Delphi 10. Array of TCustomWinSocket Access violation on usage - arrays

Question

I am making a Matchmaking server for my custom Battleship game. I am a beginner at network coding, so I am sorry in advance for dumb questions. :P
Now closer to the topic. I use default Delphi Server/Client Socket component. I have my array declared in the public section (clients: array of TCustomWinSocket;), then on the main frame startup I set the length to zero (setLength(clients, 0);). Now the confusing part for me: whenever I try to access any element of array program throws an Access Violation exception. I have checked in any known way, program NEVER exceeds the length of the array.
Here is code example which throws exception:
procedure Tmain.Button1Click(Sender: TObject);
var
i: integer;
begin
for i := 0 to length(clients) do begin
if assigned(clients[i]) then begin
showmessage(IntToStr(i));
showmessage(IntToStr(i) + ': ' + clients[i].RemoteAddress);
end;
end;
end;
Another example:
procedure Tmain.serverClientConnect(Sender: TObject; Socket: TCustomWinSocket);
begin
addLog('(' + Socket.RemoteAddress + ':' + IntToStr(Socket.RemotePort) + ') Клиент подключился');
if length(clients) <> 0 then begin
showmessage(IntToStr(length(clients)));
setLength(clients, length(clients) + 1);
showmessage(IntToStr(length(clients)));
clients[length(clients)] := Socket;
end
else if length(clients) = 0 then begin
showmessage(IntToStr(length(clients)));
clients[0] := Socket;
end;
end;
Basically, every time I use this array it gives an access violation. I can't get it, I limit the code to stay in the array length, but it does not. Or, perhaps, it is as usual my stupidity that caused it?
Any way, I could really use some help.
Thanks in advance! :)

Answer 1

You're running off the end of the array in this loop. Length(clients) is one higher than the last index, because the index into a dynamic array starts at zero. You have to either use Length(clients) - 1 or High(clients) as the terminator of your loop.
procedure Tmain.Button1Click(Sender: TObject);
var
i: integer;
begin
for i := 0 to High(clients) do begin // or Length(clients) - 1
if assigned(clients[i]) then begin
showmessage(IntToStr(i));
showmessage(IntToStr(i) + ': ' + clients[i].RemoteAddress);
end;
end;
end;
You have a similar error in your second code block as well, plus another one that's pretty clear:
procedure Tmain.serverClientConnect(Sender: TObject; Socket: TCustomWinSocket);
begin
addLog('(' + Socket.RemoteAddress + ':' + IntToStr(Socket.RemotePort) + ') Клиент подключился');
if length(clients) <> 0 then begin
showmessage(IntToStr(length(clients))); // Need - 1 or High() here
setLength(clients, length(clients) + 1);
showmessage(IntToStr(length(clients))); // Need - 1 or High() here
clients[length(clients)] := Socket;
end
else if length(clients) = 0 then begin // Second problem starts here
showmessage(IntToStr(length(clients))); // See text below
clients[0] := Socket;
end;
end;
The second error is attempting to assign to clients[0] when Length(clients) = 0, because that's an invalid index. If the array length is zero, there is no element 0, because the array is empty - there are no elements in it - not a single one; you can't assign a value to an array that has zero elements in it.
The proper code for that entire block would be something like this - you don't need both tests, to test for length = 0, or to jump through all the hoops:
procedure Tmain.ServerClientConnect(Sender: TObject; Socket; TCustomSocket);
begin
AddLog('Whatever you want to log.');
SetLength(clients, Length(clients) + 1);
Clients[High(Clients)] := Socket;
end;