错误：ISO C++禁止在类内初始化非const的静态成员。

这是头文件：employee.h

#ifndef EMPLOYEE_H
#define EMPLOYEE_H
#include 
#include 
using namespace std;
class Employee {
public:
    Employee(const string &first, const string &last) 
        : firstName(first), lastName(last) 
    { //构造函数开始
        ++counter; //每创建一个对象，计数器加一
        cout << "Employee constructor for " << firstName
             << ' ' << lastName << " called." << endl;
    }
    ~Employee() { //析构函数
        cout << "~Employee() called for " << firstName << ' '
             << lastName << endl; //返回每个对象的名字
        --counter; //计数器减一
    }
    string getFirstName() const {
        return firstName; 
    }
    string getLastName() const {
        return lastName;
    }
    static int getCount() {
        return counter;
    }
private:
    string firstName;
    string lastName;
    static int counter; //这里出错了，但是为什么呢？
};
#endif

主要程序：employee2.cpp

#include 
#include "employee2.h"
using namespace std;
int main()
{
    cout << "在实例化任何对象之前的员工数量为 "
         << Employee::getCount() << endl; //从类中调用计数器的值
    {
        Employee e1("Susan", "Bkaer"); //从Employee类初始化e1对象
        Employee e2("Robert", "Jones"); //从Employee类初始化e2对象
        cout << "在实例化对象之后的员工数量为"
             << Employee::getCount();
        cout << "\n\nEmployee 1: " << e1.getFirstName() << " " << e1.getLastName()
             << "\nEmployee 2: " << e2.getFirstName() << " " << e2.getLastName()
             << "\n\n";
    }
    cout << "\n在删除对象之后的员工数量为 "
         << Employee::getCount() << endl; //显示计数器的值
    return 0;
}