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Char_traits (part 2+)
- From: "Jack Reeves" <jackw_reeves at hotmail dot com>
- To: bkoz at redhat dot com
- Cc: libstdc++ at gcc dot gnu dot org
- Date: Fri, 16 Aug 2002 16:27:54 +0000
- Subject: Char_traits (part 2+)
- Reply-to: jack_reeves at bleading-edge dot com
I have now had a chance to review the changes for char_traits. You are
correct, I do have a problem (still). On the other hand, you might find my
recommendations closer to where you are headed – or further out in left
field – than you think.
Note: the rest if this is a philosophical point-of-view statement. You waste
your time reading it on your own conscious.
To summarize my interpretation of the Standard:
1. 21.1.1/1 is not a definition of template class char_traits<>. It is a
statement of requirements – just like numerous similar tables in the
2. 21.1.1/2 requires a declaration of template class char_traits<>. It is
not a definition either. It is also all we get.
3. 184.108.40.206 and 220.127.116.11 provide complete definitions of the explicit
specializations template<> struct char_traits<char> and template<> struct
char_traits<wchar_t> respectively. This suggests that the absence of a
complete definition in (2) is deliberate.
4. It is not possible to provide a generic implementation of char_traits
that meets the requirements of 21.1.1/1 without imposing additional
requirements on the character type that are not mandated by the Standard.
Given these points, I consider any definition of template struct
char_traits<> to be an implementation defined extension. Since an extension
can be and do whatever it wants, we could just draw the line there and let
it go at that.
Nevertheless, I had problems with the original char_traits implementation
because they were incorrect for most cases, and because of what I call the
“like a duck” problem. This is: if it looks like a ‘character traits’ class,
and calls itself a ‘character traits’ class, and is provided by the
implementation, then reasonable users can be excused when they think it is a
‘character traits’ class – and for getting annoyed when they discover that
it is not correct.
The changes address only part of this problem – now attempts to use
char_traits<MyChar> will compile, but not link unless the user provides the
definitions. First, the Standard is clear that adding explicit
specializations of templates for built-in types to namespace std results in
undefined behavior (18.104.22.168/1), so this doesn’t work for the expected
common cases. Additionally, the solution provides no useful functionality
for the user-defined types, so I do not see any point. In fact, it seems to
promise functionality that is NOT provided, so unless the requirement that
the users provide the function definitions is clearly spelled out somewhere,
I think it is actually more confusing than just going with only what the
What would I do?
This question is complicated by 3 issues:
1. char_traits is declared as a template.
2. Some of char_traits requirements are only applicable to iostreams usage.
3. The Standard requirement that a ‘character’ must be a POD type (added
between CD1 and CD2) makes some ‘character traits’ functions redundant.
Tackling these in reverse order:
(3) The ‘character traits’ functions assign (both versions), move, and copy
are redundant in the sense that no conforming program can tell if they are
used or not. Even length can be implemented generically. While tangential to
this discussion, I would recommend that the library implementation actually
NOT use them. The point for this discussion is that there is no reason NOT
to provide a generic definition of char_traits that provides these
(2) Let’s face it – the real issue is basic_string<>. Weirdo’s like me who
mess with clause 27 are rare and presumably should know what they are doing.
(3) We know that the language specifically forbids the instantiation of
ordinary template functions that are not used. This makes it possible to
provide functions that have more specific requirements than those imposed on
the template arguments in general.
Taking these things into account causes me to offer the following
A. Provide only what the Standard specifies without extensions. This has the
advantage that code will be more likely to be portable. It has the major
disadvantage that useful things like basic_string<unsigned char> will not
B. Provide A plus implementation defined explicit specializations of the
other built-in character types. This is my preference. I think it is in
keeping with both the letter and the spirit of the Standard. It allows the
useful things like declaring a basic_string<unsigned char>, but prevents
accidental usage of more esoteric types.
C. Provide a partial definition and implementation of char_traits<> that
would allow common cases such as basic_string<unsigned char>. Specifically
do not provide the declarations for those char_traits<> types and functions
that are intended only for iostreams support. Define eq, lt, compare, and
find in terms of operator== and operator< with the usual caveat that if
these operators are not available then those functions will not compile.
Strictly speaking, this is not correct, but this IS an extension. Obviously
it works for the built-in types. This is my second preferred choice. I think
it is weaker than (B) but only a little. It has the advantage that it allows
basic_string to be used as a more efficient container than vector for any
D. Provide a complete definition of template char_traits<> based on the
table in 21.1.1/1 but only provide implementations for those functions which
can be provided correctly and allow/force the user to provide the others.
For specializing basic_string this is almost equivalent to (C), but perhaps
slightly more useful. I would note that you can NOT define int_type to be an
unsigned int and have it work correctly. This is primarily why I prefer (C)
to (D). If you define int_type to be a std::pair<char_type, bool> where the
bool flag indicates EOF or not, you can make it work, but ideally the user
needs to provide his/her own type for int_type. This means that specializing
iostreams probably always requires a user defined char_traits
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