inheritance c++

Question

I have two class.

class A:

class B: public A
{
     //new function
     void setHint(...);
}

And have struct data.

typedef std::shared_ptr<A> window_ptr;
std::stack<window_ptr> m_windowsStack;
m_windowsStack.push(std::make_shared<A>("Hint"));
m_windowsStack.push(std::make_shared<B>("Game"));

And have function find in stack:

std::shared_ptr<A> WindowManager::findWindow(std::string title)
{
    ... return result;
}

And use function for find element in stack:

auto w = findWindow("Game"); //return element type B 
w->setHint(window);

But it turns out that function findWindow return type A. I get error "'class A' has no member named 'setHint' w->setHint(window);"

Do I need to declare setHint function in class A as virtual function? How do I make a variable to automatically understand that it is of type B?

Answer 1

Without understanding your entire project, I'd say the right solution is likely to add a virtual function in the base class.

class A {
 public:
  virtual void setHint(/*...*/) { /* some default implementation */}
  // or pure virtual
  virtual void setHint(/*...*/) = 0;

  virtual ~A() = default; // base class should have a virtual destructor
};

class B: public A {
 public:
  void setHint(/*...*/) override;
};

Alternatively, If you know for sure that the type pointed to the return value from findWindow is a B you can simply static_pointer_cast it down

auto w = std::static_pointer_cast<B>(findWindow("Game"));
w->setHint(window);

If you aren't sure, and A is a polymorphic type (having any virtual functions) you can dynamic_pointer_cast and check for null

if (auto w = std::dynamic_pointer_cast<B>(findWindow("Game"))) {
  w->setHint(window);
}

Answer 2

Your choices are:

1. Declare setHint virtual in class A

2. Use dynamic_pointer_cast

Thus

auto w = std::dynamic_pointer_cast<B>(findWindow("Game"));
assert(w);  // Will be nullptr if cast fails.
w->setHint(window);