/std_msgs/html/msg__gen_2cpp_2include_2std__msgs_2Header_8h_source.html。

上面的第一行是第 116 行。

这是自动生成的 ROS 消息Header_类的开始：

template <class ContainerAllocator>
struct Header_ {

这是自动生成的Header.h 的更多上下文，typedef底部有各种s：

template <class ContainerAllocator>
struct Header_ {
  typedef Header_<ContainerAllocator> Type;

  Header_()
  : seq(0)
  , stamp()
  , frame_id()
  {
  }

  Header_(const ContainerAllocator& _alloc)
  : seq(0)
  , stamp()
  , frame_id(_alloc)
  {
  }

  typedef uint32_t _seq_type;
  uint32_t seq;

  typedef ros::Time _stamp_type;
  ros::Time stamp;

  typedef std::basic_string<char, std::char_traits<char>, typename ContainerAllocator::template rebind<char>::other >  _frame_id_type;
  std::basic_string<char, std::char_traits<char>, typename ContainerAllocator::template rebind<char>::other >  frame_id;


private:
  static const char* __s_getDataType_() { return "std_msgs/Header"; }
public:
  ROS_DEPRECATED static const std::string __s_getDataType() { return __s_getDataType_(); }

  ROS_DEPRECATED const std::string __getDataType() const { return __s_getDataType_(); }

private:
  static const char* __s_getMD5Sum_() { return "2176decaecbce78abc3b96ef049fabed"; }
public:
  ROS_DEPRECATED static const std::string __s_getMD5Sum() { return __s_getMD5Sum_(); }

  ROS_DEPRECATED const std::string __getMD5Sum() const { return __s_getMD5Sum_(); }

private:
  static const char* __s_getMessageDefinition_() { return "# Standard metadata for higher-level stamped data types.n
# This is generally used to communicate timestamped data n
# in a particular coordinate frame.n
# n
# sequence ID: consecutively increasing ID n
uint32 seqn
#Two-integer timestamp that is expressed as:n
# * stamp.secs: seconds (stamp_secs) since epochn
# * stamp.nsecs: nanoseconds since stamp_secsn
# time-handling sugar is provided by the client libraryn
time stampn
#Frame this data is associated withn
# 0: no framen
# 1: global framen
string frame_idn
n
"; }
public:
  ROS_DEPRECATED static const std::string __s_getMessageDefinition() { return __s_getMessageDefinition_(); }

  ROS_DEPRECATED const std::string __getMessageDefinition() const { return __s_getMessageDefinition_(); }

  ROS_DEPRECATED virtual uint8_t *serialize(uint8_t *write_ptr, uint32_t seq) const
  {
    ros::serialization::OStream stream(write_ptr, 1000000000);
    ros::serialization::serialize(stream, seq);
    ros::serialization::serialize(stream, stamp);
    ros::serialization::serialize(stream, frame_id);
    return stream.getData();
  }

  ROS_DEPRECATED virtual uint8_t *deserialize(uint8_t *read_ptr)
  {
    ros::serialization::IStream stream(read_ptr, 1000000000);
    ros::serialization::deserialize(stream, seq);
    ros::serialization::deserialize(stream, stamp);
    ros::serialization::deserialize(stream, frame_id);
    return stream.getData();
  }

  ROS_DEPRECATED virtual uint32_t serializationLength() const
  {
    uint32_t size = 0;
    size += ros::serialization::serializationLength(seq);
    size += ros::serialization::serializationLength(stamp);
    size += ros::serialization::serializationLength(frame_id);
    return size;
  }

  typedef boost::shared_ptr< ::std_msgs::Header_<ContainerAllocator> > Ptr;
  typedef boost::shared_ptr< ::std_msgs::Header_<ContainerAllocator>  const> ConstPtr;
  boost::shared_ptr<std::map<std::string, std::string> > __connection_header;
}; // struct Header
typedef  ::std_msgs::Header_<std::allocator<void> > Header;

typedef boost::shared_ptr< ::std_msgs::Header> HeaderPtr;
typedef boost::shared_ptr< ::std_msgs::Header const> HeaderConstPtr;

有关的：

1. [不是重复]什么是 allocator<T> - 不是重复，因为我问的T是 is的具体情况，而void不是 a 的一般情况std::allocator<>。
2. [不是重复]弃用 std::allocator<void> - 不是重复，因为我不知道为什么它在 C++20 中被弃用或更改，我问的std::allocator<void>是一般情况是什么，它是什么会，以及何时/为什么使用它。
3. https://answers.ros.org/question/212857/what-is-constptr/

回答