Terje Slettebø <tslettebo@broadpark.no> writes:
Colleagues:
Are you tired of long and incomprehensible error messages when using STL, MPL, or some other modern template library? So am I!
Up to now, we've had the Boost Concept Check Library, but unfortunately, you may still get long error messages with it (in the case of Intel C++ 6.0 and STLPort, pages and pages long, although the cause of the error is usually more clear, due to words like "concept_check", "function_requires", etc. showing up in the error message), and you can't overload on concepts, i.e. having several overloaded functions taking different kinds of iterators (such as std::advance).
However, the invention of enable_if [HJW03] [JWL03] (now part of Boost) opened the door to overloading based on arbitrary properties of types. Now, what we needed was techniques for detecting type properties, which could be used to create "concept type traits". Using this, we could have done, e.g. for std::sort:
template<class Iterator> typename enable_if<is_random_access_iterator<Iterator> >::type sort(Iterator begin, Iterator end) { // ... }
enable_if doesn't always improve error messages. Sometimes you get a long list of candidate functions when what you really wanted was a message from inside just one of the overloads that indicates how concept conformance was violated.
Besides enable_if, there have also been other discoveries paving the way for constrained genericity (programming with concepts) in the later years. One of them being isAddable [KJSMA03], which could detect if there exists a "+" operator for the values "a" and "b" in "a + b", and gives a true/false result. The operator could be built-in, member function, or free function; it didn't matter.
Other work on type property introspection in C++ has been done by other people, notably Paul Mensonides, with techniques for detecting member types, (non-static) member variables and functions, static member variables and functions, etc., using SFINAE to great effect.
The original isAddable required exact overload match - no conversion on the arguments was allowed, so int + int gave true, but int + double gave false, even though 1 + 1.0 is well-formed - but using a modified version, it's possible to allow the normal promotions/conversions on the arguments, as well, letting int + double give true.
In contrast, the other mentioned detection techniques requires an exact signature match, so these two ways complement each other: The exact match way may be used where the promotions/conversions way can't be used, and vice versa.
Last spring (after the introduction of enable_if and isAddable) I realised that we now for the first time had discovered or invented all the components needed for a decent implementation of constrained genericity (or programming with concepts) in the current C++ language (as shown with the code example above)!
I therefore started on a library of type traits for detecting the components required for concept checking, which meant detecting such things as operators, constructors (to the degree it's possible), etc. The modified isAddable is a key technique used in this "operator traits" part of the library.
Isn't that what we've already been discussing in http://news.gmane.org/find-root.php?message_id=%3cchr3k4%241b1%241%40sea.gma... (http://tinyurl.com/4fx3u) ?
When this was complete, I continued with the "concept traits" part of the library, where the aim was to make type traits for all the C++ standard concepts, as well as all the Boost.MPL concepts (for the reasons that MPL is widely known and used, it's part of Boost, it could help a lot with better error messages in such metaprogramming libraries, and as an example of implementing non C++ standard concepts).
I was about halfway done with the library last summer, and then I got a new job, where I became very busy, so unfortunately I couldn't continue with it at that time. However, a while ago, I've resumed work on the library, and it's now complete.
Besides the mentioned enable_if, isAddable, and the introspection techniques, MPL is a core component of the library. Its lazy evaluation logical operators are indispensible for many of the concept traits, particularly the more complex ones. I couldn't have done it without any of you. Thank you.
MPL is also used in the unit tests, generating the test matrix on the fly, testing each trait in a list against a list of types, and comparing the result against a table. The total number of tests generated is several thousand.
Other approaches -------------------------
About a year ago, there have been proposals/discussion papers for adding support for concepts to the C++ language (see N1510, N1522 and N1536 at http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/). Using this, the above example could have been written as:
<snip real concepts> Real concepts have the advantage of being able to dispatch to the "most-refined" overload.
Nevertheless, the operator traits/concept traits library is something that can be used _now_, and which may give us more experience with programming with concepts. If the traits are used with libraries like STL or MPL, it has the potential to give drastically better diagnostics, and thereby making them easier to use. In addition, overloading on concepts is possible.
A few of the traits (is_assignable, is_default_constructible and is_copy_constructible) rely on BCCL as a "last resort" when used for user-defined types, as there is no known way of detecting these for user-defined types.
The library, including documentation, examples and tests may be found at (http://home.broadpark.no/~terjesl/operator_concept_traits.zip) (Yahoo Files is full, could we perhaps remove some of the stuff there that is now part of Boost?).
Here's the synopsis from the documentation:
Synopsis ------------- The library contains:
- 46 operator type traits, covering all the overloadable (and therefore detectable) operators, as well as detection of default constructor, copy constructor and assignment operator.
- 37 C++ standard concepts type traits, covering all the concepts of the C++ standard.
- 19 Boost.MPL concepts type traits, covering all the documented concepts of Boost.MPL.
Things have changed quite a bit since the last release of MPL. Are you sure you're up-to-date?
P.P.S. I've also seen the last days there have been discussion on the Boost list on similar facilities (the "Proposal for 'is_dereferenceable' and other templatemetafunctions" thread started by Alex Chovanec). is_dereferenceable<T> is similar to this library's has_dereference_op<T> (in the operator traits part), except that the latter doesn't test for lvalue, etc.
I think the big advantage of Daniel Wallin's last crack at that is that it provides the means to *generate* new metafunctions for detecting namespace-scope-overloadable functions (including operators) via a macro. I think providing the automation is probably more important than providing a sweeping catalog of individual metafunctions, though I could be wrong. I think I'd rather see the concept detection metafunctions in the libraries that define those concepts, rather than concentrated in one place... ... this is a similar problem to the problem of adding stream inserters and extractors, in terms of dependencies, except that most of the libraries use the MPL in one way or another already. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com