运行下列代码，屏幕上输出（ ）。

1#include <iostream>2using namespace std;3 4class shape {5protected:6   int width, height;7public:8   shape(int a = 0, int b = 0) {9      width = a;10      height = b;11   }12   virtual int area() {13      cout << "parent class area: " <<endl;14      return 0;15   }16};17 18class rectangle: public shape {19public:20   rectangle(int a = 0, int b = 0) : shape(a, b) { }21 22   int area () {23      cout << "rectangle area: ";24      return (width * height);25   }26};27 28class triangle: public shape {29public:30   triangle(int a = 0, int b = 0) : shape(a, b) { }31 32   int area () {33      cout << "triangle area: ";34      return (width * height / 2);35   }36};37 38int main() {39   shape *pshape;40   rectangle rec(10, 7);41   triangle tri(10, 5);42 43   pshape = &rec;44   pshape->area();45 46   pshape = &tri;47   pshape->area();48   return 0;49}