错误:ISO C++禁止非const静态成员的类内初始化

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) 

重载的构造函数

    : firstName(first), 

firstName重载的构造函数

      lastName(last) 

lastName重载的构造函数

    { //The constructor start
    ++counter; 

它为每个创建的对象增加一个加号;

    cout << "Employee constructor for " << firstName
         << ' ' << lastName << " called." << endl;
    }

    ~Employee() { 

析构函数cout <<"~Workee()调用"<< firstName <<"<< lastName << endl;

返回每个对象的名和姓

        --counter; 

反减一

    }

    string getFirstName() const {
        return firstName; 
    }

    string getLastName() const {
        return lastName;
    }

    static int getCount() {
        return counter;
    }
private:
    string firstName;
    string lastName;

   static int counter = 0; 

这是我得到错误的地方.但为什么？

};

主要计划:employee2.cpp

#include 
#include "employee2.h"
using namespace std;

int main()
{
    cout << "Number of employees before instantiation of any objects is "
         << Employee::getCount() << endl; 

在这里,我可以从班级中拨打计数器的价值

    { 

启动一个新的范围块

        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 << "Number of employees after objects are instantiated is"
             << Employee::getCount(); 

        cout << "\n\nEmployee 1: " << e1.getFirstName() << " " << e1.getLastName()
             << "\nEmployee 2: " << e2.getFirstName() << " " << e2.getLastName()
             << "\n\n";
    } 

结束范围块

    cout << "\nNUmber of employees after objects are deleted is "
         << Employee::getCount() << endl; //shows the counter's value
} //End of Main

问题是什么？我不知道出了什么问题.我一直在思考,但是我没有错.