Re: Cannot reset an ostrstream back to start position

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#756

From: John Harrison
Date: Sat, 07 Apr 2001 14:33

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"Nicolas Reimen" <Nicolas.reimen@acm.org> wrote in message
news:9amrrc$1v3r$1@news5.isdnet.net...
> This code :
>
> #include <string>
> #include <iostream>
> #include <strstream>
>
> using namespace std;
>
> int main(int argc, char* argv[])
> {
>  ostrstream ss;
>
>  ss << "foo "<< 1234 << ends;
>  printf("%s\n", ss.str());
>  printf("tellp %d\n", ss.tellp());
>  ss.seekp(0);
>  printf("tellp %d\n", ss.tellp());
>  ss << "bar "<< 5678 << ends;
>  printf("%s\n", ss.str());
>
>  return 0;
> }
>
> should ( from my naive point of view ) produce the following output:
>
> foo 1234
> tellp 9
> tellp 0
> bar 5678
>
> But, what I get is :
>
> foo 1234
> tellp 9
> tellp 0
> foo 1234
>
> The insert pointer is correctly set back to 0, it seems, but
> the ostrtream still does not want to overwrite what is
> already in the buffer.
>
> Can anyone help ?
>
> PS: I am using MS VC++6 and I found the advice to
> use seekp in msdn, but without more details or
> an example :-(
>

You'd be better off switching to stringstream it's much easier to handle.
However your problem is that

ss.str()

freezes the strstream, preventing further modifcation. You need to add

ss.freeze(false);

after the call to ss.str(). Without this call you also have a memory leak so
you should add it after the second call to ss.str() as well (or delete the
memory by hand).

john

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