How do i "filter" output from vector

Question

Here is the problem. I have vector<D2*>, where D2 is a Parent.

I add there childs: D3 and D4.

void readFromInput(std::vector<D2*>& vec) {
        std::cout << "\nSecond class input";
        int x = 0, y = 0, z = 0;
        std::cout << "\nEnter x: ";
        std::cin >> x;
        std::cout << "Enter y: ";
        std::cin >> y;
        std::cout << "Enter z: ";
        std::cin >> z;
        vec.push_back(new D3(x, y, z));

    }


void readFromInput(std::vector<D2*>& vec) {
        std::cout << "\nSecond class input";
        int x = 0, y = 0, z = 0;
        std::cout << "\nEnter x: ";
        std::cin >> x;
        std::cout << "Enter y: ";
        std::cin >> y;
        std::cout << "Enter z: ";
        std::cin >> z;
        vec.push_back(new D4(x, y, z));

    }

And here is how i output it.

void output(std::vector<D2*>& vec) {

    for (const auto& item : vec) {
        item->display();
        std::cout << " | ";
    }

}

display() is a virtual function, btw.

So, when i use "output" i see every single element in this vector, of course.

Is there any method to ignore, for example, elements from D3 or D4?

I mean when vec.push_back(new D3(1, 1, 1)), vec.push_back(new D4(2, 2, 2))

My ouput will be 1 1 1 | 2 2 2

Can it be 1 1 1| or 2 2 2| , using the same function?

You could provide another virtual function doing some check, returning true or false depending on you wanting to print, you could check the runtime type (rtti), or you could do a `dynamic_cast` to `Dn*` (if wanting to skip `Dn`, then you skip if you don't get a null pointer). — Aconcagua, Nov 23 '21 at 11:36
[Visitor pattern](https://en.wikipedia.org/wiki/Visitor_pattern) as another alternative... — Aconcagua, Nov 23 '21 at 11:39
It looks like you have made the "a higher-dimensional vector is a kind of lower-dimensional vector" mistake. — molbdnilo, Nov 23 '21 at 11:49
`auto d4 = dynamic_cast(vec[someIndex]);` – it's a rather slow mechanism, though. Having a good design you usually don't need to fall back to dynamic casts, so maybe you should rather rethink about the latter. Are we possibly talking about an [XY problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem)? — Aconcagua, Nov 23 '21 at 12:23
@Aconcagua, thank you so much, this works for me!! No, its not a XY problem. — acidx27x, Nov 23 '21 at 12:37

score 0 · Accepted Answer · answered Nov 23 '21 at 14:23

For your particular case you can use dynamic_cast<T>(expression). See some other stack overflow answers

You can do something like this

  void output(std::vector<D2*>& vec) {
    
        for (const auto& item : vec) {
            auto ptr = dynamic_cast<D3*>(item)
            if(ptr != nullptr){
               ptr->display();
             }else{/*Do nonthing*/}
            std::cout << " | ";
        }
    
    }

Please note that this is a bad practice!

If you want this kind of functionality you should re-think that design of your program.

How do i "filter" output from vector

1 Answers1