Refer from 1 cpp file to another C++ - c++

Question

I've got some problems with refering from 1 file to another.
I've got 2 VCL Forms:
- OpenPlotFile
- SelectElement
In my Plotfile I've got an openDialog:
void __fastcall TPlotFileForm::btn_fileselectClick(TObject *Sender)
{
AnsiString message;
//Dialog opties instellen
OpenDialog->Options << ofFileMustExist;
OpenDialog->Filter ="PPD files (*.PPD) |*.ppd |PLOT files (PLOT.*) | plot.*";
OpenDialog->FilterIndex = 2;
if (OpenDialog->Execute())
{
plotFile = OpenDialog->FileName;
Itxt_plotfile->Text=plotFile;
try
{
TFileStream *plotStream = new TFileStream(plotFile, fmOpenRead);
TStringStream *plotString = new TStringStream();
plotString->CopyFrom(plotStream, plotStream->Size);
FileBuffer = plotString->DataString;
delete plotStream;
delete plotString;
message = "Make your choice what to plot";
ListBox1->Items->Add(message);
message = "Default is everything, on insulation and with automatic weepholes at 1000 mm...";
ListBox1->Items->Add(message);
message = "Accept with the OK button...";
ListBox1->Items->Add(message);
btn_OK->Enabled=true;//Knop activeren nadat file is gekozen
}
catch(EStreamError &e)
{
ShowMessage(e.Message);
}
}
}
In this file I have a plotFile, this is the file directory. I want to get that value to another form: SelectElement
How I do it now is simple: I add AnsiString plotFile; to the OpenPlotFile.h
And I include OpenPlotFile.h in the SelectElement file.
#include "PlotFileScreen.h"
void __fastcall TSelectElementForm::FormShow(TObject *Sender)
{
element *ElementArray = new element[100];
ElementArray = GetElementInfo();
Itxt_plot_file->Text = plotFile;
Itxt_ordernumber->Text = ElementArray[0].ON;
Itxt_element_id->Text = ElementArray[0].MO;
Itxt_type->Text = ElementArray[0].SN;
Itxt_concrete->Text = ElementArray[0].el_concrete_height;
Itxt_Insulation->Text = ElementArray[0].el_iso_height;
Itxt_Length->Text = ElementArray[0].el_length;
Itxt_Width->Text = ElementArray[0].el_width;
Itxt_Weight->Text = ElementArray[0].el_weight;
Itxt_slabmark->Text = "";
Itxt_reinforce->Text = ElementArray[0].OW;
}
My program compiles and it works fine, but the odd thing is, When I debug, in both files it says: plotFile = NULL.
How can I solve this? Or how can I pass the plotFile to the other file without being NULL?

Answer 1

FYI: Global variables are ugly and should be avoided when possible. Encapsulation and abstraction are your friends. But to answer your question:
If plotFile belongs to your header file by declaring it via AnsiString plotFile; each translation unit gets its own copy. You need to define your variable in one *.cpp file and declare it as extern in your header.
From the C++ Standard:
3.5 Program and linkage
When a name has external linkage , the entity it denotes can be
referred to by names from scopes of other translation units or from
other scopes of the same translation unit.
So in your header file you have to place this:
extern AnsiString plotFile;
Define your variable in one cpp file:
AnsiString plotFile;

Answer 2

Don't use global variables for this, sooner or later you will get into trouble if you will always use this. One problem it causes is that you can use only once instance of your form because it is occupying your global variable.
There is a common solution to your problem that is I suppose identical on all platforms/systems/etc.. You have to pass your variable somehow to the form/dialog/etc class instance you want to show.
It looks like you are using Borland Builder C++, or Embarcadero. Proper way to solve your problem is to add AnsiString plotFile as a member variable of SelectElement form, and set it using accessor methods. Example code for it can be found here:
http://bcbjournal.org/articles/vol2/9810/Sharing_data_and_methods_between_forms.htm