Question

Simply put, what are the (or are there any) differences between doing say

class MyClassList : list<MyClass> { };

vs

typedef list<MyClass> MyClassList;

The only advantage that I can think of (and its what lead me to this question) is that with the derived class i can now easily forward declare MyClassList as

class MyClassList;

without compiler error, instead of

class MyClass;
typedef list<MyClass> MyClassList;

I can’t think of any differences, but this made me wonder, are there cases in which a typedef can be used that a simple derived class can’t?

Or to put it another way, is there any reason why I shouldn’t change all my typedef list<…> SomeClassList; to the simple derived class so that I can easily forward declare them?

Answer 1

Well, a typedef can only do what its name suggests while a derived class can possibly be a full-blown makeover of its base(s). So while there may not be much of a difference if you limit yourself to “just” deriving (and not add any members, or override anything, etc) as far as the compiler is concerned, there might be a big difference as far as human readers of the code are concerned.

One might wonder “why is this a derived class when a typedef would suffice”? Most people would assume that there must be a reason, so you would make life harder to the code’s future maintainers. A typedef, on the other hand, is a very specific tool and does not raise questions.

And while we ‘re on the topic of maintenance don’t forget that as most things in C++, this “nothing will go wrong as long as we are disciplined and don’t cross this line” is an open invitation to disaster. Since the compiler isn’t there to stop you, someone, someday, will cross the line.