The Boost Concept Check Library (BCCL)

The Concept Check library allows one to add explicit statement and checking of concepts in the style of the proposed C++ language extension.

Synopsis

Generic programming in C++ is characterized by the use of template parameters to represent abstract data types (or “concepts”). However, the C++ language itself does not provide a mechanism for the writer of a class or function template to explicitly state the concept that the user-supplied template argument should model (or conform to). Template parameters are commonly named after the concept they're required to model as a hint to the user, and to make the concept requirements explicit in code. However, the compiler doesn't treat these special names specially: a parameter named RandomAccessIterator is no different to the compiler than one named T. Furthermore,

• Compiler error messages resulting from incorrect template arguments can be particularly difficult to decipher. Often times the error does not point to the location of the template call-site, but instead exposes the internals of the template, which the user should never have to see.
• Without checking from the compiler, the documented requirements are oftentimes vague, incorrect, or nonexistent, so a user cannot know exactly what kind of arguments are expected.
• The documented concept requirements may not fully cover the needs of the actual template, meaning the user could get a compiler error even though the supplied template arguments meet the documented requirements.
• The documented concept requirements may be too stringent, requiring more than is really needed by the template.
• Concept names in code may drift out-of-sync with the documented requirements.

The Boost Concept Checking Library provides:

• A mechanism for inserting compile-time checks on template parameters at their point of use.
• A framework for specifying concept requirements through concept checking classes.
• A mechanism for verifying that concept requirements cover the template.
• A suite of concept checking classes and archetype classes that match the concept requirements in the C++ Standard Library.
• An alternative to the use of traits classes for accessing associated types that mirrors the syntax proposed for the next C++ standard.

The mechanisms use standard C++ and introduce no run-time overhead. The main cost of using the mechanism is in compile-time.

Every programmer writing class or function templates ought to make concept checking a normal part of their code writing routine. A concept check should be inserted for each template parameter in a component's public interface. If the concept is one of the ones from the Standard Library, then simply use the matching concept checking class in the BCCL. If not, then write a new concept checking class - after all, they are typically only a few lines long. For new concepts, a matching archetype class should also be created, which is a minimal skeleton-implementation of the concept

The documentation is organized into the following sections.

1. Introduction
2. Motivating Example
3. History
4. Publications
5. Acknowledgements
6. Using Concept Checks
7. Creating Concept Checking Classes
8. Concept Covering and Archetypes
9. Programming With Concepts
10. Implementation
11. Reference

Jeremy Siek contributed this library. Beman Dawes managed the formal review. Dave Abrahams contributed a rewrite that updated syntax to be more compatible with proposed syntax for concept support the C++ core language.

Introduction

A concept is a set of requirements (valid expressions, associated types, semantic invariants, complexity guarantees, etc.) that a type must fulfill to be correctly used as arguments in a call to a generic algorithm. In C++, concepts are represented by formal template parameters to function templates (generic algorithms). However, C++ has no explicit mechanism for representing concepts—template parameters are merely placeholders. By convention, these parameters are given names corresponding to the concept that is required, but a C++ compiler does not enforce compliance to the concept when the template parameter is bound to an actual type.

Naturally, if a generic algorithm is invoked with a type that does not fulfill at least the syntactic requirements of the concept, a compile-time error will occur. However, this error will not per se reflect the fact that the type did not meet all of the requirements of the concept. Rather, the error may occur deep inside the instantiation hierarchy at the point where an expression is not valid for the type, or where a presumed associated type is not available. The resulting error messages are largely uninformative and basically impenetrable.

What is required is a mechanism for enforcing “concept safety” at (or close to) the point of instantiation. The Boost Concept Checking Library uses some standard C++ constructs to enforce early concept compliance and that provides more informative error messages upon non-compliance.

Note that this technique only addresses the syntactic requirements of concepts (the valid expressions and associated types). We do not address the semantic invariants or complexity guarantees, which are also part of concept requirements..

Motivating Example

We present a simple example to illustrate incorrect usage of a template library and the resulting error messages. In the code below, the generic std::stable_sort() algorithm from the Standard Template Library (STL)[3, 4,5] is applied to a linked list.

  bad_error_eg.cpp:
1 #include <vector>
2 #include <complex>
3 #include <algorithm>
4 
5 int main()
6 {
7     std::vector<std::complex<float> > v;
8     std::stable_sort(v.begin(), v.end());
9 }

Here, the std::stable_sort() algorithm is prototyped as follows:

  template <class RandomAccessIterator>
  void stable_sort(RandomAccessIterator first, RandomAccessIterator last);

Attempting to compile this code with Gnu C++ produces the following compiler error:

/usr/include/c++/4.1.2/bits/stl_algo.h: In function ‘void std::
  __insertion_sort(_RandomAccessIterator, _RandomAccessIterator) [with 
  _RandomAccessIterator = __gnu_cxx::__normal_iterator<std::complex<float
  >*, std::vector<std::complex<float>, std::allocator<std::complex<
  float> > > >]’:
/usr/include/c++/4.1.2/bits/stl_algo.h:3066:   instantiated from ‘void 
  std::__inplace_stable_sort(_RandomAccessIterator, 
  _RandomAccessIterator) [with _RandomAccessIterator = __gnu_cxx::
  __normal_iterator<std::complex<float>*, std::vector<std::complex<
  float>, std::allocator<std::complex<float> > > >]’
/usr/include/c++/4.1.2/bits/stl_algo.h:3776:   instantiated from ‘void 
  std::stable_sort(_RandomAccessIterator, _RandomAccessIterator) [with 
  _RandomAccessIterator = __gnu_cxx::__normal_iterator<std::complex<float
  >*, std::vector<std::complex<float>, std::allocator<std::complex<
  float> > > >]’
bad_error_eg.cpp:8:   instantiated from here
/usr/include/c++/4.1.2/bits/stl_algo.h:2277: error: no match for 
  ‘operator<’ in ‘__val < __first. __gnu_cxx::__normal_iterator<
  _Iterator, _Container>::operator* [with _Iterator = std::complex<float
  >*, _Container = std::vector<std::complex<float>, std::allocator<
  std::complex<float> > >]()’

In this case, the fundamental error is that std:complex<float> does not model the LessThanComparable concept. Unfortunately, there is nothing in the error message to indicate that to the user.

The error may be obvious to a C++ programmer having enough experience with template libraries, but there are several reasons why this message could be hard for the uninitiated to understand:

1. There is no textual correlation between the error message and the documented requirements for std::stable_sort() and for LessThanComparable.
2. The error message is overly long, listing functions internal to the STL (e.g. __insertion_sort) that the user does not (and should not!) know or care about.
3. With so many internal library functions listed in the error message, the programmer could easily infer that the problem is in the library, rather than in his or her own code.

The following is an example of what we might expect from a more informative message (and is in fact what the Boost Concept Checking Library produces):

boost/concept_check.hpp: In destructor ‘boost::LessThanComparable<TT>::~
  LessThanComparable() [with TT = std::complex<float>]’:
boost/concept/detail/general.hpp:29:   instantiated from ‘static void boost::
  concepts::requirement<Model>::failed() [with Model = boost::
  LessThanComparable<std::complex<float> >]’
boost/concept/requires.hpp:30:   instantiated from ‘boost::_requires_<void
  (*)(boost::LessThanComparable<std::complex<float> >)>’
bad_error_eg.cpp:8:   instantiated from here
boost/concept_check.hpp:236: error: no match for ‘operator<’ in ‘((boost::
  LessThanComparable<std::complex<float> >*)this)->boost::
  LessThanComparable<std::complex<float> >::a < ((boost::
  LessThanComparable<std::complex<float> >*)this)->boost::
  LessThanComparable<std::complex<float> >::b’

This message rectifies several of the shortcomings of the standard error messages.

• The message refers explicitly to concepts that the user can look up in the STL documentation (LessThanComparable).
• The error message is now much shorter and does not reveal internal STL functions, nor indeed does it even point to std::stable_sort.
• The presence of concept_check.hpp in the error message alerts the user to the fact that the error lies in the user code and not in the library implementation.

History

The first version of this concept checking system was developed by Jeremy Siek while working at SGI in their C++ compiler and library group. That version is now part of the SGI STL distribution. The system originally introduced as the boost concept checking library differs from concept checking in the SGI STL in that the definition of concept checking classes was greatly simplified, at the price of less helpful verbiage in the error messages. In 2006 the system was rewritten (preserving backward compatibility) by Dave Abrahams to be easier to use, more similar to the proposed concept support the C++ core language, and to give better error messages.

Publications

• C++ Template Workshop 2000, Concept Checking

Acknowledgements

The idea to use function pointers to cause instantiation is due to Alexander Stepanov. We are not sure of the origin of the idea to use expressions to do up-front checking of templates, but it did appear in D&E[ 2]. Thanks to Matt Austern for his excellent documentation and organization of the STL concepts, upon which these concept checks are based. Thanks to Boost members for helpful comments and reviews.

Next: Using Concept Checks