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'.c' vs '.cpp'

Posted on 1998-04-18
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Last Modified: 2013-12-03
I have a program called 'app.c' that opens a dialog box. However when I rename the file to 'app.cpp' I get an error.

The line that gives the error is:
DialogBox ( hinstance, "SelWND", hwnd, WndDialogProc) ;
and the error message is:
error C2664: 'DialogBoxParamA' : cannot convert parameter 4 from 'int (void *,unsigned int,unsigned int,long)' to 'int (__stdcall *)(void)'

Declaration for WndDialogProc is :
BOOL    CALLBACK WndDialogProc (HWND, UINT, WPARAM, LPARAM) ;

Why does this error happen? I wanted to use classes/C++ without using MFC.
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Question by:vsinha
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7 Comments
 
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Accepted Solution

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alexo earned 60 total points
ID: 1298522
Because C++ does not implicitly convert between pointers to different function types while C does.

Your dialog procedure should be declared as:
BOOL CALLBACK WndDialogProc(HWND hwndDlg, UINT uMsg, WPARAM wParam, LPARAM lParam);

And
  #define STRICT
before you
  #include <windows.h>

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Author Comment

by:vsinha
ID: 1298523
I was missing the following line:
#define STRICT

why is it needed? what does it mean?
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LVL 10

Expert Comment

by:RONSLOW
ID: 1298524
You need to #define STRICT, which means to do strict type checking

It ensures that handle types and function prototypes much match exactly.  With C it helps catch errors, with C++ it makes code compile (as C++ already checks that function pointers match ok)

When STRICT is on, DLGPROC is defined as

typedef BOOL (CALLBACK* DLGPROC)(HWND, UINT, WPARAM, LPARAM);

When it is off it is just

typedef FARPROC DLGPROC;

where FARPROC is just typedef int (FAR WINAPI *FARPROC)();

This won't match with your WndDialogProc function in C++.  This is because in C the () in a function pointer declaration means there could be any args at all, and so arg type checking isn't done.  In C++ arg type checking is always done and () means no args at all - the same as (void).

In general is is a good idea to always use #define STRICT for both C and C++.  But it is almsot ESSENTIAL for C++. (C may compile without it, but C++ won't).

I hope this clears things up for you.
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Author Comment

by:vsinha
ID: 1298525
Thank a lot!

btw, what is the advantage of not having #define STRICT ? or, in other words why does it exist, it seems to me that all programs should always use it.
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Expert Comment

by:alexo
ID: 1298526
For compatibility with code that was (poorly) ported from Win3.x.
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Expert Comment

by:RONSLOW
ID: 1298527
Shame I didn't get offered any points for my lucid explanation .. that's life I guess :-(
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Author Comment

by:vsinha
ID: 1298528
Ronslow, check Q.10048982
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