单选题库

#include <iostream>
using namespace std;
 
class shape {
protected:
    int width, height;
public:
    shape(int a = 0, int b = 0) {
        width = a;
        height = b;
    }
    virtual int area() {
        cout << "parent class area: " << endl;
        return 0;
    }
};
 
class rectangle: public shape {
public:
    rectangle(int a = 0, int b = 0) : shape(a, b) { }
 
    int area() {
        cout << "rectangle area: ";
        return (width * height);
    }
};
 
class triangle: public shape {
public:
    triangle(int a = 0, int b = 0) : shape(a, b) { }
 
    int area() {
        cout << "triangle area: ";
        return (width * height / 2);
    }
};
 
int main() {
    shape *pshape;
    rectangle rec(10, 7);
    triangle tri(10, 5);
 
    pshape = &rec;
    pshape->area();
 
    pshape = &tri;
    pshape->area();
    return 0;
}

#include <iostream>
using namespace std;
class my_class {
	public:
		static int count;
		my_class() {
			count++;
		}
		~my_class() {
			count--;
		}
		static void print_count() {
			cout << count << " ";
		}
};
int my_class::count = 0;
int main() {
	my_class obj1;
	my_class::print_count();
	my_class obj2;
	obj2.print_count();
	my_class obj3;
	obj3.print_count();
	return 0;
}