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Week 4 Seminar Exercises ​

Complete each of the exercises below using CLion. Test each exercise after implementing it; the exercise is not complete until the program works.

  1. Create a C++ project. Cut and paste the following code that defines and uses a class.
cpp
#include <iostream>
#include <cstdlib>

using std::string;
using std::cout;
using std::endl;

class FidgetSpinner {
public:
    string colour;
    FidgetSpinner(const char *_colour);
    ~FidgetSpinner();
    void spin();
protected:
    int size;
    int spins;
};

FidgetSpinner::FidgetSpinner(const char *_colour) :
        colour(_colour), size(5), spins(0)
{
    cout << "Created a " << colour << " FidgetSpinner." << endl;
}

FidgetSpinner::~FidgetSpinner() {
    cout << "Destroyed a " << colour << " FidgetSpinner." << endl;
}

void FidgetSpinner::spin() {
    spins++;
    cout << "A " << colour << " spinner has spun " << spins << " time(s)." << endl;
}


int main() {

    FidgetSpinner v("Value Veridian"); // veridian 绿色颜料
    FidgetSpinner x();
    v.spin();
    
    FidgetSpinner *r = new FidgetSpinner("Reference Red");
    r->spin();
    delete r;

}

Test and run this program. Why does the message "Destroyed a Value Veridian FidgetSpinner" appear at the end of the program? (It is supposed to.)

NOTE

FidgetSpinner x();不能正确工作，因为C++ 的语法规则在解析时，优先认为这是一个函数声明（这是为了和 C 语言兼容）。
所以任何“看起来像函数声明”的写法，都会被编译器优先当成函数声明来处理。
但如果写：FidgetSpinner x; // 明确：对象定义，调用无参构造
编译器就能确定这是在定义对象。
但是由于我们覆盖了默认的无参构造函数，这样写也不行。

[!TIP] const是constant的简写，是C++中极为常见且重要的关键字，主要功能是设置某些参数不可修改。
static是静态变量，存储在静态存储区（存储在静态存储区的变量，如果不显式地对其进行初始化，系统会将其初始化为0），在程序执行期间，对应的存储空间不会释放，一直到程序结束才会释放。在声明后的任意位置均可调用和更改值。
另，和Java一样，C++也支持override和overload

  1. Modify the program to add a constructor to FidgetSpinner that allows both the color and the size to be specified. Modify both of the statements that construct FidgetSpinners to specify different sizes for them.

  2. Add some simple physics to the fidget spinners:

    • Add two new protected data members, velocity and angle, to the class. Both should be initialized to 0.
    • Modify the spin method to take a parameter strength. Instead of modifying spins, it should add strength to velocity.
    • Add a tick method (to represent time passing). It should do two things:
      • Add velocity to angle. Then, if angle is greater than 10, subtract 10 from it until it is less than 10, and add 1 to spins for each 10 subtracted. If spins was changed, print out the number of times the spinner has now spun.
      • Lower velocity by an amount equal to size. If this reduces it to zero or below, velocity should be set to zero, and it should print a message that the spinner has stopped, and what angle it stopped at. Modify the program to make the "reference red" spinner size 3, and spin it at strength 10; then call tick on it 5 times. It should spin twice and then stop at angle 2. (10 + (10-3) + (10-3-3) + (10-3-3-3) = 22.)

  3. Modify the main program spin both spinners and then loop calling tick on them both, until they both stop. You will need to add new methods to the class to achieve this.

  4. Modify the program to create 5 spinners in an array, spin them all, and then call tick on them until all of them are stopped. (You will need to store them in the array by reference, unless you want to initialize them all in one statement!)

  5. C++ supports static methods and properties in the same way that Java does, except that their initialization must occur outside the class definition (they must still be declared inside it). A static item a in class b is addressed as b::a. Modify the FidgetSpinner class so that the array of 5 spinners is stored inside the class and spinners are automatically added to or removed from it when they are created. If more than 5 spinners are created at once, the extras should not be stored in the array.

NOTE

关于写不写explicit的问题：
C++构造函数加不加explicit？90%程序员都踩过的坑！（附最佳实践）_explicit 关键字 可以无脑加么-CSDN博客
C++11 新特性 之 explicit关键字 - 显示构造与隐式构造_显示构造和隐式构造-CSDN博客
【重学 C++】06 | C++该不该使用 explicit
彻底理解c++的隐式类型转换 - apocelipes - 博客园
c++ 写不写explicit有无区别 - Google 搜索

参考答案:

cpp
#include <iostream>  
#include <cstdlib>  
  
using std::string;  
using std::cout;  
using std::endl;  
  
class FidgetSpinner {  
public:  
    string colour;  
    explicit FidgetSpinner(const char *_colour);  
    FidgetSpinner(const char *_colour, int _size);  
    ~FidgetSpinner();  
    void spin();  
    void spin(int strength);  
    void tick();  
    int getVelocity() const; // new: allow external code to check if spinner is still moving  
    
    // Registry of created spinners (up to MAX_STORED)    static const int MAX_STORED = 5;  
    static FidgetSpinner* stored[MAX_STORED];  
    static int storedCount;  
    
    // Accessors for the registry  
    static int getStoredCount();  
    static FidgetSpinner* getStoredAt(int idx);  
    
protected:  
    int size;  
    int spins;  
    int velocity;  
    int angle;  
};  
  
FidgetSpinner* FidgetSpinner::stored[FidgetSpinner::MAX_STORED] = { nullptr };  
int FidgetSpinner::storedCount = 0;  
  
FidgetSpinner::FidgetSpinner(const char *_colour) :  
        colour(_colour), size(5), spins(0), velocity(0), angle(0)  
{  
    cout << "Created a " << colour << " FidgetSpinner." << endl;  
}  
FidgetSpinner::FidgetSpinner(const char *_colour, const int _size) :  
        colour(_colour), size(_size), spins(0), velocity(0), angle(0)  
{  
    cout << "Created a " << colour << " FidgetSpinner." << endl;  
    // Add to registry if space remains  
    if (storedCount < MAX_STORED) {  
        stored[storedCount++] = this;  
    }  
}  
  
FidgetSpinner::~FidgetSpinner() {  
    cout << "Destroyed a " << colour << " FidgetSpinner." << endl;  
    // Remove from registry if present and compact the array  
    for (int i = 0; i < storedCount; ++i) {  
        if (stored[i] == this) {  
            // shift left  
            for (int j = i; j + 1 < storedCount; ++j) stored[j] = stored[j + 1];  
            stored[storedCount - 1] = nullptr;  
            --storedCount;  
            break;  
        }  
    }  
}  
  
void FidgetSpinner::spin() {  
    spins++;  
    cout << "A " << colour << " spinner has spun " << spins << " time(s)." << endl;  
}  
void FidgetSpinner::spin(int strength) {  
    // Previously this incremented the spins counter; change to add to velocity  
    velocity += strength;  
    cout << "A " << colour << " spinner with size " << size << " was given strength " << strength << "." << endl;  
}  
  
void FidgetSpinner::tick()  
{  
    // Add velocity to angle  
    int previousVelocity = velocity;  
    angle += velocity;  
    
    // For every full 10 units of angle, reduce angle by 10 and increment spins  
    int spinsBefore = spins;  
    while (angle >= 10) {  
        angle -= 10;  
        spins += 1;  
    }  
    angle = angle % 10;  
    spins = (int)(angle / 10);  
    
    // If spins changed, print the updated spins count  
    if (spins != spinsBefore) {  
        cout << "A " << colour << " spinner with size " << size << " has spun " << spins << " time(s)." << endl;  
    }  
    
    // Lower velocity by an amount equal to size  
    velocity -= size;  
    if (velocity <= 0) {  
        // If spinner was moving and now stopped, print once  
        if (previousVelocity > 0) {  
            cout << "A " << colour << " spinner has stopped at angle " << angle << "." << endl;  
        }  
        velocity = 0;  
    }  
}  
  
int FidgetSpinner::getVelocity() const {  
    return velocity;  
}  
  
int FidgetSpinner::getStoredCount() {  
    return storedCount;  
}  
  
FidgetSpinner* FidgetSpinner::getStoredAt(int idx) {  
    if (idx < 0 || idx >= storedCount) return nullptr;  
    return stored[idx];  
}  
  
int main() {  
    // FidgetSpinner *FidgetSpinners[5];  
    // Create 5 spinners (they will automatically register)    
    FidgetSpinner *s1 = new FidgetSpinner("Reference Red", 3);  
    FidgetSpinner *s2 = new FidgetSpinner("Blue Bolt", 4);  
    FidgetSpinner *s3 = new FidgetSpinner("Green Glide", 5);  
    FidgetSpinner *s4 = new FidgetSpinner("Yellow Yaw", 6);  
    FidgetSpinner *s5 = new FidgetSpinner("Purple Pulse", 7);  
    
    // Give each spinner an initial strength (these are the same strengths as before)  
    int strengths[5] = {10, 9, 8, 7, 6};  
    for (int i = 0; i < 5; ++i) {  
        FidgetSpinner* cur = FidgetSpinner::getStoredAt(i);  
        if (cur) cur->spin(strengths[i]);  
    }  
    
    // Tick all registered spinners until all have stopped  
    bool anyMoving = true;  
    while (anyMoving) {  
        anyMoving = false;  
        int count = FidgetSpinner::getStoredCount();  
        for (int i = 0; i < count; ++i) {  
            FidgetSpinner* cur = FidgetSpinner::getStoredAt(i);  
            if (cur && cur->getVelocity() > 0) {  
                cur->tick();  
                anyMoving = true;  
            }  
        }  
    }  
    
    // Clean up (destroy created spinners)  
    delete s1;  
    delete s2;  
    delete s3;  
    delete s4;  
    delete s5;  
    
    return 0;  
    
}