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Asked: 2026-06-08T00:08:04+00:00

Some background first: I am working on some legacy code that implements UI interfaces.

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Some background first:

I am working on some legacy code that implements UI interfaces. The code is structured in a way that it has a base class (actually it’s a struct) WinBase which is then derived to all sorts of graphical objects.

WinBase:
 - Button
 - DialogWin
 - (etc...)
 - EditWin

WinBase has a member WinBase **children that is used to keep tabs on all of the children of each object. These children are created and destroyed during runtime in the standard new[]/delete[] fashion.

All methods/functions of these objects assume that they are receiving a pointer to a WinBase object, and dynamic_cast<> is used to ensure/validate that the appropriate pointers are being used.

A later change to the code added two more classes/structs (ListBox and TextBox), that are derived from EditWin which in turn is derived from WinBase.

So… I have pointers to WinBase that being casted to ListBox and TextBox. As I said, dynamic_cast<> is used throughout the code, but I am not sure how safe this is when a function like this constructor is called.

TextBox(WinBase *pw,(...));

How safe is it to cast directly from WinBase to ListBox and vice versa (for instance)?

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1 Answer

  1. Editorial Team

    First of all, why would you want to cast a WinBase to a ListBox when a ListBox is given to a method expecting a WinBase? If you need a cast there, you should re-think the design, as methods expecting the WinBase should operate only on WinBase's interface.

    If, and only if, ListBox is derived from WinBase (either directly or indirectly), you can always cast from a ListBox* to a WinBase* (or reference), because a ListBox IS A WinBase.

    Casting the other way around is NOT safe, because you will never know if the WinBase* you got is actually a ListBox* or something else and that calls for trouble. You can of course use dynamic_cast<> to test if it really is a ListBox*, but any dynamic cast calls for a review of your design.

    Especially that kind of down-cast calls for trouble in a constructor, because when you are not getting what you wanted, what is your code supposed to do? Throwing exceptions, as the only way out is not such a good idea and at its best avoided at all (unless you know what you are doing).

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