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Bjarne Stroustrup On Concepts, C++0x
An anonymous reader writes "Danny Kalev has an interview with Bjarne Stroustrup about the failure of concepts, the technical issues of concepts, whether the ISO committee's days are over, and whether C++ is at a dead-end. 'I don't think that concepts were doomed to fail. Also, I don't think concepts were trying to fix many things or to transform C++ into an almost new language. They were introduced to do one thing: provide direct language support to the by-far dominant use of templates: generic programming. They were intended to allow direct expression of what people already state in comments, in design documents, and in documentation. Every well-designed template is written with a notion of what is required from its arguments. For good code, those requirements are documented (think of the standard's requirements tables). That is, today most templates are designed using an informal notion of concepts.'"
It is a big deal. The two most important things concepts were going to do was make generic programming in C++ (1) explicit and (2) accessible.
Currently, generic programming in C++ is supported through a number of template meta-programming patterns and practices, most of which exist as Boost tribal knowledge - hail King Dave!* It is implicit in many library designs, but there is nothing enforcing it at the language level. If you're not familiar with the concepts of generic programming (pun intended), it's easy to mistake what's going on in the standard libraries as something else. This is especially true if your primary use of the STL is to have polymorphic container classes - you might just design a generic extension to another language that completely misses the point of generic programming (see Java Generics).
At a more fundamental level, a lot of the power in generic programming comes from the specializations that are possible when you meet type requirements. Right now, there is no way, outside the documentation, to state requirements and possible specializations. Making this explicit in the language makes it clear to the library user what the requirements are and what specializations are available.
This leads into the accessibility problem. Generic Programming using template meta-programming is difficult. To use it, you have to understand both the template system and generic programming. The former is defined in the standard, but the latter, as mentioned above, is tribal knowledge. By making generic programming explicit in the language, it immediately becomes accessible to a large number of developers who didn't have the time, patience, or fortitude (dealing with the Boost mailing list requires a large supply of this) to become proficient with template-based generic programming.
As an analogy: consider object-oriented programming in C. Prior to (and even after) C++, lots of OO code was been developed in C. But, each object system was different and based on local best-practices. C++ (and Objective-C) took those practices and codified them into a language extension. As soon as that occurred, one method for OO was standardized. Developers no longer had to implement their own object systems and adhere to documented (but not language enforced) policies. And, with a standard set of rules around this flavor of OO, many other developers felt comfortable jumping in the the fray.
Concepts in C++ should have had the same effect for Generic Programming in C++ that C++ had for Object Oriented Programming in C. The should have democratized generic programming and brought forth a renaissance in C++ library design. Instead, petty politics killed the most exciting change to C-like languages in years.
-Chris
*(Dave - I mean that in the nicest sense... you've done a great job with Boost (oh, we need to jam again, too)).