C: Attempting to declare a variable results in an undeclared variable error

Question

I was just working on a little programming project, and I got a fairly common error about something being undeclared:

MP_HighLevelData.c:230:15: error: ‘RemovedUser’ undeclared (first use in this function)

Thinking I had just forgotten to declare the variable, I went along to the line in my source file, and found that the error was pointing at this line of code:

User *RemovedUser;

Odd, I can't declare a new variable because it doesn't exist? I'm sure it's not specifically this line of code that's at fault, so here's a more complete code snippet. I'd really like to know what I've done wrong.

void RemoveUserFromGameRoom(User *User) {

  if (User->GameRoom != NULL) {
    GameRoom *GameRoom = User->GameRoom;

    if (GameRoom->Owner == User) {

      // We should delete the whole game room, since the owner is leaving and a new owner isn't chosen automatically
      while (GameRoom->UsersHead != NULL) { // Awesome way of looping while there are users left in the room
        // We need to get rid of all the users in this game room, including the owner, before we can remove it
        User *RemovedUser;
        RemovedUser = GameRoom->UsersHead->User;
        DeleteUserPtrFromGameRoom(GameRoom->UsersHead); // Remove reference to the user from the game room
        RemovedUser->GameRoom = NULL; // Remove reference to the game room from the user (needs to be set afterwards, whoops)
      }

      // All the users have been kicked out, now we can take care of the game room
      FreeRIDfromGameCategory(GameRoom->RID, User->GameCategory);
      ClearGameRoomName(GameRoom);
      DeleteGameRoomFromGameCategory(GameRoom, User->GameCategory);

    } else {
      UserPtr *UserPtr = GameRoom->UsersHead;

      while (UserPtr != NULL) {
        if (UserPtr->User == User) {
          DeleteUserPtrFromGameRoom(UserPtr);
          User->GameRoom = NULL;
          break;
        }

        UserPtr = UserPtr->next;
      }
    }

  }

}

Answer 1

Usually, when faced with the decision of 'Type' or 'Variable of type', the compile will always assume 'Variable of type', which is why accessing User as object works.

However, at the same time, declaring a new object with the type User fails to work, because for the compiler, that's a variable, not a type.

In short: Rename your variable User to anything but the type name (i.e. UserObject or something), and your code should work just fine in that regard.

For the clarification, this is my suggested fix:

void RemoveUserFromGameRoom(User *myUser) {

    if (myUser->GameRoom != NULL) {
        GameRoom *GameRoom = myUser->GameRoom;
        //More code to come
    }
    //Some more code
 }