Returning temporary object and binding to const reference

Possible Duplicate:
Does a const reference prolong the life of a temporary?

My compiler doesn't complain about assigning temporary to const reference:

string foo() {
  return string("123");
};

int main() {
  const string& val = foo();
  printf("%s\n", val.c_str());
  return 0;
}

Why? I thought that string returned from foo is temporary and val can point to object which lifetime has finished. Does C++ standard allow this and prolongs the lifetime of returned object?