Apr 96 Top 10
Volume Number: 12
Issue Number: 4
Column Tag: Symantec Top 10
Symantec Top 10 [TOKEN:61441]
By Andy McFarland, Symantec Technical Support
Note: Source code files accompanying article are located on MacTech CD-ROM orsource code disks.
Q: I have written a constructor for my CPanorama derived class so I can pass some
specific initial values, but now all the parts of the object I specified in the Visual
Architect are coming up NULL or garbage. What’s going on?
A: When it comes time to instantiate a class created in Visual Architect, a TCL macro
new_by_name is used. It calls a (default) constructor if you have written one, or
simply an implied (default-default) constructor if you have not. In either case,
the information you specified in the VA is read in from the resource file with
GetFrom() after all the constructors in the inheritance chain for your class have
been called.
VA can call only default constructors. This includes constructors with no
parameters and those for which all the parameters have default values. If you
write a constructor that requires that at least one value be passed, you will be
creating an object separate from the one VA is going to create. You may still
initialize non-TCL members which may have been added to your VA-generated
class either in the body of a default constructor or as parameters with default
values.
Override GetFrom() if you need to use information from the VA resource file in
your intitialization. For instance, a Rect frameRect might be used to save you
from having to use the LongToQDRect() macro in toolbox calls that require a
Rect. It is perfectly valid (and may be simpler) to put all of your
construction/initialization code into GetFrom(), and not write a constructor at
all.
Q: VA conveniently creates Save and SaveAs file menu items. What do I actually
have to do to save my document to a file?
A: CDocument has a data member itsFile, a CFile*, which will point to whatever
species of CFile you actually create, such as CDatafile, CResFile or
CPictFile, etc.
PickFileName() is called from there, which puts up a StandardPutFile()
what you need to overide in your document class, i.e. in CMain. At this point you
need to allocate space for itsFile if you haven’t already.
Now just use all the nifty methods inherited from CFile, CDataFile and perhaps
CPictFile to do things like creating, opening, closing, reading and writing to the
file and changing its name. For simple file I/O, you may never have to call a File
Manager routine directly.
Q: I’m trying to use the TCL macro new_by_name() with a VA-generated class, but
it’s not working. What’s up?
A: At the beginning of the file you will see TCL_DEFINE_CLASS_M1(Cfoo,x_Cbar).
This is the macro that makes the class an RTTI class (type “Run-Time Type
Identification” into THINK Reference for a full explanation.) VA defaults to the
M1 suffix, which must be changed to D1 if new_by_name() is to be used. Note that
most people will rarely, if ever, need to use new_by_name() instead of
TCL_NEW().
Q: Is it possible to create and use a routine descriptor for a member function?
When I do so, the arguments are assigned in the wrong order.
A: No. There are various problems involved. There are some implied parameters to
member functions, like the this pointer (a pointer to the object). Further, the
parameters are passed in a different order (Pascal style) but stack cleanup is
done C-style.
Q: I’m getting error -1708, “Couldn’t complete the last command because the
AppleEvent was not handled”, when trying to build a library.
A: This usually results from the ToolServer not being installed properly. Check to
be sure there is an alias for ToolServer in the (Tools) folder. Also, be sure to
rename the alias as “ToolServer”. You may also need to copy the Toolserver
folder from the CD, Apple Software:Tools: Toolserver1.1.1, to your hard
drive.
Q: How do I get the Standard Console to accept single keystrokes without a Carriage
Return?
A: Use the csetmode() function.
#include // For Constant EXIT_SUCCESS
#include // For C++ I/O, cin and cout
//#include // If you use straight C I/O
#include // For cgetmode
main()
{
char c;
printf( "Type a character: ");
csetmode(C_CBREAK,stdin); // Allow single key inputs
c = getchar();
printf( "\nYou typed: '%c'", c, "\n" );
return 0;
}
Q: When I bring my SPM project up to date, a Build Errors window shows up but is
empty. What’s going on?
A: Your code is probably generating compiler warnings that you aren’t seeing
because the Build Errors window is set to hide them. Click on the Show
Warnings button.
Q: Can I use exception handling without the TCL?
A: To use exception handling without the Think Class Library, you must include
BRLib and Exceptions.cp in your project. Also, you must compile with the
directive:
#define NO_TCL
The four macros used to make exception handling work correctly are:
AUTO_DESTRUCT_OBJECT
TCL_NEW
TCL_END_CONSTRUCTOR
TCL_START_DESTRUCTOR
The macro AUTO_DESTRUCT_OBJECT will guarantee that the destructor is called
for an automatic object on the stack. A destructor will work only on a completely
constructed object. TCL_END_CONSTRUCTOR helps the compiler to determine the
complete construction of an object.
See the example below to see how the macros are used.
class funClass TCL_AUTO_DESTRUCT_OBJECT //macro in class header
{
public:
funClass() {
// no arg constructor
cout << "In constructor." << endl;
char * myStr = new char[64]; [TOKEN:12079] allocate memory
TCL_END_CONSTRUCTOR [TOKEN:12079] End of the constructor
}
virtual ~funClass() { [TOKEN:12079] virtual destructor
TCL_START_DESTRUCTOR [TOKEN:12079] Beginning of the destructor
cout << "In destructor." <
delete [] myStr; [TOKEN:12079] deallocate memory
}
};
Q: Okay. I’ve done everything I’m supposed to do to use exception handling without
the TCL, but I get a bunch of link errors when I try to use TCL_NEW.
A: Rebuild BR_Lib.o with
#define TCL_UNSAFE_ALLOCATION.
Q: Why does writing a text handle using CSaver cause garbage in the stream when I
read it in?
A: When you create an arbitrary length handle and then fill only part of it, the
whole handle length is written (and then read) and so you end up with garbage.
To get CSaver to work correctly with handles, you need either to create a handle
the right size to begin with, or to resize it when assigning the text to it. As an
example we will stream out a CEditText object.
Within the myContents.cp file:
// Don’t bother allocating space for the handle.
In GetFrom(), stream in the text:
stream >> myMainText;
In PutTo(), stream out the handle and kill it:
stream << myMainText;
DisposHandle(myMainText);
In CMain.cp::WindowToContents, construct the handle, and fill it:
long len = fMain_Pano3->GetLength(); // get the length we want
itsContents->myMainText = NewHandleCanFail( len );
BlockMove( *fMain_Pano3->GetTextHandle(),
itsContents->myMainText, len );
Special thanks to Michael Hopkins, Craig Conner, Glenn Austin, Scott Morison,
and Mark Baldwin.
Referenced by (1):