Question

我写了一个程序，询问用户三个值，然后计算其他值。

但不知何故，它会返回垃圾。例如：

Track.h

class Track : public World
{
    public:

        friend class Vehicle;

        Track();
        virtual ~Track();
        float GetLenght() {return m_Lenght;}
        float GetSlope() {return m_Slope;}
        void SetSlope(float value) {m_Slope = value;}
        float GetAcceleration();
    protected:
    private:
        const float m_Lenght = 2; //m
        float m_Slope;
        float m_TrackAcceleration;
};

Track.cpp

#include "Track.h"
#include "World.h"
#include <math.h>
#include "Vehicle.h"

#define PI 3.14159265359

[...]

float Track::GetAcceleration() {
    World *Earth = new World();
    Vehicle *Car = new Vehicle();
    m_TrackAcceleration = Earth->Accelerate(Car->GetWeight())*sin(1*PI/180);
    return m_TrackAcceleration;
}

的main.cpp

int main()
{
    World *Earth = new World();
    Track *Track1 = new Track();
    Vehicle *Car = new Vehicle();
    Mass *Mass1 = new Mass();

    float MassWeight, VehicleWeight, Slope;

    cout << "Mass Weight:"; cin >> MassWeight;
    cout << "Vehicle Weight:"; cin >> VehicleWeight;
    cout << "Slope:"; cin >> Slope;

    Mass1->SetWeight(MassWeight);
    Car->SetWeight(VehicleWeight);
    Track1->SetSlope(Slope);

    cout << "Acceleration Force:" << Track1->GetAcceleration() << endl << endl;
};

Vehicle.h

class Vehicle : public World
{
    public:

        friend class Track;

        Vehicle();
        virtual ~Vehicle();
        float GetWeight() {return m_VehicleWeight;}
        void SetWeight(float value) {m_VehicleWeight = value;}
        float GetSpeed(float seconds);
        float GetAcceleration();
        float GetDistance(float seconds);
    protected:
    private:
        float m_VehicleWeight;
        float m_Speed;
        float m_Distance;
        float m_VehicleAcceleration;
};

World.h

class World
{
    public:
        World();
        virtual ~World();
        float GetGravity (){return m_Gravity;}
        float Accelerate (float mass);
    protected:
    private:
        const float m_Gravity = 9.81; // m/s^2
        float m_WorldAcceleration;
};

应返回0,171208....，但会返回1.91825e-039 Mass Weight=1，Vehicle Weight=1和Slope=1。

对此有何看法？

Answer 1

我认为问题在于您不了解在本地范围内创建对象时会发生什么。出于某种原因，您的代码会创建两组独立的Earth和Car对象。

float Track::GetAcceleration() 
{
    World *Earth = new World();   // this is a brand new World object
    Vehicle *Car = new Vehicle();  // this is a brand new Vehicle object

    // now you're calcuating acceleration with these brand new objects 
    m_TrackAcceleration = Earth->Accelerate(Car->GetWeight())*sin(1*PI/180);
    return m_TrackAcceleration;
}

你不仅有内存泄漏，你正在创建新对象，而且我们不知道新构造的对象有什么值，因为你没有发布World或{{1的构造函数的代码}}。

然后在main（）中执行此操作：

Vehicle

好的，您将值分配给int main() { World *Earth = new World(); // a brand new object Track *Track1 = new Track(); // another brand new object Vehicle *Car = new Vehicle(); // yet another Mass *Mass1 = new Mass(); // and another float MassWeight, VehicleWeight, Slope; cout << "Mass Weight:"; cin >> MassWeight; cout << "Vehicle Weight:"; cin >> VehicleWeight; cout << "Slope:"; cin >> Slope; Mass1->SetWeight(MassWeight); Car->SetWeight(VehicleWeight); Track1->SetSlope(Slope); // This call knows nothing about the Earth or Car objects you created in main(). cout << "Acceleration Force:" << Track1->GetAcceleration() << endl << endl; }中声明的对象，但是当您调用main()时，该函数会创建两个与{{1}中创建的对象无关的全新对象}}。您对main（）中创建的对象的引用在哪里？

首先，您需要意识到C ++不是Java。您不需要GetAcceleration来创建对象。

main

此代码中现在没有内存泄漏。

现在，在GetAcceleration中，除非将它们作为参数传递，否则如何知道在main（）中创建的对象？

new

来自int main() { World Earth; // a brand new object Track Track1; // another brand new object Vehicle Car; // yet another Mass Mass1; // and another float MassWeight, VehicleWeight, Slope; cout << "Mass Weight:"; cin >> MassWeight; cout << "Vehicle Weight:"; cin >> VehicleWeight; cout << "Slope:"; cin >> Slope; Mass1.SetWeight(MassWeight); Car.SetWeight(VehicleWeight); Track1.SetSlope(Slope); // This call knows nothing about the Earth or Car objects you created in main(). cout << "Acceleration Force:" << Track1.GetAcceleration() << endl << endl; }的来电将如下所示：

  float Track::GetAcceleration(World& theWorld, Vehicle& theVehicle) 
  {
    m_TrackAcceleration = theWorld.Accelerate(theVehicle.GetWeight())*sin(1.0*PI/180);
    return m_TrackAcceleration;
  }

Answer 2

您的Car车辆刚刚创建。假设它没有将成员初始化为不同于零的值，则在乘法后应该得到零值。

1.91825e-039是接近零的值。差异可能是您使用斜坡地球加速度等进行代数的结果。

至少你的问题没有说明为什么人们应该期待不同的价值。

简单计算返回错误的值

2 个答案: