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

/***************************
 *filename:hello.cc

 *filedescription:test

 *writer:warrior

 *date:2015/4/5

 *mail:[e-mail]
 **************************/

//include
#include "hello.h"


using namespace std;
//define


//variable

//class


//function

/*********************
 *function name:void hello()

 *function discription:

 *parameter:
    input: NULL
    output: NULL
 ********************/
void Printer::Hello()
{
    char name[10];
    cout<<"Please input your name :"<<endl;
    cin>>name;
    cout<<"Hello C++!"<<endl;
    cout<<"I am "<<name<<" ."<<endl;
};

hello.h

#ifndef _HELLO_H_
#define _HELLO_H_
//include
#include <iostream>
#include <sstream>

class Printer
{
    private:
        int Serial_Number;
    public:
        void Hello();
};
#endif

main.cc

/***************************
 *filename:main.cc

 *filedescription:test

 *writer:warrior

 *date:2015/4/5

 *mail:[e-mail]
 **************************/
//include
#include "main.h"

using namespace std;
//define


//variable


//function

/*********************
 *function name:int main()

 *function discription:the main function

 *parameter:
    input: NULL
    output: NULL
 ********************/
int main()
{
    Printer My_Printer;
    My_Printer.Hello();
    return 0;
}

main.h

#ifndef _MAIN_H_
#define _MAIN_H_

#include <stdio.h>
#include "hello.h"
#endif

makefile

#makefile demo V1.0
#author: warrior
#mail:[e-mail]
#date:2015/4/5

CXX := g++
CFLAGS := -c -g -W -Wall
DIR := ./

SOURCES := $(shell ls *.cc)
OBJS := $(patsubst %.cc,%.o,$(SOURCES))
TARGET := target


${TARGET} : ${OBJS}
    ${CXX} -o $@ $^

%.o : %.cc
    $(CXX) ${CFLAGS} -o $@ $<




.PHONY : clean
clean :
    -rm -f *.o target