Code working inline, but when in class - c++

Question

I'm writing program using Boost::Asio, I want to implement simple chat. Currently I'm struggling with problem that when I put some code inline of class function it's working, but when same code is provided by another class object is not. I think it could be connected to Boost::Asio, but I'm not sure:
void Connection::session(socket_ptr sock)
{
try{
for(;;) {
char mesg[1024];
boost::system::error_code error;
size_t length = sock->read_some(boost::asio::buffer(mesg), error);
if (error == boost::asio::error::eof){
disconnected(sock);
break;
}
else if (error)
throw boost::system::system_error(error);
message msg(mesg,length);
char *data;
data = msg.getData();
std::cout << "In session " << strlen(data) << " " << data;
/*This is working
string s_msg,s_data;
s_msg = mesg;
s_data = s_msg.substr(2,length);
std::vector<char> writable(s_data.size() + 1);
std::copy(s_data.begin(), s_data.end(), writable.begin());
std::cout << "In session " << strlen(&writable[0]) << " " << &writable[0];
send_all(sock,&writable[0],strlen(&writable[0]));
*/
send_all(sock,data,strlen(data));
}
}
catch (std::exception& e){
std::cerr << "Exception in thread: " << e.what() << "\n";
}
}
Class message that is only parsing data
message::message(char *c_msg, size_t length)
{
msg = c_msg;
id = msg.at(0);
data = msg.substr(2,length);
}
char* message::getData()
{
std::vector<char> writable(data.size() + 1);
std::copy(data.begin(), data.end(), writable.begin());
std::cout << data;
std::cout << &writable[0];
return &writable[0];
}
So when using class message this line:
std::cout << "In session " << strlen(data) << " " << data;
I get:
st String //this is cout from message getData
st String //this is cout from message getData
In session 0
With inline version:
In session 11 st String
So, in session function string is empty although message cout shows something opposite.
I don't know if it's important, but this function is invoked as new thread from main.
Regards,
Piotr

Answer 1

You're returning the address of a temporary:
char* message::getData()
{
std::vector<char> writable(data.size() + 1);
//...
return &writable[0];
}
This is undefined behaviour.
I'm assuming that data is just a std::string. You could do this instead:
const char* message::getData() const
{
return data.c_str();
}