# Wednesday, 12 August 2009
There was a bit of a kerfuffle in the C++ standards world in mid July arising from the summer meeting of the ISO C++ committee. Agreement couldn't be reached on how to implement concepts. In the end it was decided not to include them in the next version of the standard.

What are concepts? They improve the experience for template consumers. C++ templates are amazingly powerful, but using them can involve a lot of guesswork and a lot of swearing. They're pretty easy to write, and just by typing a line of code you create a requirement, a pre-requisite, for anyone who wants to use your template. If you write a template that takes two instances of class T called t1 and t2, and then somewhere it says t1 + t2 or t1 < t2, then your template can only be used with types T that have the operator + or the operator <. Anyone who uses it for some other type is going to get an error message that might be useful, but might be your typical nasty template-related error message. That makes templates easy to write, but hard to consume. Short of actually reading through the code for the template, you can't know before you try compiling whether or not your consuming code actually meets the requirements for using the template.

Generics in .NET deal with this through constraints - the generic writer identifies the interfaces that must be implemented by any type that will use the generic - IComparable or IEnumerable etc. If you want to use the generic, you  implement the interface, and the compiler will help you get that done with readable error messages.

So that was the heart of concepts. Some sort of decoration you would put on your template to say "here is what I expect of those that use me." How important are they in general, and how important to C++0x?
  • concepts’ presence or absence in C++0x just won’t make much difference to most users - Herb Sutter
  • [concepts were yanked out and that is] a major setback for C++, but not a disaster - Bjarne Stroustrup
  • removing them was our best available option both for C++0x and for concepts as a language feature - Doug Gregor
  • Not the end of the world, but disappointing nevertheless - Alex Fabijanic
What do I think? I think that template consumers fall into two large categories - those who consume templates they wrote themselves, and those who consume the STL. If you wrote it, I hope you know what it needs, and if it's the STL, well you can paste your opaque error message into a search engine and someone's blog will tell you what you need to add to your class to use that template. I don't think there are a lot of people consuming poorly documented and sparsely used templates. I agree that concepts could result in that sector growing some day, but that few developers are walking around wishing for them or feeling the pain of not having them. I'm sure it's a disappointment to everyone who put so much work in to them, and who will continue to work on them towards inclusion in a future version of the standard, but the rest of us can relax and get ready to see the other C++0x goodies included in a standard where x might actually fit in a hex digit.


Wednesday, 12 August 2009 14:59:55 (Eastern Daylight Time, UTC-04:00)  #    Comments [0]