correct usage of ostringstream

The two functions foo() and bar() are trying to do the same thing. foo
doesn't work but bar does. My question "Is bar a leagal function or is
it treading into the undefined behaviour and I'm simply getting away
with it"? I have tried the code both in VC 6 and VC 2005. Both exhibit
the same behaviour.

As long as I use the ostringstream::str() function as the first output
variable it seems to work. As soon as it appears in a different
location, the code will crash.

#include "stdafx.h"
#include <string>
#include <sstream>

using namespace std;

std::string SomeFunc( const std::string &str )
{
   // Iterate through string and calculate a checksum
   return " checksum ";
}

void foo()
{
   const char stx = '\2';
   const char etx = '\3';

   std::ostringstream os;

   os << stx
      << "One"
      << SomeFunc(os.str().substr(1)) // crashes here
      << etx;

   std::string done = os.str();
}

void bar()
{
   const char stx = '\2';
   const char etx = '\3';

   std::ostringstream os;

   os << stx
      << "One";
   os << SomeFunc(os.str().substr(1)) // works but why?
      << etx;

   std::string done = os.str();
}

int main(int argc, char * argv[])
{
   argc; argv;

   foo(); // doesn't work
   bar(); // works fine
   return 0;
}