Improve company productivity with a Business Account.Sign Up

x
?
Solved

Inheritance and 'virtual'

Posted on 2004-08-26
8
Medium Priority
?
324 Views
Last Modified: 2013-11-15
class Map
{
   void Put(const char*, const char*);
   // ...
}

class Settings : public Map
{
   Put(const char*, const char*);
   // ...
}


Obviously this code is wrong as Map::Put() is not virtual. What I am wanting is for Settings to be able to do a small bit of processing *before* passing the parameters up to Map::Put(), but don't want the implementation of Map::Put() to be completely overridable. I don't want to have to change the inheritance to private.

Is this acheivable?
--Rob
0
Comment
Question by:boycy
  • 4
  • 3
8 Comments
 
LVL 13

Expert Comment

by:SteH
ID: 11900839
class Map
{
   virtual void Put(const char*, const char*);
   // ...
}

class Settings : public Map
{
   Put(const char*, const char*);
   // ...
}


Settings::Put (const char* pc1, const char* pc2)
{
  //do your processing here
  Map::Put (pc1, pc2);
}
0
 
LVL 6

Author Comment

by:boycy
ID: 11900949
Quote "don't want the implementation of Map::Put() to be completely overridable".

--Rob
0
 
LVL 13

Accepted Solution

by:
SteH earned 300 total points
ID: 11900974
Can you then specify what you exactly want to do. You can make the function non virtual and do the same.
class Map
{
   void Put(const char*, const char*);
   // ...
}

class Settings : public Map
{
   void Put(const char*, const char*);
   // ...
}


Settings::Put (const char* pc1, const char* pc2)
{
  //do your processing here before calling
  Map::Put (pc1, pc2);
}

or do you need only parts of the functionality of Map::Put implemented? In that case you could seperate that part into another function of Map and call this function from Map::Put and Settings::Put.
0
Free Tool: SSL Checker

Scans your site and returns information about your SSL implementation and certificate. Helpful for debugging and validating your SSL configuration.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

 
LVL 6

Author Comment

by:boycy
ID: 11901016
Overriding a non-virtual function will compile but the behaviour is undefined.
I want all of Map::Put() to be run as part of Settings::Put() after some extra checking.

Basically, the second char* for Map::Put() may be 0 and I need it to stay that way, and Map::Put() should not be completely re-implementable therefore shouldn't be virtual.
Settings::Put() needs to impose a restriction that the second char* may *not* be 0 before passing the parameters on to Map::Put().

Cheers,
Rob
0
 
LVL 13

Expert Comment

by:SteH
ID: 11901067
Why is the behaviour undefined. A virtual function will do a check at runtime which version to call. A non virtual function is linked at compile time. So you can only call Settings::Put using a pointer to or and object of class Settings. You need to check that you have a proper object for calling it. Both of these functions are defined in different namespaces (the classes) so what is undefined?
0
 
LVL 6

Author Comment

by:boycy
ID: 11901155
OK, I take it back - I'd read a couple of hours ago that overriding non-virtuals causes undefined behaviour, but just read in another place that it's legal (but immoral!).

Settings::Put(const char*psz1, const char *psz2)
{
   if (psz2 == 0)
      throw ...;
   Map::Put(psz1, psz2);
}

Assuming that Map::Put is a non-virtual function, is this completely legal valid code which won't produce any nasty behaviour or do anything unexpected?

Rob
0
 
LVL 2

Expert Comment

by:carribus
ID: 11901209
virtual functions are provided for polymorphism, i.e. using a base class pointer to call overridden functions in derived classes... so in your case:

Map*   pMapObj = new Settings;

pMap->Put();

if your Put function was virtual in Map, then in the above code, the Settings::Put() function would be called. However, if the Map function is NOT virtual, the Map::Put() function will be called.

SteH is correct. You literally override the Map::Put() function in the Settings class and then call via scoping into the Map::Put() function.
0
 
LVL 6

Author Comment

by:boycy
ID: 11901330
OK, cheers both.
0

Featured Post

Free Tool: Path Explorer

An intuitive utility to help find the CSS path to UI elements on a webpage. These paths are used frequently in a variety of front-end development and QA automation tasks.

One of a set of tools we're offering as a way of saying thank you for being a part of the community.

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

Join & Write a Comment

The article will include the best Data Recovery Tools along with their Features, Capabilities, and their Download Links. Hope you’ll enjoy it and will choose the one as required by you.
Microsoft will be releasing the Windows 10 Creators Update in just a matter of weeks. Are you prepared? Follow these steps to ensure everything goes smoothly and you don't lose valuable data on your PC.
The goal of the tutorial is to teach the user how to use functions in C++. The video will cover how to define functions, how to call functions and how to create functions prototypes. Microsoft Visual C++ 2010 Express will be used as a text editor an…
The viewer will learn how to use the return statement in functions in C++. The video will also teach the user how to pass data to a function and have the function return data back for further processing.

601 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question