Robert Ramey wrote:
[Noticing that the Boost Concept Check library (BCCL) is not used as much as one might expect from the documentation, however starting to appreciate the value of concepts, and wondering whether there is really a discrepancy between the value of concepts and the extent of the adoption of the BCCL.]
If this isn't a good abstract of your email, please say so. Assuming it's good enough: I wondered about this too. I'll share my personal conclusions with you and hope it helps. For a start, I think the most important purpose of concepts is documentation. You define your concept once, at least in natural language and optionally also through the BCCL formalisms, and after that you can use just a single word again and again to refer to an entire set of operations that should be supported, as well as their semantics. If you adopt this view, it's not so strange for a library to use concepts in the documentation (verbally) but not in the code (as compile-time checks). A secondary value of concepts is that they can make compiler errors more readable. Because the BCCL was developed (almost?) entirely for this purpose, from its documentation it may seem a bit like improving compiler errors is the primary purpose of concepts in general. I think the use of the BCCL (contrary to the use of concepts for documentation) can also have disadvantages, i.e. in terms of maintainability and readability of the code. Hence, when concept requirements are fairly straightforward, the templates in question are so short that the code would almost double if you add a BOOST_CONCEPT_REQUIRES, or the compiler errors aren't too hard to understand after all, this may be enough reason for a library developer to not use the BCCL even though they do use concepts in the documentation. Note, that most (if not all) Boost libraries that don't use the BCCL still use concepts in the names of template arguments or in comments as a form of within-code documentation. For example, I'm quite sure I've seen this in the Boost.Random and the Boost.MinMax header files. Also note that concepts were voted out of the C++0x standard proposal about a year ago (IIRC). This might be partly due to the same reasons that many Boost libraries don't use compile-time concept checks. HTH, Julian