继承和成员函数

Question

Consider I have series of derived classes for example as listed below:

class A
{
...
}

class B1 : public A //there may be many B's here say, B2, B3 etc
{
 ...
}
class C1 : public B1 //may be more C's as well
{
...
}

I would like to put all of the objects in single container, thus all would be of type class A .

Suppose I would like to add a function to class C1 , what would be the best way to achieve this? My options would be introducing it in the base class A and write the needed implementation in C1 , or, I could introduce it in C1 and do dynamic casting to access it. Which one is preferred? Is dynamic casting too expensive? (My main constrain is the run time.I have a flag in the base class to indicate what type of derived object it is, thus I do not have to dynamic cast every object in the container. Does adding unnecessary functions to base class can result in bad instruction cache use?)

Answer 1

You don't tell us the purpose of the new function in C1 , and this does affect the answer, but as rough guidelines:

• If the new function is a general behavior that you may need on any object and C1 happens to be the first user, definitely just add the interface to A .
• If the new function is specific to the C series of classes but it can follow some general pattern (for example post-processing), add a post_process method to A , override it in C1 , and have that method call private implementation methods of C1 to do the actual specific post-processing task.
• If neither of these are the case you may wish to reconsider your use of inheritance as it's possible you're using it to represent a relationship other than substitution.

Answer 2

adding a virtual function to your base class A is better because:

• you should avoid dynamic cast especially in performance sensitive code. Please see Performance of dynamic_cast?

• you should avoid having conditions to examine the object type (eg is it A, B1, or C1 ?) before performing a type-specific operation. Not only because it's slow, but also because if you do so, every time you add a new object type (eg C2) you will need to check all those conditions to see if they need to be updated.