Unexpected behaviour while using ostringstream

Question

Getting different output in following cases

std::string temp, temp1 = "foo", temp2 = "bar";
    std::vector<char> test;
    std::ostringstream s;
    s << temp1;
    temp = s.str();
    std::copy(s.str().begin(), s.str().end(), std::back_inserter(test));
    std::copy(temp2.begin(), temp2.end(), std::back_inserter(test));
    std::cout << &test[0];

Output : foo

 std::string temp, temp1 = "foo", temp2 = "bar";
    std::vector<char> test;
    std::ostringstream s;
    s << temp1;
    temp = s.str();
    std::copy(temp.begin(), temp.end(), std::back_inserter(test));
    std::copy(temp2.begin(), temp2.end(), std::back_inserter(test));
    std::cout << &test[0];

Output : foobar can somebody explain why this happened

Answer 1

The streams str function returns the string by value.

That means the two s.str() calls will return two different strings, and their respective begin and end iterators will be for different strings, making the std::copy call invalid and lead to undefined behavior.