Why subclass method isn't called?

f() needs to be declared virtual in the base class A:

class A {
public:
    virtual void f() {
        cout << "A";
    }
};

The other languages you already worked with may default to virtual methods, but C++ doesn't (don't pay for what you don't use: virtual methods incur an indirection when calling them which means they are slightly slower than normal method calls).

By adding virtual, binding will be postponed to runtime (called dynamic binding) and which f() function call will be decided on the type of the value.

Because you have not declared function f() as virtual, binding is static (at compilation time) and will use the type of variable (but not value) to determine which f() to call. So in your present code statment a->f(); calls the A class's f() because a is pointer to the A class.

In order to achieve polymorphic behavior, the method of the base class must be virtual.

So in class A you need to change void f() to virtual void f().

The function must be declared virtual to be able to override it:

#include <iostream>
using namespace std;

class A {
public:
    virtual void f() {// Here you must define the virtual.
        cout << "A";
    }
};

class B : public A {
public:
    virtual void f() { //Here the "virtual" is optional, but a good practice
        cout << "B";
    }
};

int main() {
    A *a = new B();
    a->f();
    return 0;
}