C++多继承场景报'Vehicle':base class undefined错误排查

报'Vehicle':base class undefined以及后续连锁编译错误，核心是三类低级写法错误：

• 派生类头文件声明继承Vehicle时，没有提前引入Vehicle.h，编译器看不到Vehicle的类定义，自然无法识别基类
• 错误在多个头文件、cpp文件中#include了.cpp实现文件：C++中.cpp是独立编译单元，永远不要被其他文件include，否则必然触发重复定义链接错误
• 文件名和引用名不匹配：汽油车类头文件命名为Gasoline.h，但其他代码中引用的是GasolineVehicle.h
• 多层继承时上层头文件缺依赖：HybridVehicle.h使用了ElectricVehicle和GasolineVehicle类，但没有引入两个父类的头文件

1. 统一修正所有头文件，只include依赖的头文件、不碰cpp #

ElectricVehicle.h #

新增基类头文件引用，修正后代码：

#pragma once
#ifndef ElectricVehicle_h
#define ElectricVehicle_h
#include <iostream>
#include "Vehicle.h" // 必须引入，否则编译器不知道Vehicle是什么

class ElectricVehicle : virtual public Vehicle {
public:
    float maximumCharge;
    float currentCharge;
    float engineEfficiency;

public:
    ElectricVehicle(float max, float eff);
    float calculateRange();
    float percentEnergyRemaining();
    void drive(float km);
    ~ElectricVehicle();
};
#endif

GasolineVehicle.h #

先把原文件名Gasoline.h重命名为GasolineVehicle.h，和其他文件的引用名保持一致，再新增基类头文件引用：

#pragma once
#include <iostream>
#include "Vehicle.h" // 必须引入基类定义

#ifndef GasolineVehicle_h
#define GasolineVehicle_h

class GasolineVehicle : virtual public Vehicle {
public:
    float maximumGasoline;
    float currentGasoline;
    float engineEfficiency;
    GasolineVehicle(float max, float eff);
    ~GasolineVehicle();
    float calculateRange();
    float percentEnergyRemaining();
    void drive(float km);
};
#endif

HybridVehicle.h #

新增两个直接父类的头文件引用：

#pragma once
#ifndef HybridVehicle_h
#define HybridVehicle_h
#include "ElectricVehicle.h"
#include "GasolineVehicle.h"

class HybridVehicle: public GasolineVehicle, public ElectricVehicle {
public:
    HybridVehicle(float gMax, float gEff, float eMax, float eEff);
    ~HybridVehicle();
    float calculateRange();
    float percentEnergyRemaining();
    void drive(float km);
};
#endif

2. 修正所有cpp文件，删除所有对.cpp的引用，只include对应头文件 #

所有cpp文件去掉无意义的#pragma once（该宏仅用于头文件防重复包含，cpp不需要）：

Vehicle.cpp #

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

Vehicle::~Vehicle() {
    cout << "In vehicle destructor" << endl;
}

ElectricVehicle.cpp #

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

ElectricVehicle::ElectricVehicle(float maxEnergy, float rating) {
    engineEfficiency = rating;
    maximumCharge = maxEnergy; 
    currentCharge = maxEnergy;
}

ElectricVehicle::~ElectricVehicle() {
    cout << "In Electric vehicle destructor" << endl;
}

float ElectricVehicle::calculateRange() {
    return (currentCharge * 100 / engineEfficiency);
}

float ElectricVehicle::percentEnergyRemaining() {
    return (currentCharge * 100.0f / maximumCharge);
}

void ElectricVehicle::drive(float km) {
    if (currentCharge < 0) {
        cout << "Your Car is out of energy";
        return;
    }
    currentCharge -= (km / 100) * engineEfficiency;
}

GasolineVehicle.cpp #

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

GasolineVehicle::GasolineVehicle(float maxEnergy, float rating) {       
    engineEfficiency = rating;
    maximumGasoline = maxEnergy;
    currentGasoline = maxEnergy;
}

GasolineVehicle::~GasolineVehicle() {
    cout << "In Gasoline vehicle destructor" << endl;
}

float GasolineVehicle::calculateRange() {
    return currentGasoline * 100 / engineEfficiency;
}

float GasolineVehicle::percentEnergyRemaining() {
    return (currentGasoline * 100.0f / maximumGasoline);
}

void GasolineVehicle::drive(float km) {
    if (currentGasoline < 0){
        cout << "Your Car is out of energy";
        return;
    }
    currentGasoline -= (km / 100) * engineEfficiency;
}

HybridVehicle.cpp #

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

HybridVehicle::HybridVehicle(float maxGasoline, float gasefficiency, float maxcharge, float electricefficiency) :GasolineVehicle(maxGasoline, gasefficiency), ElectricVehicle(maxcharge, electricefficiency) {}
    
HybridVehicle::~HybridVehicle() {
    cout << "In Hybrid vehicle destructor" << endl;
}

float HybridVehicle::calculateRange() {
    return (currentCharge / ElectricVehicle::engineEfficiency) * 100 + (currentGasoline / GasolineVehicle::engineEfficiency) * 100;
}

float HybridVehicle::percentEnergyRemaining() {
    return ((currentCharge + currentGasoline) / (maximumCharge + maximumGasoline)) * 100.0f;
}

void HybridVehicle::drive(float km) {
    if (currentGasoline + currentCharge < 0) {
        cout << "Your Car is out of energy";
        return;
    }
    else if (currentCharge > 0) {
        currentCharge -= (km / 100) * ElectricVehicle::engineEfficiency;
    }
    else
    {
        currentGasoline -= (km / 100) * GasolineVehicle::engineEfficiency;
    }
}

入口文件Week2.cpp #

修正错误的类型强转，去掉对cpp的引用：

#include "Vehicle.h"
#include "ElectricVehicle.h"
#include "GasolineVehicle.h"
#include "HybridVehicle.h"
#include <iostream>
using namespace std;

Vehicle* testVehicle(Vehicle* pVehicle, const char* vehicleName) {
    cout << vehicleName << " s range is: " << pVehicle->calculateRange() << endl;
    pVehicle->drive(150);
    cout << vehicleName << " s energy left is: " << pVehicle->percentEnergyRemaining() << endl;
    cout << vehicleName << " s range is now: " << pVehicle->calculateRange() << endl;

    return pVehicle;
}

int main(int argc, char** argv)
{
    //50L of gas, 7.1 L/100km
    delete testVehicle(new GasolineVehicle(50, 7.1), "Corolla");

    //42 L of gas, 4.3 L/100km, 8.8kWh, 22 kWh/100km
    delete testVehicle((Vehicle*)new HybridVehicle(42, 4.3, 8.8, 22.0), "Prius"); // 原代码强转为GasolineVehicle*不符合多态逻辑，改为基类Vehicle*

    //75 kWh, 16 kWh/100km
    delete testVehicle(new ElectricVehicle(75, 16), "Tesla 3");

    return 0;
}

编译时需要把所有cpp文件加入编译列表，不要只编译Week2.cpp，要将Vehicle.cpp/ElectricVehicle.cpp/GasolineVehicle.cpp/HybridVehicle.cpp/Week2.cpp统一编译链接即可正常运行。

内容的提问来源于stack exchange，提问作者Alice Smith