Re: std::string in C-style in interface

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 10 Sep 2010 03:34:33 -0700 (PDT)

Message-ID:

<5d52f1cc-051c-4255-bd8d-2b076a81d011@z25g2000vbn.googlegroups.com>

On Sep 10, 9:36 am, James Kanze <james.ka...@gmail.com> wrote:

On Sep 9, 11:49 pm, Seungbeom Kim <musip...@bawi.org> wrote:

On 2010-09-08 23:53, Kai Wen wrote:

On 2010-09-09, SG <s.gesem...@gmail.com> wrote:

  vector<char> temp;
  temp.reserve(thestring.size()+1);
  temp.assign(thestring.begin(),thestring.end());
  temp.push_back('\0');
  foo(&temp[0]);
  thestring = &temp[0];

But I think this method is too complex. :-(

Maybe the four lines before calling foo can be shortened to two:
const char *b = thestring.c_str(), *e = b + thestring.size() + 1;
vector<char> temp(b, e);

Or even to:
        std::vector<char> temp(thestring.begin(), thestring.end());
Similarly, the last line could just as easily be:
        thestring.assign(temp.begin(), temp.end());

The best solution depends on context, however. If the legacy
function is actually returning a string value in the buffer (and
doesn't use the initial contents), something like the following
may be appropriate:

        std::string
        getCurrentWorkingDirectory()
        {
                std::vector<char> results( 1000 );
                errno = 0;
                while ( getcwd(&results[0], results.size() ) == NULL
                                && errno == ERANGE) {
                        results.resize( 2 * results.size() );
                        errno = 0;
                }
                if ( errno != 0 )
                        throw SomeError();
                return std::string( results.begin(), results.end() );
        }

(I've used the Posix function getcwd as an example, but of
course there are tons of functions for which this would be
appropriate, but under Posix and under Windows.)

Sorry for responding to my own posting, but there's an obvious
bug in the above: the return should be:
    return std::string( &results[0] );

For that matter, you could just use std::string instead of
std::vector; you'd then want to add:
    results.resize( strlen( results.c_str() ) );
before returning (or return a new string:
    return std::string( results.c_str() );

--
James Kanze