ostringstream and sgetn() weirdness

From:

whatdoineed2do@yahoo.co.uk

Newsgroups:

comp.lang.c++

Date:

Mon, 02 Jul 2007 08:33:14 -0700

Message-ID:

<1183390394.279724.139740@k79g2000hse.googlegroups.com>

hi,

i have come aross a strange problem with a bit of code that uses a
ostringstream to build up a string and i extract the string into a
user buf via the sgetn() call instead of via the str() method.
however, it appears that when the contents extracted via sgetn() are
invalid for the first extract.

below is a test prog that demonstrates the issue. we are using Forte
7.0 on solaris 5.8; i dont have access to gcc (or any other c++
compiler). can someone tell me where i'm going wrong?
thanks
ray

    #include <strings.h>

    #include <iostream>
    #include <sstream>
    #include <exception>
    #include <string>

    using namespace std;

    int main(int argc, char* argv[])
    {
        try
        {
            ostringstream dump;
            size_t sz = 0;

            if (argc == 1) {
                dump << "huh?";
                sz = dump.tellp();
                char tmp[5];
                memset(tmp, 0, 5);
                dump.rdbuf()->sgetn(tmp, 4);
                tmp[4] = NULL;
                dump.seekp(0);

                // this comes out as empty string!!!
                cout << "init str='" << tmp << "'" << endl;
            }

            const char* what = "abcdefghijklmnopqrstuvwxyz";
            dump << what;
            sz = dump.tellp();

            char* s = new char[sz+1];
            memset(s, 0, sz+1);
            dump.rdbuf()->sgetn(s, sz);
            s[sz] = NULL;
            dump.seekp(0);

            cout << "inserted string is ok: " << (strcmp(what, s) ==
0 ? "yes" : "no") << endl;
            what = "aBcDeFgHiJkLmNoPqRsTuVwXyZ";
            dump << what;
            sz = dump.tellp();
            memset(s, 0, sz+1);
            dump.rdbuf()->sgetn(s, sz);
            s[sz] = NULL;
            cout << "inserted shorter string is ok: " << (strcmp(what,
s) == 0 ? "yes" : "no") << endl;

            delete [] s;
        }
        catch (const exception& ex)
        {
            cerr << ex.what() << endl;
        }

        return 0;
    }