Using "super" in C++


Question

My style of coding includes the following idiom:

class Derived : public Base
{
   public :
      typedef Base super; // note that it could be hidden in
                          // protected/private section, instead

      // Etc.
} ;

This enables me to use "super" as an alias to Base, for example, in constructors:

Derived(int i, int j)
   : super(i), J(j)
{
}

Or even when calling the method from the base class inside its overridden version:

void Derived::foo()
{
   super::foo() ;

   // ... And then, do something else
}

It can even be chained (I have still to find the use for that, though):

class DerivedDerived : public Derived
{
   public :
      typedef Derived super; // note that it could be hidden in
                             // protected/private section, instead

      // Etc.
} ;

void DerivedDerived::bar()
{
   super::bar() ; // will call Derived::bar
   super::super::bar ; // will call Base::bar

   // ... And then, do something else
}

Anyway, I find the use of "typedef super" very useful, for example, when Base is either verbose and/or templated.

The fact is that super is implemented in Java, as well as in C# (where it is called "base", unless I'm wrong). But C++ lacks this keyword.

So, my questions:

  • is this use of typedef super common/rare/never seen in the code you work with?
  • is this use of typedef super Ok (i.e. do you see strong or not so strong reasons to not use it)?
  • should "super" be a good thing, should it be somewhat standardized in C++, or is this use through a typedef enough already?

Edit: Roddy mentionned the fact the typedef should be private. This would mean any derived class would not be able to use it without redeclaring it. But I guess it would also prevent the super::super chaining (but who's gonna cry for that?).

Edit 2: Now, some months after massively using "super", I wholeheartedly agree with Roddy's viewpoint: "super" should be private. I would upvote his answer twice, but I guess I can't.

1
188
7/17/2011 7:27:57 AM

Accepted Answer

Bjarne Stroustrup mentions in Design and Evolution of C++ that super as a keyword was considered by the ISO C++ Standards committee the first time C++ was standardized.

Dag Bruck proposed this extension, calling the base class "inherited." The proposal mentioned the multiple inheritance issue, and would have flagged ambiguous uses. Even Stroustrup was convinced.

After discussion, Dag Bruck (yes, the same person making the proposal) wrote that the proposal was implementable, technically sound, and free of major flaws, and handled multiple inheritance. On the other hand, there wasn't enough bang for the buck, and the committee should handle a thornier problem.

Michael Tiemann arrived late, and then showed that a typedef'ed super would work just fine, using the same technique that was asked about in this post.

So, no, this will probably never get standardized.

If you don't have a copy, Design and Evolution is well worth the cover price. Used copies can be had for about $10.

138
7/13/2012 4:56:25 PM

I've always used "inherited" rather than super. (Probably due to a Delphi background), and I always make it private, to avoid the problem when the 'inherited' is erroneously omitted from a class but a subclass tries to use it.

class MyClass : public MyBase
{
private:  // Prevents erroneous use by other classes.
  typedef MyBase inherited;
...

My standard 'code template' for creating new classes includes the typedef, so I have little opportunity to accidentally omit it.

I don't think the chained "super::super" suggestion is a good idea- If you're doing that, you're probably tied in very hard to a particular hierarchy, and changing it will likely break stuff badly.


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