运行下列代码，屏幕上输出（ ）。
1 #include <iostream>
2 using namespace std;
3
4 class shape {
5 protected:
6 int width, height;
7 public:
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
18 class rectangle: public shape {
19 public:
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
28 class triangle: public shape {
29 public:
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
38 int 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 }