Memory leaks and JSON data - json

Question

I have an app that reads a JSON data string from disk to create a string grid listing stocks (ticker, number held, cost) and then calls a stockbroker API to fill in current price and value. It works but the code below is causing memory leaks but whilst there are many internet posts on how to access data in a JSONArray none I could find discuss how to free allocated memory. Can anybody help ? I use Delphi Berlin 10.1 and Indy 10.6 if it matters. There are 3 pages of data, some stocks are on 2 pages, sTickers is a stringlist populated with each stock ticker (about 10 of them) and there are about 14 total stock holdings on the 3 stringgrids (tabGrids[0..2] which are on a page control.
The problem code is:
JSONArray := TJSONObject.ParseJSONValue(s) as TJSONArray;
if JSONarray = nil then
begin
ShowMessage('An error occurred reading the data file, section = [' +
iniSection[tabpage] + '].');
continue;
end;
for row := 0 to JSONArray.Count - 1 do
begin
s := (JSONArray.Items[row] as TJSONObject).Get('ticker').JSONValue.Value;
SL.Add(s);
end;
CombineStrings(sTickers, SL);
tabGrids[tabpage].RowCount := SL.Count + 2; //set row count
for row := 1 to SL.Count do
begin //add fixed data to each grid row
tabGrids[tabpage].Cells[0, row] := SL[row - 1];
tabGrids[tabpage].Cells[4, row] := (JSONArray.Items[row - 1] as TJSONObject).Get('qty').JSONValue.Value;
tabGrids[tabpage].Cells[6, row] := (JSONArray.Items[row - 1] as TJSONObject).Get('cost').JSONValue.Value;
tabGrids[tabpage].Cells[1, row] := (JSONArray.Items[row - 1] as TJSONObject).Get('name').JSONValue.Value;
if not tryStrToFloat(tabGrids[tabpage].Cells[4, row], qty) then qty := 0;
if not tryStrToFloat(tabGrids[tabpage].Cells[6, row], price) then price := 0;
tabGrids[tabpage].Cells[6, row] := FloatToStr(qty*price/100);
end;
tabGrids[tabpage].Width := tabGrids[tabpage].ColWidths[0] +
tabGrids[tabpage].ColWidths[1] + tabGrids[tabpage].ColWidths[2] +
tabGrids[tabpage].ColWidths[3] + tabGrids[tabpage].ColWidths[4] +
tabGrids[tabpage].ColWidths[5] + tabGrids[tabpage].ColWidths[6] + 18;
SL.Clear;
end;
JSONArray.Free;
I assume the (JSONArray.Items[row] as TJSONObject).Get('ticker').JSONValue.Value lines are allocating memory that I am not releasing but I do not see how to release it. Or maybe there is a better way to get the data.

Answer 1

You mention 3 pages of data. You are leaking 2 x TJasonArray, and you are freeing one TJasonArray on the last line right after an end; that doesn't have a corresponding begin.
From that I draw the conclusion that you have a loop (that you did not show) that you run three times. On each time you create a TJasonArray but you free only one, after the loop end;, Move the JSonArray.Free; to before the end;.

Answer 2

To do it simpler, you can just use a TJSONValue to parse and free it at end, without free array :
var
Value: TJsonValue
// ...
begin
Value := TJSONObject.ParseJSONValue(s);
try
Ar := Value.FindValue('item') as TJSONArray;
finally
FreeAndNil(Value);
end;
end;
Instead of :
tabGrids[tabpage].Cells[1, row] := (JSONArray.Items[row - 1] as TJSONObject).Get('name').JSONValue.Value;
You can do
tabGrids[tabpage].Cells[1, row] := JSONArray.Items[row - 1].GetValue<string>('name');
Then, you have a end; before you free the array, if it's a upper loop you will have a memory leaks. Use try finally block to free it and the TStringList.