为什么老是报错啊啊啊啊啊啊啊？

main.cpp

#include <iostream>

#include "ClassStudent.h"
using namespace std;
/* run this program using the console pauser or add your own getch, system("pause") or input loop */

int main(int argc, char** argv) {
    Student a(1,'m','m');
    Student::showCount();
    return 0;
}

ClassStudent.cpp

#include "ClassStudent.h"
using namespace std;
    int Student::count=0;
    void Student::showCount()
    {
        cout<<count<<endl;
    }

ClassStudent.h

class Student{
    public:
        Student(int num=0,char name='a',char sex='f'):num(num),name(name),sex(sex){
            count++;
        }
        static void showCount();
    private:
        int num;
        char name;
        char sex;
        static int count;
};

ClassStudent.cpp里增加

#include <iostream>