未解析的外部，创建实例时出现类错误

Question

我试着写一个汽车类，里面有一个名为＆＃34;引擎＆＃34;在里面         我在创建引擎实例时遇到问题（命名为＆＃34; myengine＆＃34;）         *错误代码表示我有未解决的外部         我会很感激任何帮助/评论

    **this is the car header**

    #include "engine.h"
    #ifndef _car_
    #define _car_





     class car
    {
        engine myengine;                      //here is the error
        char* brandname="mybrand";
        char* model="mymodel";



    };




    #endif

**this is the engine header**


#ifndef _engine_
#define _engine_

class engine
{
public:

    float volume;
    float currentfuel;
    float maxfuel;
    bool activated;     //off
    engine();
     engine(float volume, float currentfuel, float maxfuel, bool activated) { 10, 0, 100, 0; }
    ~engine();
    void setfuel(float fuel);
    float getfuel();
    float getmaxfuel();
    void setmaxfuel(float maxfuel);
    float getvolume();
    void setvolume(float volume);
    void setactivated(bool active);
    bool getactivated();
    void activate(bool active);





};


#endif

**this is the engine cpp**

#include "engine.h"
#include <iostream>
using namespace std;

void engine::setfuel(float fuel)
{
    cout<<"enter fuel amount"<<endl;
    cin >> fuel;

    if (fuel < 0)
    {
        cout << "the amount of fuel cannot be less than 0";
    }
    else fuel = currentfuel;



}