我现在知道使用ROS话题消息的功能包的命名空间是在哪里定义的了

确实是会基于msg文件生成对应头文件，打开头文件，一切都明了了。

比如这是DroneState.msg对应生成的头文件DroneState.h，是在devel文件夹下面，是编译生成的。这里一开头就明确定义了工作空间namespace，以后再看到prometheus_msgs::DroneSate就不会有疑问了，我一直对这个细节抓着不放彻底弄明白弄清楚是对的，是可以真正完全弄清楚的。看来msg文件可以写那么简单完全是为了方便我们啊，实际的头文件内容还是非常多的。

// Generated by gencpp from file prometheus_msgs/DroneState.msg
// DO NOT EDIT!#ifndef PROMETHEUS_MSGS_MESSAGE_DRONESTATE_H
#define PROMETHEUS_MSGS_MESSAGE_DRONESTATE_H#include <string>
#include <vector>
#include <map>#include <ros/types.h>
#include <ros/serialization.h>
#include <ros/builtin_message_traits.h>
#include <ros/message_operations.h>#include <std_msgs/Header.h>
#include <geometry_msgs/Quaternion.h>namespace prometheus_msgs
{
template <class ContainerAllocator>
struct DroneState_
{typedef DroneState_<ContainerAllocator> Type;DroneState_(): header(), connected(false), armed(false), landed(false), mode(), time_from_start(0.0), position(), velocity(), attitude(), attitude_q(), attitude_rate()  {position.assign(0.0);velocity.assign(0.0);attitude.assign(0.0);attitude_rate.assign(0.0);}DroneState_(const ContainerAllocator& _alloc): header(_alloc), connected(false), armed(false), landed(false), mode(_alloc), time_from_start(0.0), position(), velocity(), attitude(), attitude_q(_alloc), attitude_rate()  {(void)_alloc;position.assign(0.0);velocity.assign(0.0);attitude.assign(0.0);attitude_rate.assign(0.0);}typedef  ::std_msgs::Header_<ContainerAllocator>  _header_type;_header_type header;typedef uint8_t _connected_type;_connected_type connected;typedef uint8_t _armed_type;_armed_type armed;typedef uint8_t _landed_type;_landed_type landed;typedef std::basic_string<char, std::char_traits<char>, typename ContainerAllocator::template rebind<char>::other >  _mode_type;_mode_type mode;typedef float _time_from_start_type;_time_from_start_type time_from_start;typedef boost::array<float, 3>  _position_type;_position_type position;typedef boost::array<float, 3>  _velocity_type;_velocity_type velocity;typedef boost::array<float, 3>  _attitude_type;_attitude_type attitude;typedef  ::geometry_msgs::Quaternion_<ContainerAllocator>  _attitude_q_type;_attitude_q_type attitude_q;typedef boost::array<float, 3>  _attitude_rate_type;_attitude_rate_type attitude_rate;typedef boost::shared_ptr< ::prometheus_msgs::DroneState_<ContainerAllocator> > Ptr;typedef boost::shared_ptr< ::prometheus_msgs::DroneState_<ContainerAllocator> const> ConstPtr;}; // struct DroneState_typedef ::prometheus_msgs::DroneState_<std::allocator<void> > DroneState;typedef boost::shared_ptr< ::prometheus_msgs::DroneState > DroneStatePtr;
typedef boost::shared_ptr< ::prometheus_msgs::DroneState const> DroneStateConstPtr;// constants requiring out of line definitiontemplate<typename ContainerAllocator>
std::ostream& operator<<(std::ostream& s, const ::prometheus_msgs::DroneState_<ContainerAllocator> & v)
{
ros::message_operations::Printer< ::prometheus_msgs::DroneState_<ContainerAllocator> >::stream(s, "", v);
return s;
}template<typename ContainerAllocator1, typename ContainerAllocator2>
bool operator==(const ::prometheus_msgs::DroneState_<ContainerAllocator1> & lhs, const ::prometheus_msgs::DroneState_<ContainerAllocator2> & rhs)
{return lhs.header == rhs.header &&lhs.connected == rhs.connected &&lhs.armed == rhs.armed &&lhs.landed == rhs.landed &&lhs.mode == rhs.mode &&lhs.time_from_start == rhs.time_from_start &&lhs.position == rhs.position &&lhs.velocity == rhs.velocity &&lhs.attitude == rhs.attitude &&lhs.attitude_q == rhs.attitude_q &&lhs.attitude_rate == rhs.attitude_rate;
}template<typename ContainerAllocator1, typename ContainerAllocator2>
bool operator!=(const ::prometheus_msgs::DroneState_<ContainerAllocator1> & lhs, const ::prometheus_msgs::DroneState_<ContainerAllocator2> & rhs)
{return !(lhs == rhs);
}} // namespace prometheus_msgsnamespace ros
{
namespace message_traits
{template <class ContainerAllocator>
struct IsFixedSize< ::prometheus_msgs::DroneState_<ContainerAllocator> >: FalseType{ };template <class ContainerAllocator>
struct IsFixedSize< ::prometheus_msgs::DroneState_<ContainerAllocator> const>: FalseType{ };template <class ContainerAllocator>
struct IsMessage< ::prometheus_msgs::DroneState_<ContainerAllocator> >: TrueType{ };template <class ContainerAllocator>
struct IsMessage< ::prometheus_msgs::DroneState_<ContainerAllocator> const>: TrueType{ };template <class ContainerAllocator>
struct HasHeader< ::prometheus_msgs::DroneState_<ContainerAllocator> >: TrueType{ };template <class ContainerAllocator>
struct HasHeader< ::prometheus_msgs::DroneState_<ContainerAllocator> const>: TrueType{ };template<class ContainerAllocator>
struct MD5Sum< ::prometheus_msgs::DroneState_<ContainerAllocator> >
{static const char* value(){return "2e58976f7e6bb6fb3559cb004475bce0";}static const char* value(const ::prometheus_msgs::DroneState_<ContainerAllocator>&) { return value(); }static const uint64_t static_value1 = 0x2e58976f7e6bb6fbULL;static const uint64_t static_value2 = 0x3559cb004475bce0ULL;
};template<class ContainerAllocator>
struct DataType< ::prometheus_msgs::DroneState_<ContainerAllocator> >
{static const char* value(){return "prometheus_msgs/DroneState";}static const char* value(const ::prometheus_msgs::DroneState_<ContainerAllocator>&) { return value(); }
};template<class ContainerAllocator>
struct Definition< ::prometheus_msgs::DroneState_<ContainerAllocator> >
{static const char* value(){return "std_msgs/Header header\n"
"\n"
"## 机载电脑是否连接上飞控，true已连接，false则不是\n"
"bool connected\n"
"## 是否解锁，true为已解锁，false则不是\n"
"bool armed\n"
"## 是否降落，true为已降落，false则代表在空中\n"
"bool landed\n"
"## PX4飞控当前飞行模式\n"
"string mode\n"
"\n"
"## 系统启动时间\n"
"float32 time_from_start             ## [s]\n"
"\n"
"## 无人机状态量：位置、速度、姿态\n"
"float32[3] position                 ## [m]\n"
"float32[3] velocity                 ## [m/s]\n"
"float32[3] attitude                 ## [rad]\n"
"geometry_msgs/Quaternion attitude_q ## 四元数\n"
"float32[3] attitude_rate            ## [rad/s]\n"
"================================================================================\n"
"MSG: std_msgs/Header\n"
"# 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 seq\n"
"#Two-integer timestamp that is expressed as:\n"
"# * stamp.sec: seconds (stamp_secs) since epoch (in Python the variable is called 'secs')\n"
"# * stamp.nsec: nanoseconds since stamp_secs (in Python the variable is called 'nsecs')\n"
"# time-handling sugar is provided by the client library\n"
"time stamp\n"
"#Frame this data is associated with\n"
"string frame_id\n"
"\n"
"================================================================================\n"
"MSG: geometry_msgs/Quaternion\n"
"# This represents an orientation in free space in quaternion form.\n"
"\n"
"float64 x\n"
"float64 y\n"
"float64 z\n"
"float64 w\n"
;}static const char* value(const ::prometheus_msgs::DroneState_<ContainerAllocator>&) { return value(); }
};} // namespace message_traits
} // namespace rosnamespace ros
{
namespace serialization
{template<class ContainerAllocator> struct Serializer< ::prometheus_msgs::DroneState_<ContainerAllocator> >{template<typename Stream, typename T> inline static void allInOne(Stream& stream, T m){stream.next(m.header);stream.next(m.connected);stream.next(m.armed);stream.next(m.landed);stream.next(m.mode);stream.next(m.time_from_start);stream.next(m.position);stream.next(m.velocity);stream.next(m.attitude);stream.next(m.attitude_q);stream.next(m.attitude_rate);}ROS_DECLARE_ALLINONE_SERIALIZER}; // struct DroneState_} // namespace serialization
} // namespace rosnamespace ros
{
namespace message_operations
{template<class ContainerAllocator>
struct Printer< ::prometheus_msgs::DroneState_<ContainerAllocator> >
{template<typename Stream> static void stream(Stream& s, const std::string& indent, const ::prometheus_msgs::DroneState_<ContainerAllocator>& v){s << indent << "header: ";s << std::endl;Printer< ::std_msgs::Header_<ContainerAllocator> >::stream(s, indent + "  ", v.header);s << indent << "connected: ";Printer<uint8_t>::stream(s, indent + "  ", v.connected);s << indent << "armed: ";Printer<uint8_t>::stream(s, indent + "  ", v.armed);s << indent << "landed: ";Printer<uint8_t>::stream(s, indent + "  ", v.landed);s << indent << "mode: ";Printer<std::basic_string<char, std::char_traits<char>, typename ContainerAllocator::template rebind<char>::other > >::stream(s, indent + "  ", v.mode);s << indent << "time_from_start: ";Printer<float>::stream(s, indent + "  ", v.time_from_start);s << indent << "position[]" << std::endl;for (size_t i = 0; i < v.position.size(); ++i){s << indent << "  position[" << i << "]: ";Printer<float>::stream(s, indent + "  ", v.position[i]);}s << indent << "velocity[]" << std::endl;for (size_t i = 0; i < v.velocity.size(); ++i){s << indent << "  velocity[" << i << "]: ";Printer<float>::stream(s, indent + "  ", v.velocity[i]);}s << indent << "attitude[]" << std::endl;for (size_t i = 0; i < v.attitude.size(); ++i){s << indent << "  attitude[" << i << "]: ";Printer<float>::stream(s, indent + "  ", v.attitude[i]);}s << indent << "attitude_q: ";s << std::endl;Printer< ::geometry_msgs::Quaternion_<ContainerAllocator> >::stream(s, indent + "  ", v.attitude_q);s << indent << "attitude_rate[]" << std::endl;for (size_t i = 0; i < v.attitude_rate.size(); ++i){s << indent << "  attitude_rate[" << i << "]: ";Printer<float>::stream(s, indent + "  ", v.attitude_rate[i]);}}
};} // namespace message_operations
} // namespace ros#endif // PROMETHEUS_MSGS_MESSAGE_DRONESTATE_H

我任打开一个其他的话题消息的头文件，每个都写了namespace prometheus_msgs

我似乎也搜到了对应的头文件

http://docs.ros.org/en/diamondback/api/geometry_msgs/html/Pose_8h_source.html

00001 /* Auto-generated by genmsg_cpp for file /tmp/buildd/ros-diamondback-common-msgs-1.4.0/debian/ros-diamondback-common-msgs/opt/ros/diamondback/stacks/common_msgs/geometry_msgs/msg/Pose.msg */
00002 #ifndef GEOMETRY_MSGS_MESSAGE_POSE_H
00003 #define GEOMETRY_MSGS_MESSAGE_POSE_H
00004 #include <string>
00005 #include <vector>
00006 #include <ostream>
00007 #include "ros/serialization.h"
00008 #include "ros/builtin_message_traits.h"
00009 #include "ros/message_operations.h"
00010 #include "ros/message.h"
00011 #include "ros/time.h"
00012
00013 #include "geometry_msgs/Point.h"
00014 #include "geometry_msgs/Quaternion.h"
00015
00016 namespace geometry_msgs
00017 {
00018 template <class ContainerAllocator>
00019 struct Pose_ : public ros::Message
00020 {
00021   typedef Pose_<ContainerAllocator> Type;
00022
00023   Pose_()
00024   : position()
00025   , orientation()
00026   {
00027   }
00028
00029   Pose_(const ContainerAllocator& _alloc)
00030   : position(_alloc)
00031   , orientation(_alloc)
00032   {
00033   }
00034
00035   typedef  ::geometry_msgs::Point_<ContainerAllocator>  _position_type;
00036    ::geometry_msgs::Point_<ContainerAllocator>  position;
00037
00038   typedef  ::geometry_msgs::Quaternion_<ContainerAllocator>  _orientation_type;
00039    ::geometry_msgs::Quaternion_<ContainerAllocator>  orientation;
00040
00041
00042 private:
00043   static const char* __s_getDataType_() { return "geometry_msgs/Pose"; }
00044 public:
00045   ROS_DEPRECATED static const std::string __s_getDataType() { return __s_getDataType_(); }
00046
00047   ROS_DEPRECATED const std::string __getDataType() const { return __s_getDataType_(); }
00048
00049 private:
00050   static const char* __s_getMD5Sum_() { return "e45d45a5a1ce597b249e23fb30fc871f"; }
00051 public:
00052   ROS_DEPRECATED static const std::string __s_getMD5Sum() { return __s_getMD5Sum_(); }
00053
00054   ROS_DEPRECATED const std::string __getMD5Sum() const { return __s_getMD5Sum_(); }
00055
00056 private:
00057   static const char* __s_getMessageDefinition_() { return "# A representation of pose in free space, composed of postion and orientation. \n\
00058 Point position\n\
00059 Quaternion orientation\n\
00060 \n\
00061 ================================================================================\n\
00062 MSG: geometry_msgs/Point\n\
00063 # This contains the position of a point in free space\n\
00064 float64 x\n\
00065 float64 y\n\
00066 float64 z\n\
00067 \n\
00068 ================================================================================\n\
00069 MSG: geometry_msgs/Quaternion\n\
00070 # This represents an orientation in free space in quaternion form.\n\
00071 \n\
00072 float64 x\n\
00073 float64 y\n\
00074 float64 z\n\
00075 float64 w\n\
00076 \n\
00077 "; }
00078 public:
00079   ROS_DEPRECATED static const std::string __s_getMessageDefinition() { return __s_getMessageDefinition_(); }
00080
00081   ROS_DEPRECATED const std::string __getMessageDefinition() const { return __s_getMessageDefinition_(); }
00082
00083   ROS_DEPRECATED virtual uint8_t *serialize(uint8_t *write_ptr, uint32_t seq) const
00084   {
00085     ros::serialization::OStream stream(write_ptr, 1000000000);
00086     ros::serialization::serialize(stream, position);
00087     ros::serialization::serialize(stream, orientation);
00088     return stream.getData();
00089   }
00090
00091   ROS_DEPRECATED virtual uint8_t *deserialize(uint8_t *read_ptr)
00092   {
00093     ros::serialization::IStream stream(read_ptr, 1000000000);
00094     ros::serialization::deserialize(stream, position);
00095     ros::serialization::deserialize(stream, orientation);
00096     return stream.getData();
00097   }
00098
00099   ROS_DEPRECATED virtual uint32_t serializationLength() const
00100   {
00101     uint32_t size = 0;
00102     size += ros::serialization::serializationLength(position);
00103     size += ros::serialization::serializationLength(orientation);
00104     return size;
00105   }
00106
00107   typedef boost::shared_ptr< ::geometry_msgs::Pose_<ContainerAllocator> > Ptr;
00108   typedef boost::shared_ptr< ::geometry_msgs::Pose_<ContainerAllocator>  const> ConstPtr;
00109 }; // struct Pose
00110 typedef  ::geometry_msgs::Pose_<std::allocator<void> > Pose;
00111
00112 typedef boost::shared_ptr< ::geometry_msgs::Pose> PosePtr;
00113 typedef boost::shared_ptr< ::geometry_msgs::Pose const> PoseConstPtr;
00114
00115
00116 template<typename ContainerAllocator>
00117 std::ostream& operator<<(std::ostream& s, const  ::geometry_msgs::Pose_<ContainerAllocator> & v)
00118 {
00119   ros::message_operations::Printer< ::geometry_msgs::Pose_<ContainerAllocator> >::stream(s, "", v);
00120   return s;}
00121
00122 } // namespace geometry_msgs
00123
00124 namespace ros
00125 {
00126 namespace message_traits
00127 {
00128 template<class ContainerAllocator>
00129 struct MD5Sum< ::geometry_msgs::Pose_<ContainerAllocator> > {
00130   static const char* value()
00131   {
00132     return "e45d45a5a1ce597b249e23fb30fc871f";
00133   }
00134
00135   static const char* value(const  ::geometry_msgs::Pose_<ContainerAllocator> &) { return value(); }
00136   static const uint64_t static_value1 = 0xe45d45a5a1ce597bULL;
00137   static const uint64_t static_value2 = 0x249e23fb30fc871fULL;
00138 };
00139
00140 template<class ContainerAllocator>
00141 struct DataType< ::geometry_msgs::Pose_<ContainerAllocator> > {
00142   static const char* value()
00143   {
00144     return "geometry_msgs/Pose";
00145   }
00146
00147   static const char* value(const  ::geometry_msgs::Pose_<ContainerAllocator> &) { return value(); }
00148 };
00149
00150 template<class ContainerAllocator>
00151 struct Definition< ::geometry_msgs::Pose_<ContainerAllocator> > {
00152   static const char* value()
00153   {
00154     return "# A representation of pose in free space, composed of postion and orientation. \n\
00155 Point position\n\
00156 Quaternion orientation\n\
00157 \n\
00158 ================================================================================\n\
00159 MSG: geometry_msgs/Point\n\
00160 # This contains the position of a point in free space\n\
00161 float64 x\n\
00162 float64 y\n\
00163 float64 z\n\
00164 \n\
00165 ================================================================================\n\
00166 MSG: geometry_msgs/Quaternion\n\
00167 # This represents an orientation in free space in quaternion form.\n\
00168 \n\
00169 float64 x\n\
00170 float64 y\n\
00171 float64 z\n\
00172 float64 w\n\
00173 \n\
00174 ";
00175   }
00176
00177   static const char* value(const  ::geometry_msgs::Pose_<ContainerAllocator> &) { return value(); }
00178 };
00179
00180 template<class ContainerAllocator> struct IsFixedSize< ::geometry_msgs::Pose_<ContainerAllocator> > : public TrueType {};
00181 } // namespace message_traits
00182 } // namespace ros
00183
00184 namespace ros
00185 {
00186 namespace serialization
00187 {
00188
00189 template<class ContainerAllocator> struct Serializer< ::geometry_msgs::Pose_<ContainerAllocator> >
00190 {
00191   template<typename Stream, typename T> inline static void allInOne(Stream& stream, T m)
00192   {
00193     stream.next(m.position);
00194     stream.next(m.orientation);
00195   }
00196
00197   ROS_DECLARE_ALLINONE_SERIALIZER;
00198 }; // struct Pose_
00199 } // namespace serialization
00200 } // namespace ros
00201
00202 namespace ros
00203 {
00204 namespace message_operations
00205 {
00206
00207 template<class ContainerAllocator>
00208 struct Printer< ::geometry_msgs::Pose_<ContainerAllocator> >
00209 {
00210   template<typename Stream> static void stream(Stream& s, const std::string& indent, const  ::geometry_msgs::Pose_<ContainerAllocator> & v)
00211   {
00212     s << indent << "position: ";
00213 s << std::endl;
00214     Printer< ::geometry_msgs::Point_<ContainerAllocator> >::stream(s, indent + "  ", v.position);
00215     s << indent << "orientation: ";
00216 s << std::endl;
00217     Printer< ::geometry_msgs::Quaternion_<ContainerAllocator> >::stream(s, indent + "  ", v.orientation);
00218   }
00219 };
00220
00221
00222 } // namespace message_operations
00223 } // namespace ros
00224
00225 #endif // GEOMETRY_MSGS_MESSAGE_POSE_H
00226

细看一下它类里面其实就是写个结构体，所以可以像下面这么写

geometry_msgs::Pose pose

pose.position.x

pose是对象名,position可能是结构体，感觉似乎又不对

讲道理一个消息就是一种类，你包含了其他的消息就是包含了其他的类啊，

所以你看pose这个消息的头文件里面是include了的poin和quaternion这两个消息的头文件的

上面写的struct似乎也是定义类的。

这应该就是一个函数。这应该是多重继承的写法

http://m.biancheng.net/view/2277.html

这也应该是继承的写法。

我想起java是不能多重继承的，得用接口，是不是C++可以多重继承。

而且我刚刚发现上面正是多重继承下的构造函数！！！！！Pose_()正是构造函数！！！！！

http://m.biancheng.net/view/2277.html

所以ROS里面一个消息类型是由其他小消息类型组合而成的，在C++层面上是通过多重继承实现的。

http://docs.ros.org/en/diamondback/api/geometry_msgs/html/PoseStamped_8h.html

从这里也可以看出 geometry_msgs 确实就是个功能包，我理解得没有错，话题消息的定义都是写在一个专门的功能包里面的

http://wiki.ros.org/geometry_msgs

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