how does c++ compiler know doing practical duty call?

here an instance polymorphism (edited readability):

// summary bottom class
#include <iostream>
using namespace std;

class polygon {
    protected:
        int width;
        int height;
    public:
        vacant set_values(int a, int b) { extent = a; tallness = b; }
        practical int area(void) =0;
};

class rectangle: open polygon {
    public:
        int area(void) { relapse extent * height; }
};

class triangle: open polygon {
    public:
        int area(void) { relapse extent * tallness / 2; }
};

int sure () {
    rectangle rect;
    triangle trgl;
    polygon * ppoly1 = &rect;
    polygon * ppoly2 = &trgl;
    ppoly1->set_values (4,5);
    ppoly2->set_values (4,5);
    cout << ppoly1->area() << endl; // outputs 20
    cout << ppoly2->area() << endl; // outputs 10
    relapse 0;
}

my doubt does compiler know ppoly1 rectangle ppoly2 triangle, repremand area() function? out looking during "polygon * ppoly1 = ▭" line meaningful rect rectangle, nonetheless wouldn't work cases, it? something this?

cout << ((polygon *)0x12345678)->area() << endl;

assuming you're certified opening futile area memory.

i exam out nonetheless i can't mechanism i'm during moment.

(i wish i'm blank something obvious...)