c++ OOP .. iterating over list of derived objects

**rodrigorules** · 05-09-2010

If I have a base class with some pure virtual functions

and 2 derived classes which have their own additional member functions how would I iterate over a list of these and only use the ones that are of a certain derived class?

for example.. If i have Vehicle class , and 2 derived classes Car and Bike.

I create a list

Code:

list<Vehicle*> myrides;

now only Car has the drive() member function... what would be the cleanest way to iterate over the list and only access the drive() function for which are of type Car (ignoring the bikes which dont have drive() )..

Thanks! ..

( or am I doing OOP wrong? lol)

**tabstop** · 05-09-2010

[20] Inheritance -- virtual functions, C++ FAQ Lite

**anon** · 05-09-2010

It is sort of pointless to cram everything to a list<Base*>, if you now need to figure out which is which. Keep Cars in one list and Bikes in another?

**jeffcobb** · 05-09-2010

C++ Programming/RTTI - Wikibooks, collection of open-content textbooks

This will get you started but a lighter method would be to have a className() method in the base class and have each derived class provide a different name:
Base::className() returns "car"
Derived::className() returns "toyota"
DerivedFromDerived::className() returns "corolla"

**iMalc** · 05-10-2010

In OOP it is sometimes perfectly acceptable to give a definition of a virtual method that does nothing. Whilst many of the derived classes may have some special behaviour to perform, one or some of them may not have anything they need to do at all.

For example imagine a drawing program which can represent circles, squares, triangles, and hexagons. Now imagine that there is a "rotate" method which takes an arbitrary angle to rotate the object by, and rotates the object about its center. Assuming that there are no non-uniform scaling methods here, what action would the circle class need to take? None. Does this mean that a circle should not be derived from shape? No, it probably should still be derived from shape.

Depending on what "Bike" really is, it may be perfectly acceptable to provide a "drive" method that similarly does nothing.

**VirtualAce** · 05-11-2010

It is sort of pointless to cram everything to a list<Base*>,

It depends on how many heterogenous objects you intend on cramming into the list. By nature of your statement you would need separate containers for each derived type which in itself can also become a bit cumbersome. It really depends on the requirement being fulfilled and the design of the system.

**Usul** · 05-11-2010

It sounds like you need C++ code to do what the following does in lua:

Code:

if base.drive then base.drive() end

Dunno how though.

**Elysia** · 05-12-2010

Originally Posted by jeffcobb

C++ Programming/RTTI - Wikibooks, collection of open-content textbooks

This will get you started but a lighter method would be to have a className() method in the base class and have each derived class provide a different name:
Base::className() returns "car"
Derived::className() returns "toyota"
DerivedFromDerived::className() returns "corolla"

Avoid RTTI. Usually it means you have a poor design. Plus it leaves you implementation defied land if you try to get names for your types (ie classes).

**Xupicor** · 01-11-2011

Sorry for raising 1 month old thread, but isn't it a textbook example of a need for the Visitor pattern?

**VirtualAce** · 01-11-2011

It is a textbook example of bumping very old threads and an example of when I should close a thread.

Closed.

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