Re: What is the output of this program?

From:

James Kanze <kanze.james@neuf.fr>

Newsgroups:

comp.lang.c++.moderated

Date:

9 Jul 2006 17:10:33 -0400

Message-ID:

<e8rdiv$tp7$1@nntp.aioe.org>

dnguyenk@yahoo.com wrote:

//...
string ToUpper(const string& s1)
{
    string s2;
    for (int i = 0; i < s1.size(); ++i)
        if (isalpha(s1[i])) s2[i] = toupper(s1[i]);
    return s2;
}

int main()
{
      string v1[5] = {"This", "is", "a", "small", "quizz"};
      string v2[5];
      for (int i = 0; i < 5; ++i) v2[i] = ToUpper(v1[i]);

      //..Print out v2. What is the output?

     return 0;
}
A mistake made me stare at the screen for a while ...

Could you give some hints to avoid similar mistakes
systematically?

Any decent implementation of the STL, in debug mode, would have
caused a core dump. Which should have made the error pretty
obvious.

Using the usual STL idioms would have avoided the core dump in
your specific case: iterators and the function push_back should
be almost second nature if you're using the STL.

Note too that your samples doesn't begin to test the function.
There's another frequent problem present, and even replacing the
assignment to s2[i] with a push_back won't remove the undefined
behavior. The most usable function toupper is in <locale>, not
<locale.h>, and takes two arguments. A more usual solution for
something this frequent would be a functional object---why the
standard contains a function std::toupper, rather than a class
with the same name, I don't know. At the very least, I'd
extract the std::ctype<char> from the locale once, before the
loop, rather than each time through. Something like:

     std::string
     toUpper( std::string const& in )
     {
         std::string result ;
         result.reserve( in.size ) ;
         typedef std::ctype< char >
                             CType ;
         CType const& ctype
                 = std::use_facet< CType >( std::locale() ) ;
         for ( std::string::const_iterator iter = in.begin() ;
                 iter != in.end() ;
                 ++ iter ) {
             result.push_back( ctype.toupper( *iter ) ) ;
         }
         return result ;
     }

(Of course, this isn't thread safe. To make it thread safe,
you'd need to make a copy of the locale you're using. Another
reason why I prefer the functional object in this case.)

Note too that this really doesn't work in general, because in
many code sets, some (often most) of the lower case characters
are multi-byte encoded. Any reasonable test set will include
things like "M??e" (which should give "M?SSE"---or possibly
"M?SZE" in certain contexts---after toupper). The conversion of
"?" to "SS" is a bit awkward, no matter what you do, but "?" to
"?" also fails if UTF-8 is being used.

--
James Kanze kanze.james@neuf.fr
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