Doc. no. J16/03-0141=WG21/N1588
Date: 14 November 2003
Project: Programming Language C++
Reply to: Beman Dawes <firstname.lastname@example.org>
Requirements and Objectives
Granularity of macros
Transition between the TR and future standards
Proposed additions to the TR
Because the Library Technical Report (TR) is non-normative, implementers are permitted to ship partial implementations or to not implement the TR at all. At least one vendor has already begun shipping a partial implementation.
Users wishing to write portable programs that use TR components must therefore determine which TR components are actually available.
For example, several Boost libraries already use a component availability macro (BOOST_HAS_HASH) to determine if a non-standard hash map is available. If present, they use hash maps to speed execution, but otherwise rely on std::map.
Similarly, users of the Library TR code may wish to code something like this:
# include <some_std_lib_header> # ifdef __TR12345_UNORDERED__ # include <unordered_map> # else # include <map> # endif
(For brevity, the example above ignores namespace and other naming issues.)
Such usage is portable if the header and component macro are specified as part of the Library TR. If not specified by the TR, then users are forced to develop their own configuration mechanisms to determine TR component availability. This would be a serious burden for users wishing to write portable code.
A second use case might occur when user code requires std::tr1 components, and wishes to issue an explicit error message when they are not present:
# include <some_std_lib_header> # ifndef __TR12345_UNORDERED__ # error Standard Library Technical Report <unordered_map> header required # endif
Note: This proposal does not deal with the question of how users request
std::tr1 components be made available by an implementation. That is an
orthogonal issue, and is already dealt with in the TR working paper.
To provide a useful level of functionality to users without unduly burdening implementers, a design needs to consider the following as at least objectives if not downright requirements:
Much of the design is driven by the Requirements and Objectives.
The <limits> header is chosen as the location of the macros since it best meets requirements (2), (5). Requirement (5) implies the macros are defined in existing headers, so a new header can't be used.
The proposed granularity is based on intuition rather than any hard-and-fast rule. If the granularity is too fine, implementations will diverge through honest confusion, and the names will be hard to remember. If the granularity is too coarse, there will be a mismatch between what components some implementers will wish to actually deliver and what components the macros identify.
Pete Becker expressed concern over the fine granularity of the macros in c++std-lib-12274. Discussion followed.
Given that implementers are already shipping partial implementations of the TR, I continue to propose moderately macros with some granularity. I'm not adverse to reducing the granularity somewhat, but I feel it greatly benefits users to have some granularity.
Several LWG members expressed concern over the transition between the TR and future standards.
Two notes have been added to the proposal to make it clear to users that in
the transition from the TR to future standards, the TR components will not
remain in namespace
std::tr1 and the configuration macros will
The proposed text below uses
__TRn as a macro name prefix, where
n is to be replaced by the ISO/IEC number assigned to the Technical Report.
Other prefixes are possible.
[Note: Should any library components from this technical report become
part of a future version of the C++ standard, they will be placed in a namespace
std::tr1. --end note]
The availability of components described in this technical report are reported by the presence of the macros described in the Component Availability Macros Table. Each macro shall be defined if and only if the entire component it identifies is available. Each macro definition shall appear in the standard's <limits> header and in all other standard library or technical report headers containing the component or partial component that it identifies . The value of each macro is unspecified.
[Note: These macros will not appear in future versions of the C++
standard. Future versions of the standard can be identified by the value of the
__cplusplus. --end note]
[Note to editor: replace n below by the ISO/IEC number assigned to the Technical Report.]
||All. [note - not defined unless all TR components are available. - end note]|
||Reference wrappers ([tr.util.refwrp.synopsis])|
||Smart Pointers ([tr.util.smartptr])|
||Function objects and higher-order programming. ([tr.func])|
||Metaprogramming and type traits ([tr.meta])|
||Random number generation ([tr.rand])|
||Mathematical special functions ([tr.rand])|
||Tuple types ([tr.tuple])|
||Unordered associative containers ([tr.hash]) [note - also defined in <functional>. - end note]|
||Regular expressions ([tr.re])|
||Fixed size array wrapper ([TBD])|
||Iterator concepts, facades, and adaptors ([TBD])|
||C99 library ([TBD])|
© Copyright Beman Dawes, 2003.
Revised 14 November, 2003