'='：左操作数必须是l值

当我编译下面的代码时，我得到了“error C2106：'='：左操作数必须是l值”at“m.msg_body [i] .id =一世;”。当我评论这一行时，没有错误。我的代码有什么问题？'='：左操作数必须是l值

static const short MSG_DATA_MAX = 10; 

struct MsgBodyData 
{ 
    int id; 
    string value; 
}; 

class MsgBody 
{ 
public: 
    MsgBody() 
    { 
     len = 0;  
    } 
    MsgBody(MsgBody & msg_obj); 
    ~MsgBody() {} 

    int length() { return len; } 
    void setLength(int _len) { len = _len; } 

    MsgBodyData operator[](short index) 
    { 
     if(index > -1 && index < MSG_DATA_MAX) 
      return data[index]; 

     MsgBodyData dump_data; 
     return dump_data; 
    } 

    void operator=(MsgBody & msg_obj) 
    { 
     len = msg_obj.length(); 
     for(int i = 0; i < len; ++i) 
      data[i] = msg_obj[i]; 
    } 

private: 
    int len; 
    MsgBodyData data[MSG_DATA_MAX]; 
}; 

MsgBody::MsgBody(MsgBody &msg_obj) 
{ 
    operator=(msg_obj); 
} 

struct Msg 
{ 
    int msg_id; 
    string msg_title; 
    MsgBody msg_body; 
}; 


void sendMsg() 
{ 
    Msg m; 
    m.msg_id = 1; 
    m.msg_title = "test_msg"; 

    int size = MSG_DATA_MAX; 

    for(int i = 0; i < size; ++i) 
    { 
     m.msg_body[i].id = i; // HERE I GOT ERROR !!! 
     m.msg_body[i].value = "test_value"; 
    } 
}

来源

2012-08-08 Lwin Htoo Ko