Question

I am an absolute beginner to C++ and Qt.

I am broadcasting different data via TCP using Qt. The process of sending and retrieving data works fine but I have problems on interpreting the data on the receivers side.

The data is represented in different structs which have in common that they have a commandId and a state. The rest could be anything like an error message, a filename or something else. This code is not written on my own and I am not allowed to change it (e.g: define and implement a common interfaces.)

typedef struct
{
  uint8_t commandId;
  State state;
  //special data
  QString errorMessage;
} Command1;

typedef struct
{
  uint8_t commandId;
  State state;
  //special data
  uint8_t amountSensors;
} Command2;

enum State {
  STATID_PAUSE = 50000
  STATID_RECORD = 50001
  STATID_PLAY = 50002
  STATID_ERROR = 50003
}

The sender is converting a struct into a QByteArray this way:

Command1 example;
example.commandId = 134;
example.state = STATID_ERROR;

char *p_Char;
p_char = reinterpret_cast<char*>(&example);
QByteArray qba(p_char, sizeof(p_char));

Now I have to write a receiver, but the receiver doesn’t know what he gets (Command1, Command2 or something else). He would be able to interpret if he could read out the commandId and the state.

At this moment I am able to read out the commandId like this:

commandId = static_cast<uint8_t>(qba[0]);

but how could I read out the State which is an enum?

Answer 1

State values will take the size of an int. which means to access it you will do :

 State state = (State) (*( reinterpret_cast<const int*>(qba.constData()+1)) );

First you reinterpret the const char pointer as an const int pointer, then you deference it (which means you obtain the value), and you cast this value as an State.

In order to access other variables you will start at index 1 + sizeof(int) = 1+ sizeof(State)

See this thread about the size of an enum.