Why does deleting a TStringList pointer throw an exception? - c++

Question

I am using Embarcadero C++ Builder.
I have a function that declares a TStringList, uses it throughout the function, then deletes the object at the end of the function.
I have been using this code happily as a 32 bit application and have converted it to a 64 bit application, and now I get an "Invalid Pointer Operation" exception when trying to delete the TStringList. Any ideas?
The odd thing is I had the same trouble with another function that uses a character pointer (using new to create heap memory space) and a delete operation. I ended up creating a local buffer with stack space for that function, but I'm stuck with this one since I would like to use the TStringList object.
Here is the code:
String ReadUserConfig(String ConfigString) {
String UserConfigPath = AppDrive + "\\DC\\userconfig.csv";
TStringList *List = new TStringList;
if (FileExists(UserConfigPath)) { // file present, parse it
try {
List->LoadFromFile(UserConfigPath);
delete List;
}
catch(...) {
ShowMessage("Exception in ReadUserConfig()");
return ReturnString;
}
for (int i = 0; i < List->Count; ++i) {
String thisLine = List->Strings[i];
/* search for ConfigString in this line */
if ((thisLine.Pos(ConfigString) != 0) &&
(thisLine.Pos("USER_CONFIG") != 0)) {
/* grab everything right of ConfigString */
thisLine = thisLine.SubString
(thisLine.Pos(ConfigString) + ConfigString.Length() + 1,
thisLine.Length());
ReturnString = thisLine.Trim();
i = List->Count;
}
}
}
delete List; /* CAUSES INVALID POINTER EXCEPTION */
return ReturnString;
}

Answer 1

As stated in comments, your code has logic bugs in it, causing you to delete the List twice, or leak it completely.
Try something more like this instead:
String ReadUserConfig(String ConfigString) {
String UserConfigPath = AppDrive + "\\DC\\userconfig.csv";
try {
TStringList *List = new TStringList;
try {
if (FileExists(UserConfigPath)) { // file present, parse it
List->LoadFromFile(UserConfigPath);
for (int i = 0; i < List->Count; ++i) {
String thisLine = List->Strings[i];
/* search for ConfigString in this line */
if ((thisLine.Pos(ConfigString) != 0) &&
(thisLine.Pos("USER_CONFIG") != 0)) {
/* grab everything right of ConfigString */
thisLine = thisLine.SubString(thisLine.Pos(ConfigString) + ConfigString.Length() + 1, thisLine.Length());
ReturnString = thisLine.Trim();
break;
}
}
}
}
__finally {
delete List;
}
}
catch(const Exception &e) {
ShowMessage("Exception in ReadUserConfig()\n" + e.Message);
}
catch(...) {
ShowMessage("Exception in ReadUserConfig()");
}
return ReturnString;
}
Or, use a std::auto_ptr (pre C++11) or std::unique_ptr (C++11 and later) instead of a try/finally block:
#include <memory>
String ReadUserConfig(String ConfigString) {
String UserConfigPath = AppDrive + "\\DC\\userconfig.csv";
try {
//std::auto_ptr<TStringList> List(new TStringList);
std::unique_ptr<TStringList> List(new TStringList);
if (FileExists(UserConfigPath)) { // file present, parse it
List->LoadFromFile(UserConfigPath);
for (int i = 0; i < List->Count; ++i) {
String thisLine = List->Strings[i];
/* search for ConfigString in this line */
if ((thisLine.Pos(ConfigString) != 0) &&
(thisLine.Pos("USER_CONFIG") != 0)) {
/* grab everything right of ConfigString */
thisLine = thisLine.SubString(thisLine.Pos(ConfigString) + ConfigString.Length() + 1, thisLine.Length());
ReturnString = thisLine.Trim();
break;
}
}
}
}
catch(const Exception &e) {
ShowMessage("Exception in ReadUserConfig()\n" + e.Message);
}
catch(...) {
ShowMessage("Exception in ReadUserConfig()");
}
return ReturnString;
}