如何在其他类中使用参数化构造函数创建类的对象？

Question

我想在Area类中创建一个DataArea类的对象并在main函数中初始化数据。 但是我的代码工作的唯一方法是在Area类中初始化数据。

另外，我不知道我是否正确地制作了对象。 请指导我。 我的代码如下：

#include<iostream>
using namespace std;
class DataArea
{
public:
    int radius, length, width, base, heigth;
    DataArea(int l, int w, int b, int h, int r)
    {
        length = l;
        width = w;
        radius = r;
        heigth = h;
        base = b;
    }
};

class Area
{
public:
    DataArea* s = new DataArea(3, 4, 5, 6, 7);
    float AreaCirle()
    {
        return 3.142 * s->radius * s->radius;
    }
    float AreaRectangle()
    {
        return s->length * s->width;
    }
    float AreaTraingle()
    {
        return (s->base * s->heigth) / 2;
    }
};

class print_data : public Area
{
public:
    void print()
    {
        cout << "Area of Circle is: " << AreaCirle() << endl;
        cout << "Area of Rectangle is: " << AreaRectangle() << endl;
        cout << "Area of Traingle is: " << AreaTraingle() << endl;
    }
};

int main()
{
    //DataArea da(3, 4, 5, 6, 7);
    print_data m;
    m.print();
}

Answer 1

在我看来， Area课程对于您想要实现的目标来说是多余的。 您可能应该将方法直接放在DataArea类中。 然后，您可以根据需要创建任意数量的DataArea对象...

像这样：

class DataArea
{
    public:
        int radius, length, width, base, heigth;
        DataArea(int l , int w , int b , int h , int r )
        {
            length = l;
            width = w;
            radius = r;
            heigth = h;
            base = b;
        }



        float AreaCirle()
        {
            return 3.142 * radius * radius;
        }
        float AreaRectangle()
        {
            return length * width ;
        }
        float AreaTraingle()
        {
            return (base * heigth)/2;
        }
};

int main(int argc, char **argv)
{
    DataArea area1 (1,2,3,4,5);
    DataArea area2 (8,2,3,4,5);

    std::cout << area1.AreaCirle() << std::endl;
    std::cout << area2.AreaCirle() << std::endl;
}

您可能难以理解这个概念的原因是：您正在定义一个类并实例化一个对象。 有时这些术语可以互换使用，但在这种情况下，这是一个重要的区别。

如果您希望您的方法对某个其他类进行操作，那么您应该创建接受该类作为参数的方法。 否则，它是不必要的复杂。

Answer 2

如果您不在Area类之外使用它，您的DataArea基本上是绝对的。 同样， print_data类可以替换为operator<<重载。

以下是更新后的代码，其中的注释将引导您完成。

#include <iostream>    

// DataArea (optionally) can be the part of Area  class
struct DataArea /* final */
{
    float length, width, base, height, radius;

    DataArea(float l, float w, float b, float h, float r)
        : length{ l }  // use member initializer lists to initlize the members
        , width{ w }
        , base{ b }
        , height{ h }
        , radius{ r }
    {}
};

class Area /* final */
{
    DataArea mDataArea;  // DataArea  as member
public:
    // provide a constructor which initialize the `DataArea` member
    Area(float l, float w, float b, float h, float r)
        : mDataArea{ l, w, b, h, r }  // member initializer 
    {}

    // camelCase naming for the functions and variables
    // mark it as const as the function does not change the member
    float areaCirle() const /* noexcept */
    {
        return 3.142f * mDataArea.radius * mDataArea.radius;
    }

    float areaRectangle() const /* noexcept */
    {
        return mDataArea.length * mDataArea.width;
    }

    float areaTraingle() const /* noexcept */
    {
        return (mDataArea.base * mDataArea.height) / 2.f;
    }

    // provide a operator<< for printing the results
    friend std::ostream& operator<<(std::ostream& out, const Area& areaObject) /* noexcept */;
};

std::ostream& operator<<(std::ostream& out, const Area& areaObject) /* noexcept */
{
    out << "Area of Circle is: " << areaObject.areaCirle() << "\n";
    out << "Area of Rectangle is: " << areaObject.areaRectangle() << "\n";
    out << "Area of Traingle is: " << areaObject.areaTraingle() << "\n";
    return out;
}

int main()
{
    // now construct the Area object like this
    Area obj{ 3, 4, 5, 6, 7 };
    // simply print the result which uses the operator<< overload of the Area class
    std::cout << obj;
}

输出：

Area of Circle is: 153.958
Area of Rectangle is: 12
Area of Traingle is: 15