如何將子類對象添加到其父類（C ++）的向量中？

Question

我有各種形狀類，包括矩形，圓形，正方形，五邊形等，都是Shape類的所有子類，並且所有這些類都有一個getArea（）方法來返回其面積。 我正在嘗試制作一個Shape的矢量，可以將所有這些形狀的面積相加，以便可以使用一個函數來計算總面積。 這是我遇到麻煩的代碼的一部分。 我還沒有完成getTotalArea方法的編寫，但這很容易。 我只需要知道如何命名該方法（getTotalArea方法）以及要使用的引用或指針。 同樣，我可以獲得main方法底部的唯一方法是在矢量中使用Shape的指針，並在將子對象添加到矢量中時引用它們：

vector<Shape*> shapes;
shapes.push_back(&circle);

有任何想法嗎？ 非常感謝您的幫助。

#include <iostream>
#include "shape.h"
#include "point.h"
#include <vector>
using namespace std;
double getTotalArea(vector<Shape> sh)
{
  return 0;
}
int main(int argc, char** argv)
{
  vector<Point> my_points;
  my_points.push_back(Point(2, 3));
  my_points.push_back(Point(1, 1));
  my_points.push_back(Point(3, 1));
  cout << "Points for a triangle:\n\n";
  Triangle triangle(my_points);
  cout << triangle << "area = " << triangle.getArea();
  my_points.clear();

  my_points.push_back(Point(2, 7));
  my_points.push_back(Point(9, 7));
  my_points.push_back(Point(2, 3));
  my_points.push_back(Point(9, 3));
  cout << "\n\nPoints for a rectangle:\n\n";
  Rectangle rectangle(my_points);
  cout << rectangle << "area = " << rectangle.getArea();
  my_points.clear();

  my_points.push_back(Point(0, 5));
  my_points.push_back(Point(5, 5));
  my_points.push_back(Point(0, 0));
  my_points.push_back(Point(5, 0));
  cout << "\n\nPoints for a square:\n\n";
  Square square(my_points);
  cout << square << "area = " << square.getArea();
  my_points.clear();

  my_points.push_back(Point(0, 5));
  my_points.push_back(Point(6, 6));
  my_points.push_back(Point(10, 5));
  my_points.push_back(Point(8, 3));
  my_points.push_back(Point(1, 2));
  cout << "\n\nPoints for a pentagon:\n\n";
  Pentagon pentagon(my_points);
  cout << pentagon << "area = " << pentagon.getArea();
  my_points.clear();

  my_points.push_back(Point(3, 5));
  cout << "\n\nLength of semi-major axis and length of semi-minor axis for an oval (put in a Point object)):\n\n";
  Oval oval(my_points);
  cout << oval << "area = " << oval.getArea();
  my_points.clear();

  cout << "\n\nRadius of a circle (first parameter of a Point object - second is ignored):\n\n";
  my_points.push_back(Point(12, 0));  //the second argument here can also be NULL
  Circle circle(my_points);
  cout << circle << "area = " << circle.getArea();
  my_points.clear();

  vector<Shape*> shapes;
  shapes.push_back(&circle);
  shapes.push_back(&rectangle);
  shapes.push_back(&square);
  shapes.push_back(&pentagon);
  shapes.push_back(&oval);
  shapes.push_back(&circle);
  double totalArea = getTotalArea(shapes);

  return 0;
}

感謝你們中的一些人為我指出了正確的方向，我解決了我遇到的問題。 這就是我得到的，並且可以正常使用：

  //a function outside the main method
  void displayTotalArea(vector<Shape*>& sh)
  {
     double total = 0;

     for (int x = 0; x < sh.size(); x++)
     {
       total += sh[x]->getArea();
     }
     cout << "\n\nTotal area of all shapes = " << total;
  }

  //in the main method
  vector<Shape*> shapes;
  shapes.push_back(&pentagon);
  shapes.push_back(&triangle);
  shapes.push_back(&rectangle);
  shapes.push_back(&square);
  shapes.push_back(&pentagon);
  shapes.push_back(&oval);
  shapes.push_back(&circle);
  displayTotalArea(shapes);

Answer 1

如果我了解您的工作，那么可以這樣做：

double getTotalArea(const vector<Shape*>& sh)
{
    double tot = 0;

    for (Shape* s : sh)
        tot += s->getArea();

    return tot;
}

將子級添加到容器中（然后使用vector而不是vector，這樣做會更好，因為當向量超出范圍時，使用非指針版本會釋放形狀）：

Triangle t;
Oval v;
std::vector<Shape> container;
container.push_back(t);
container.push_back(v);

這樣編譯就可以了：

#include <iostream>
#include <vector>

class A {};
class B : public A {};
class C : public A {};

int main()
{
    std::vector<A> vec;
    vec.push_back(B());
    vec.push_back(C());

    B instance;
    vec.push_back(instance);

    return 0;
}