C++ Builder Tutorial: TStringList load from / write to file

You can read from / write to a text file with list components such as TMemo, TListBox, and so on. This is done with the functions LoadFromFile and SaveToFile, for example:

   Memo1->Lines->LoadFromFile(ATextFile);

Instead, in our example we'll use a TStringList object that reads its strings from a text file. A stringlist is a non-visual component.

We want to display individual strings by entering their "index" and clicking a button.

Because the stringlist must be available at any time, we have to construct it at start-up and destroy it before the application terminates:

• In the unit file for the application's main form, there must be a field of type TStringList in the form's declaration. In our example, a field called SL.
• An event handler for the main form's OnCreate event (executes before the form appears) should create a stringlist and assign it to the field that was declared earlier.
• The stringlist must be destroyed before the application terminates, i.e. in the form's OnDestroy event.

Building the example

1. Create a new VCL Forms application. As usual, it's best to save all the files in a new empty folder, such as \StringList.
2. In unit1.h, declare a field named SL by adding a line under private:

   private:  // User declarations
     TStringList *SL;

3. Through the Object Inspector, create an event handler for OnCreate of Form1 and complete it as follows:

   void __fastcall TForm1::FormCreate(TObject *Sender)
   {
     SL = new TStringList;
     SL->LoadFromFile("C:\\StringList\\List.txt");
   }

   Note: because in C++ the symbol \ acts as the "escape" character, we have to write \\ if we want to specify a backslash.
4. Create an event handler for the OnClose event of Form1 and complete it as follows:

   void __fastcall TForm1::FormDestroy(TObject *Sender, TCloseAction &Action)
   {
     delete SL;  // destroy the stringlist before the program terminates
   }

5. In a text editor (such as NotePad) create a text file with a few lines.
   Save it as List.txt in folder \StringList.
6. Add three components to Form1: a TButton, a TEdit and a Tlabel.
7. Set the Text property of Edit1 to 0.
   Change the property NumbersOnly to True.
8. Double click Button1 and complete the event handler as follows:

   void __fastcall TForm1::Button1Click(TObject *Sender)
   { 
     int i;
     i = StrToInt(Edit1->Text);
     if (i > SL->Count)
       ShowMessage("Line does not exist");
     else
       Label1->Caption = SL->Strings[i];
   }

9. Save all and test your program.

You can also write a text file by using the TStringList's SaveToFile() method:

   SL->SaveToFile("C:\\StringList\\List2.txt");