Re: C++ - how to convert string to uppercase/lowercase

From:

Rolf Magnus <ramagnus@t-online.de>

Newsgroups:

comp.lang.c++

Date:

Thu, 18 Dec 2008 18:57:15 +0100

Message-ID:

<gie2tr$ic2$01$1@news.t-online.com>

Triple-DES wrote:

On 18 Des, 11:04, James Kanze <james.ka...@gmail.com> wrote:

On Dec 17, 10:46 pm, Noah Roberts <n...@nowhere.com> wrote:

sean_in_rale...@yahoo.com wrote:

You have to create my_toupper (or use a cast) due to a
C++ wart.

What wart?

To begin with, the fact (inherited from C) that you can't call
toupper with a char without encurring undefined behavior.

I don't think that's strictly correct.
As long as the argument is an int whose value can be represented as an
unsigned char, (a value between 0 and CHAR_MAX) it should be well-defined.

Yes. As long as you _ensure_ that the function will never be called with a
character outside that range, the behaviour is well-defined.

Like his original code, it has undefined behavior. Unlike his
original code, there's also a very good chance that it won't
compile. There are two problems: the use of toupper is
ambiguous; this is easily resolved by including <ctype.h>
instead of <cctype>, and specifying ::toupper. The second
problem is more subtle: ::toupper takes an int as argument, not
a char, and it has a pre-condition that the value is in the
range 0...UCHAR_MAX (or EOF). If plain char is signed (as is
all too often the case), calling it with a plain char results in
undefined behavior. (There are other problems with this
solution, e.g. ::toupper uses mutable global state, which may
cause problems in a multithreaded environment. But they don't
apply here.)

For the reason above, I think this is well-defined unless any of the
characters in "hello world" has a negative numerical value. Which
makes the example "possibly UB", not "guaranteed UB".

I'm not sure if it's useful at all to talk about "guaranteed UB",
considering that UB means that nothing is guaranteed.