MessageIn message("Hello world!"); int length = message.getLength();
MessageIn message; message.readFromSocket(socket); int length = message.getLength();In this example, a `MessageIn` object is created and then data is read from a socket using the `readFromSocket()` function. After the data is read, the `getLength()` function is called to get the length of the message in bytes, which is returned and assigned to the variable `length`. Based on the use of the `MessageIn` class and the `getLength()` function, it is likely that the package library being used is the cpp-netlib package.