questions in c++ (polymerphism, range-based for loop)

Question

Please understand that I've only begun learning C++. I must be missing some basic understanding because I can't configure:

why does the following code changes its output when I replace virtual void f() in class A with void f()? (it prints B when void, C when virtual void)

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

class B:public A{
public:
    void f(){
    cout<<"B";}
    };
class C:public B{
public:
    void f(){cout<<"C";}
    };
int main(){
    B *p = new C;
    p->f();
    }

Why does the following code print more than "CPP!"?

int main (){
     int x[10] = {1,2,3,4,5,6,7,8,9,10};
for(int y:x){
switch(y){
    case 1:cout<<"C";
    case 3:cout<<"P";
    case 7:cout<<"P";
    case 8:cout<<"!";
}

}

I look forward to hearing back from you experts..

Both questions are very basic. Have you tried to look it up? — HolyBlackCat, Nov 28 '17 at 11:13
Furthermore you could learn the answers to both questions by reading [a good beginners book or two](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). Hint for the second question: It's not the `for` loop that's the problem, it's the `switch` (or rather each `case`). — Some programmer dude, Nov 28 '17 at 11:14
See https://stackoverflow.com/questions/2391679/why-do-we-need-virtual-functions-in-c and https://stackoverflow.com/questions/33222437/switch-case-statement-without-break — Gaurav Sehgal, Nov 28 '17 at 11:15
1) [virtual keyword for functions in derived classes. Is it necessary?](https://stackoverflow.com/questions/4895294/c-virtual-keyword-for-functions-in-derived-classes-is-it-necessary) — Bo Persson, Nov 28 '17 at 11:18

BobMorane · Accepted Answer · 2017-11-28T13:57:39.653

why does the following code changes its output when I replace virtual void f() in class A with void f()? (it prints B when void, C when virtual void)

When a class inherits from another (like in your case B and C from A and B respectively), the virtual keyword means that the method it is applied to (f()) in your case) can be re-defined for child classes. In your case, it allows classes B and C to have their own personal implementations for f(), through polymorphism. Notice how in your main() you create a C object, and not a B:

B *p = new C;

it seems confusing because the address of that C object is stored in a B pointer, but is is legal in C++ because a C object is in fact B object, because it inherits (has a "is a" relationship) from B. When you call f()on that B pointer storing a C object, polymorphism is enabled because the method is virtual: it selects the C-version of the f() method, and that is why you see a C printed.

When you remove the virtual polymorphism is disabled the the C object is seen as a B object, and the B-version of f() is selected. Then you see "B" on your screen.

Why does the following code print more than "CPP!"?

From this, it says that:

The switch statement has a somewhat peculiar syntax inherited from the early times of the first C compilers, because it uses labels instead of blocks. In the most typical use (shown above), this means that break statements are needed after each group of statements for a particular label. If break is not included, all statements following the case (including those under any other labels) are also executed, until the end of the switch block or a jump statement (such as break) is reached.

So, try this instead:

int main ()
{
    int x[10] = {1,2,3,4,5,6,7,8,9,10};
    for(int y:x)
    {
        switch(y)
        {
            case 1:cout<<"C";break; // <- notice the 'break'!
            case 3:cout<<"P";break;
            case 7:cout<<"P";break;
            case 8:cout<<"!";break;
        }
    }
}

questions in c++ (polymerphism, range-based for loop)

1 Answers1