Calling a member function from a separate class

[pelleas]

Hello to all!

1a. I'm a newbie having trouble accessing a pointer from one .cpp file which is defined within a member function found in a separate .cpp file. I'm looking for a line or two of sample correct syntax, and/or a philosophical hint or two about structuring such requests.

Consider the following code snippet from "theFrame.c pp" (written for VC++ 2005 Express), attached. Here, a pointer 'grid' is defined, which is then used a few lines later to call wxWidgets 'spreadsheet' functions. This code works in context, creating a spreadsheet 100 rows x 13 columns, and then modifying an axis label.

wxGrid* MyFrame::Create Grid()
{
wxGrid* grid = new wxGrid(this, wxID_ANY,
wxPoint(0,0),
wxSize(150,250) ,
wxNO_BORDER | wxWANTS_CHARS);

// Create a Grid

grid->CreateGrid( 0, 0 );
grid->AppendRows(100 );
grid->AppendCols(13) ;

// Name the columns
grid->SetLabelValue( wxHORIZONTAL,_T ("ItemID"),0 );

1b. I'd like to access 'grid' from another class, in another file. For instance, in "DrawSpreadshee t.cpp" I'd like to call "grid->CreateGrid( 0, 0 );" as above. I suspect "extern wxGrid *grid" is not a valid way to bring the 'grid' pointer into this new file -- it generates an unresolved external message from the compiler. How can I access the 'grid' pointer from classes in other files?

Thanks in advance for any insight you might be able to shed...

p.s.--The attached code references wxGrid within the wxWidgets library.

[Banfa]

Your variable

wxGrid* grid;

is declared in the scope of the function

wxGrid* MyFrame::Create Grid()

It will never be viewable by any code external to this function except functions that are called by this function and passed grid as a parameter.

To access grid from any external code (in this file or another file) it needs to be declared at a different scope. You have 2 choices

1. Declare grid at file scope. Declare grid at the top of the file, then you can use a simple extern statement in your other c files (or preferably a header) to access it.

2. Declare grid at class scope. Declare grid as a member of MyFrame (possibly public). Then external files can access it through the current instance of MyFrame either directly or using a member function to return it.


Since global data is frowned upon in C++ (and C as well to some extent) I would suggest option 2, make it a member of MyFrame.

[pelleas]

Your variable
2. Declare grid at class scope. Declare grid as a member of MyFrame (possibly public). Then external files can access it through the current instance of MyFrame either directly or using a member function to return it.

Thanks, Banfa, for answering my (silly) question with great kindness...that 's much appreciated.

I realize that global variables are discouraged in C++, so I'd like to declare 'grid' as a public member of MyFrame, as you suggest. Here's what I attempted: (watch for "-->")

---

(excerpt from "theFrame.h ")

class MyFrame : public wxFrame
{

public:
MyFrame(wxWindo w* parent,
wxWindowID id,
const wxString& title,
const wxPoint& pos = wxDefaultPositi on,
const wxSize& size = wxDefaultSize,
long style = wxDEFAULT_FRAME _STYLE | wxSUNKEN_BORDER );

~MyFrame(); //prototype for destructor

--> wxGrid* grid;
...

I'm attempting to refer to data in 'DrawSpreadshee t.cpp', as below (watch for "-->")

(excerpt from "DrawSpreadshee t.cpp")

#include <stdio.h>
#include "theApp.h"
--> #include "theFrame.h "
#include "DrawSpreadshee t.h"
#include "wxIrrlichtWind ow.h"
#include "Protocol.h "
#include "ClientSocket.h "

#include "wx/grid.h"
#include "wx/generic/gridctrl.h"

extern MyApp& wxGetApp();
--> extern wxGrid* grid;

DrawSpreadsheet ::DrawSpreadshe et()
{
// Name a column as a test
grid->SetLabelValue( wxHORIZONTAL,_T ("TEST"),0);
...

I clearly have my syntax wrong and am receiving unresolved external symbol error from the compiler from DrawSpreadsheet .cpp. Would it be possible for you point out my errors?

Thanks again for your help!

[Banfa]

In excerpt from "DrawSpreadshee t.cpp" look for <--

--> #include "theFrame.h " <-- this is good and what is required

--> extern wxGrid* grid; <-- this is completely unrequired


DrawSpreadsheet :rawSpreadsheet ()
{
// Name a column as a test
grid->SetLabelValue( wxHORIZONTAL,_T ("TEST"),0); <-- wrong syntax

<-- you need an instance or pointer to a MyFrame object

MyFrame *pFrame = GetMyFrame();

pFrame->grid->SetLabelValue( wxHORIZONTAL,_T ("TEST"),0);

...
}


For the purposes of this example I have created the imaginary function

MyFrame *GetMyFrame()

You will have to find a way to implement/replace it.