TITLE: std lib headers and portability

(Newsgroups: comp.std.c++, 1 Mar 2000)

BOURGUIGNON: David Bourguignon
 
> As a beginner, I need a piece of advice concerning portability of the
> std lib headers.
> 
> Let's take an example. I use two different compilers:
> - on one hand, CC (SGI MIPSpro compiler) provides the std lib <limits>
> header but do not provide the <cmath> one ;
> - on the other hand, g++ doesn't provide <limits> but do provide
> <cmath>.

CLAMAGE: Steve Clamage <stephen.clamage@sun.com>

The <limits> header is specific to C++. The <limits.h> header is inherited
from the C library and has completely different content.  The related C++
header is <climits>, not <limits>.

BOURGUIGNON:

> What is the best strategy to adopt when someone want to write portable
> code with #include of the std lib headers? Just forget it and only use
> good old C <xxx.h> headers?

CLAMAGE:

C++ inherits the 18 standard C headers in their <xxx.h> form. The C++
standard also specifies alternative <cxxx> versions. The latter versions
put all the standard declarations only in the "std" namespace. The <xxx.h>
versions typically contain declarations other than those specified by
the C and C++ standards. (For example, POSIX and XOpen specify
additional content for some of those headers, and various operating
systems typically add OS-specific declarations for historical reasons.)
Whether these additional declarations appear in the <cxxx> headers, and
whether those declarations are in namespace std, depends on the
implementation.

The C++ standard also specifies 32 additional headers that are unique to
C++. These include what is informally known as the "STL", string classes,
iostream classes, and miscellaneous runtime support declarations. These
standard C++ headers do not have a <xxx.h> form. Some C++ implementations
provide .h versions of some of those 32 headers, but you can't depend
on any particular content (because they are not standard).

Since not all C++ implementations provide the <cxxx> versions of the
18 C headers, your code will be more portable if you use the <xxx.h>
forms. For the remaining 32 C++ headers, use the <xxx> form for
portability. If the implementation provides the C++ standard library
at all, it will provide the <xxx> headers.

(Private Correspondence, 20 April 2000)

TRIBBLE: David R Tribble [mailto:david@tribble.com] 

Unfortunately, use of the <xxx.h> C++ headers has been deprecated (see
Annex D, "Compatibility Features"), i.e., they are not guaranteed to be
part of the C++ standard in future revisions.

It's a catch-22; on the one hand, current C++ implementations haven't yet
caught up with the C++98 standard, so you are safer using the C header
names (<xxx.h>), but on the other hand, future C++ implementations might
not be required to support the C headers, so you are investing in the
future by not using them.

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