为什么这不起作用？ （C ++）

Question

我有这个代码来计算形状的面积。我不明白为什么我会收到错误＆＃34;在＆＃39; double＆＃39;＆＃34;之前预期不合格的ID和＃34;期望（在双＆＃34之前;在第44行为＆＃34;类三角::三角形（双s1，双s2，双s3）：多边形（s1，s2，s3,0.0）{}。＆ ＃34;非常感谢任何帮助。提前谢谢。

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

    //Polygon constructor
    Polygon :: Polygon(double side1, double side2, double side3, double side4)
    {
        s1 = side1;
        s2 = side2;
        s3 = side3;
        s4 = side4;
    }
    //get area method
    double Polygon :: getArea()
    {
        float length, width, area;

        if (s1 == s2)
            {
                length = s1;
                width = s3;
            }
        else if (s1 == s3)
            {
                length = s1;
                width = s2;
            }
        else if (s1 == s4)
            {
                length = s1;
                width = s3;
            }
        area = length * width;
        return area;
    }
    //Triangle class
    class Triangle :: Triangle(double s1, double s2, double s3) : Polygon(s1,       s2, s3, 0.0) {}

    double Triangle :: getArea()
    {
        float s, area;

            s = (s1 + s2 + s3)/2;
            area = sqrt(s * (s - s1) * (s - s2) * (s - s3));

            return area;
    }

Answer 1

您需要在此处删除class这个词：

class Triangle :: Triangle

当然，您需要确保在此之前声明class Triangle。

Answer 2

使用以下代码段。

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

    //Polygon constructor
    Polygon :: Polygon(double side1, double side2, double side3, double side4)
    {
        s1 = side1;
        s2 = side2;
        s3 = side3;
        s4 = side4;
    }
    //get area method
    double Polygon :: getArea()
    {
        float length, width, area;

        if (s1 == s2)
            {
                length = s1;
                width = s3;
            }
        else if (s1 == s3)
            {
                length = s1;
                width = s2;
            }
        else if (s1 == s4)
            {
                length = s1;
                width = s3;
            }
        area = length * width;
        return area;
    }
    //Triangle class
     Triangle :: Triangle(double s1, double s2, double s3) : Polygon(s1,       s2, s3, 0.0) {}

    double Triangle :: getArea()
    {
        float s, area;

            s = (s1 + s2 + s3)/2;
            area = sqrt(s * (s - s1) * (s - s2) * (s - s3));

            return area;
    }