Issue with stringstream data getting corrupt.

Question

I have an issue using stringstreams

When I run this code the first printf is fine but at some point it gets contaminated and prints out a shorter string.

string CJpsURI::getURIwithSipIpPort()
{
    stringstream ss;
    ss << "sip:" << m_ipAddr << ":" << m_portAddr;
    string out = ss.str();
}

main() {

    CJpsURI localIp();
    localIp.setIpPort("192.168.88.1", 5060);

    char *s = localIp.getURIwithSipIpPort().c_str();

    printf("This is the sip port: %s", s);  // this may be ok  -- sip:192.168.88.1:5060
    // do some stuff
    printf("This is the sip port: %s", s);  // wrong; -- sip:192.168.8/030/004

}

It almost appears that the *s is pointing to the out string on the stack which gets destroyed. But that should not be happening since I am returning out and not a reference to out.

But this seems to work.

string CJpsURI::getURIwithSipIpPort()
{
    string out = (boost::format("sip:%1%:%2%") % m_ipAddr % m_portAddr).str();
    return out;
}

main() {

    CJpsURI localIp();
    localIp.setIpPort("192.168.1.1", 5060);

    char *s = localIp.getURIwithSipIpPort().c_str();

    printf("This is the sip port: %s", s);  // this may be ok
    // do some stuff
    printf("This is the sip port: %s", s);  // this will be ok too;

}

Any ideas would be appreciated

Answer 1

You have two problems:

1. As remyabel and Galik pointed out: Your function does not return. So I would Concur with Galik that you need to adapt your function to:

string CJpsURI::getURIwithSipIpPort()
{
 stringstream ss;
 ss << "sip:" << m_ipAddr << ":" << m_portAddr;
 return ss.str();
}

2. A char* points to an array of chars. Here you want the array found inside the string returned by getURIwithSipIpPort. But that memory will be released as soon as this line ends! There is nothing hanging on to it. So what you really need to do is:

   string s{localIp.getURIwithSipIpPort()};

Answer 2

Your first function does not return anything:

string CJpsURI::getURIwithSipIpPort()
{
    stringstream ss;
    ss << "sip:" << m_ipAddr << ":" << m_portAddr;
    string out = ss.str(); // This does not get returned!
}

Try this:

string CJpsURI::getURIwithSipIpPort()
{
    stringstream ss;
    ss << "sip:" << m_ipAddr << ":" << m_portAddr;
    return ss.str(); // Now it returns something!
}

Answer 3

The C++ standard says:

§ 6.7/2 [..] Flowing off the end of a function is equivalent to a return with no value; this results in undefined behavior in a value-returning function.

The second problematic statement is:

char *s = localIp.getURIwithSipIpPort().c_str();

First, the conversion from const char* to char* is deprecated. Use const char*. Second, don't do this in the first place. If you want to prolong the lifetime of a temporary, use a const reference.

const std::string& s = localIp.getURIwithSipIpPort().c_str();