错误的输出(继承）

>我正在尝试使用继承运行程序，但在匹配所需输出时遇到麻烦。我将在下面发布所有内容。

头文件：

    class Vehicle {
    public:
            virtual void accelerate() = 0;
            virtual void brake() = 0;
            virtual std::ostream& display(std::ostream& ostr) const =0;
    };

class Car: public Vehicle {
            int speed_;
            int maxSpeed_;
    protected:
            void speed(int value);
            int maxSpeed() const;
    public:
            int speed() const;
            Car(int m = 100);
    };

    class SportCar: public Car{
            int noOfPassengers_;
    public:
            SportCar();
            SportCar(int speed, int pass);
            // implementations of Vehicle's pure virtual methods
            void accelerate();
            void brake();
            std::ostream& display(std::ostream& os) const;
    };
    class PickupTruck: public Car {
            int loadAmount_;
            char loadedMaterial_[31];
    public:
            PickupTruck();
            void load(const char* loadedMaterial, int loadAmount);
            void unload();
            bool isEmpty()const;
            // implementations of Vehicle's pure virtual methods
            void accelerate();
            void brake();
            std::ostream& display(std::ostream& os) const;
    };

CPP 文件：

void Car::speed(int value) {
            speed_ = value;
            if (value > maxSpeed_ ) {
                    maxSpeed_ = value;
            }
            else if (value < 0) {
                    speed_ = 0;
            }
    }
    int Car::maxSpeed()const {
            return maxSpeed_;
    }
    int Car::speed() const {
            return speed_;
    }
    Car::Car(int m) {
            maxSpeed_ = m;
            speed_ = 0;
    }


    SportCar::SportCar() {
            noOfPassengers_ = 1;
    }
    SportCar::SportCar(int speed, int pass): Car(speed) {
            noOfPassengers_ = pass;
    }
    void SportCar::accelerate() {
            speed(speed() + 40);
    }
    void SportCar::brake() {
            speed(speed() - 10);
    }
    std::ostream& SportCar::display(std::ostream& os) const {
            if (speed() > 0) {
                    os << "This sport car is carrying" << noOfPassengers_ << "passengers and is traveling at a speed of " << speed() << "km/h";
            }
            else {
                    os << "This sport car is carrying" << noOfPassengers_ << "passengers and is parked";
            }
            return os;
    }

    PickupTruck::PickupTruck() {
            loadAmount_ = 0;
            loadedMaterial_[0] = char(0);
    }
    bool PickupTruck::isEmpty()const {
            return loadAmount_ == 0;
    }
    void PickupTruck::load(const char* loadedMaterial, int loadAmount) {
            std::strcpy(loadedMaterial_, loadedMaterial);
            loadAmount_ = loadAmount;
    }
    void PickupTruck::accelerate() {
            speed(speed() + 20);
    }
    void PickupTruck::brake()
    {
            speed(speed() - 5);
    }
    void PickupTruck::unload() {
            loadAmount_ = 0;
    }
    std::ostream& PickupTruck::display(std::ostream& os) const
    {
            os << "This pickup truck is ";
            if (isEmpty()) {
                    os << "not carrying any load";
            }
            else {
                    os << "carrying " << loadAmount_ << " kgs of " << loadedMaterial_;
            }
            if (speed() > 0) {
                    os << ", traveling at the speed of " << speed() << " km/h.";
            }
            else {
                    os << " and is parked.";
            }
            return os;
    }

主要（下面有所需的输出）：

void drive(Car& cDrive) {
    cDrive.accelerate();
    cDrive.brake();
    cDrive.display(cout);
}
void stop(Car& cStop) {
    if (cStop.speed() > 0) {
            cStop.brake();
    }
    cStop.display(cout);
}
int main()
{
    SportCar Tesla(240, 2);
    PickupTruck Ford;
    Tesla.display(cout) << endl;
    Ford.display(cout) << endl;
    Ford.load("Bricks", 3500);
    drive(Tesla);
    drive(Ford);
    stop(Tesla);
    stop(Ford);
    Ford.unload();
    Tesla.display(cout) << endl;
    Ford.display(cout) << endl;
    return 0;
}
/* output
This sport car is carrying 2 passengers and is parked.
This pickup truck is not carrying any load and is parked.
This sport car is carrying 2 passengers and is traveling at a speed of 30   km/h.
This pickup truck is carrying 3500 kgs of Bricks, traveling at the speed of 15 km/h.
This sport car is carrying 2 passengers and is parked.
This pickup truck is carrying 3500 kgs of Bricks and is parked.
This sport car is carrying 2 passengers and is parked.
This pickup truck is not carrying any load and is parked.
*/

我得到的错误：

在输出的第 5 行中：输出应为：这辆跑车载有2名乘客并停放。但您的输出是：这辆跑车搭载 2 名乘客，以 20 公里/小时的速度行驶。

我已经坚持了一段时间了，任何正确方向的帮助都会很棒。谢谢！