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?

Just cast the result, i.e. auto w = findWindow("Game"); -> auto w = static_cast<B*>(findWindow("Game")); — George
– George, Commented Jan 6, 2017 at 8:33
class A: is an error, please write real code. Also I am not sure if you intended setHint to be a variadic function — M.M
– M.M, Commented Jan 6, 2017 at 8:35
@M.M class A: {} it's abstract class, my class big and there is a lot of unnecessary variables for this question \ functions. :) — Tipok
– Tipok, Commented Jan 6, 2017 at 8:38

Ryan Haining · Accepted Answer · 2017-01-06 22:53:21Z

5

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);
}

edited Jan 6, 2017 at 22:53

answered Jan 6, 2017 at 8:35

Ryan Haining

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5 Comments

M.M Over a year ago

another option is dynamic_cast<B&>(*w).setHint(window); - this will throw an exception instead of undefined behaviour in case it is not the expected type

Ryan Haining Over a year ago

@M.M right, but not if OP is trying to not add virtual functions

M.M Over a year ago

He said in a comment to the question that A is an abstract class (implying it already contains virtual functions)

Holt Over a year ago

You might want to use cast operators for shared_ptr instead of casting the raw pointer stored.

Ryan Haining Over a year ago

@Holt indeed, that does eliminate the extra variable

Martin Bonner supports Monica · Accepted Answer · 2017-01-06 08:35:40Z

2

Your choices are:

Declare setHint virtual in class A
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);

answered Jan 6, 2017 at 8:35

Martin Bonner supports Monica

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