My first CXX problem:"aggregate has incomplete type and cannot be defined"

When I test to code in CPP, I meet a problem. And It's error:aggregate ‘Printer My_Printer’ has incomplete type and cannot be defined.
    After my debug I realized that there is an error in my statement in class. I statement my class in hello.cc file is fault, And when I move the statement into hello.h file the error is disappear.

hello.cc

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1. /***************************
   
2.  *filename:hello.cc
   
3. 
   
4.  *filedescription:test
   
5. 
   
6.  *writer:warrior
   
7. 
   
8.  *date:2015/4/5
   
9. 
   
10.  *mail:[e-mail]
    
11.  **************************/
    
12. 
    
13. //include
    
14. #include "hello.h"
    
15. 
    
16. 
    
17. using namespace std;
    
18. //define
    
19. 
    
20. 
    
21. //variable
    
22. 
    
23. //class
    
24. 
    
25. 
    
26. //function
    
27. 
    
28. /*********************
    
29.  *function name:void hello()
    
30. 
    
31.  *function discription:
    
32. 
    
33.  *parameter:
    
34.     input: NULL
    
35.     output: NULL
    
36.  ********************/
    
37. void Printer::Hello()
    
38. {
    
39.     char name[10];
    
40.     cout<<"Please input your name :"<<endl;
    
41.     cin>>name;
    
42.     cout<<"Hello C++!"<<endl;
    
43.     cout<<"I am "<<name<<" ."<<endl;
    
44. };

hello.h

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1. #ifndef _HELLO_H_
   
2. #define _HELLO_H_
   
3. //include
   
4. #include <iostream>
   
5. #include <sstream>
   
6. 
   
7. class Printer
   
8. {
   
9.     private:
   
10.         int Serial_Number;
    
11.     public:
    
12.         void Hello();
    
13. };
    
14. #endif

main.cc

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1. /***************************
   
2.  *filename:main.cc
   
3. 
   
4.  *filedescription:test
   
5. 
   
6.  *writer:warrior
   
7. 
   
8.  *date:2015/4/5
   
9. 
   
10.  *mail:[e-mail]
    
11.  **************************/
    
12. //include
    
13. #include "main.h"
    
14. 
    
15. using namespace std;
    
16. //define
    
17. 
    
18. 
    
19. //variable
    
20. 
    
21. 
    
22. //function
    
23. 
    
24. /*********************
    
25.  *function name:int main()
    
26. 
    
27.  *function discription:the main function
    
28. 
    
29.  *parameter:
    
30.     input: NULL
    
31.     output: NULL
    
32.  ********************/
    
33. int main()
    
34. {
    
35.     Printer My_Printer;
    
36.     My_Printer.Hello();
    
37.     return 0;
    
38. }

main.h

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1. #ifndef _MAIN_H_
   
2. #define _MAIN_H_
   
3. 
   
4. #include <stdio.h>
   
5. #include "hello.h"
   
6. #endif

makefile

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1. #makefile demo V1.0
   
2. #author: warrior
   
3. #mail:[e-mail]
   
4. #date:2015/4/5
   
5. 
   
6. CXX := g++
   
7. CFLAGS := -c -g -W -Wall
   
8. DIR := ./
   
9. 
   
10. SOURCES := $(shell ls *.cc)
    
11. OBJS := $(patsubst %.cc,%.o,$(SOURCES))
    
12. TARGET := target
    
13. 
    
14. 
    
15. ${TARGET} : ${OBJS}
    
16.     ${CXX} -o $@ $^
    
17. 
    
18. %.o : %.cc
    
19.     $(CXX) ${CFLAGS} -o $@ $<
    
20. 
    
21. 
    
22. 
    
23. 
    
24. .PHONY : clean
    
25. clean :
    
26.     -rm -f *.o target