Don't catch what you can't handle

Fri, 07 Feb 2014 00:10:41 +0000

One of the cardinal rules of C++ exception handling is “don’t catch what you can’t handle”. Of course there are always, er, exceptions to the rule, but the basic principle always holds. The consequences of violating the rule are less severe in the .NET world, but even there it’s a good rule of thumb.

C++ function declaration tips

Wed, 06 Feb 2013 22:59:29 +0000

Creating and calling functions is one of the most fundamental tasks in C++ programming. The function declaration serves as the primary description of a function’s interface - or contract - with callers, thereby making it the most important piece of code documentation. A good function declaration should convey as much information as possible. I’m sure an entire book could be written on the topic of function interface design, but I’ll just touch briefly on a few factors to consider when designing a function’s interface.

Using /delayload to specify dependent DLL path

Sat, 26 Jan 2013 01:03:48 +0000

Let’s say you have an ObjectARX module with an implicit dependency on another DLL. Your installer puts both your ARX module and the dependent DLL into a private application folder. Windows must be able to resolve the implicit DLL dependency at load time, otherwise it will fail to load your ARX module. Alas, Windows will not automatically search for the dependent DLL in your application folder, even though your ARX module is located there. Therefore your ARX module won’t load because the dependent DLL cannot be resolved.

Polymorphic bit flags in C++

Wed, 19 Dec 2012 11:55:45 +0000

I often encounter cases in ObjectARX programming where numerous boolean flags need to be persisted as part of an AutoCAD database object. For filing and for passing around to other functions, it’s most efficient to package those bit flags into a single unsigned integer. Below is an example that demonstrates how to use a union inside a containing class that can be simultaneously used as either a single unsigned integer or as individual booleans.

Checking function return values

Thu, 30 Aug 2012 15:48:36 +0000

A lot of you are guilty of not checking return values from API function calls. I’ll bet you have a good excuse, like “it’s a lot of extra typing for no reason”, or “that function should never return an error code”.

Visual Studio 2010 Native Multi-Targeting

Thu, 07 Oct 2010 02:16:19 +0000

I was hoping that Microsoft would address the need to target earlier versions of Visual Studio within the Visual Studio 2010 IDE, but since they decided to punt, I tackled the problem myself. I’ve released my solution as an open source project named Daffodil for Visual Studio.

ObjectARX 2010: Dealing With Missing Exports

Sat, 28 Mar 2009 16:54:00 +0000

In the new ObjectARX 2010 SDK, Autodesk has added some new virtual member functions that are not exported as they should be. For example, the AcGiFaceData class has had two new virtual functions added for setting and getting the face transparency:

class AcGiFaceData: public AcRxObject
{
//[… deleted for brevity]
ACDB_PORT virtual AcDbObjectId materials() const;
ACDB_PORT virtual AcGiMapper mappers() const
virtual void setTransparency(const AcCmTransparency transparency);
virtual AcCmTransparency transparency() const
private:
AcGiImpFaceData mpAcGiImpFaceData;
};

As you can see, whoever added the new functions neglected to prefix them with the ACDB_PORT macro. ACDB_PORT evaluates to __declspec(export), which tells the compiler to export the function. Since the macro is missing, the new functions are not exported from acdb18.dll.

Since these are virtual functions, you won’t have any problems calling them through a pointer to an AcGiFaceData object that was constructed by AutoCAD. The problem arises when you derive a class from AcGiFaceData. Since the functions are not exported, the linker has no way of resolving their address for creating the virtual function table of your derived class. This results in linker errors:

acrxEntryPoint.obj : error LNK2001: unresolved external symbol “public: virtual void __thiscall AcGiFaceData::setTransparency(class AcCmTransparency const )” (?setTransparency@AcGiFaceData@@UAEXPBVAcCmTransparency@@@Z)
acrxEntryPoint.obj : error LNK2001: unresolved external symbol “public: virtual class AcCmTransparency * __thiscall AcGiFaceData::transparency(void)const " (?transparency@AcGiFaceData@@UBEPAVAcCmTransparency@@XZ)
Following is an example that results in these errors:

class AcGiFaceDataEx: public AcGiFaceData
{
public:
AcGiFaceDataEx() {}
~AcGiFaceDataEx() {}
} Test;

The only solution is to provide an implementation of the missing functions. In this case, it could be accomplished by something like this:

class AcGiFaceDataEx: public AcGiFaceData
{
AcCmTransparency mpTransparency;
public:
AcGiFaceDataEx() : mpTransparency( NULL ) {}
~AcGiFaceDataEx() { delete mpTransparency; }
virtual void setTransparency(const AcCmTransparency transparency)
{
delete mpTransparency;
mpTransparency = (transparency? new AcCmTransparency( transparency ) : NULL);
}
virtual AcCmTransparency transparency() const { return mpTransparency; }
} Test;
This will fix the linker errors, but there is no guarantee that it will work as intended. AutoCAD might access its internal transparency value directly without calling through the member functions, which means it would never “see” the transparency set through the replacement member functions. Furthermore, the addition of the new pointer member changes the size of the class, which causes AcGiFaceDataEx arrays to have a different memory footprint than AcGiFaceData arrays. Lastly, what if Autodesk fixes the problem in a future AutoCAD service pack?

The ideal solution should not change the size of the class. It should check at runtime whether the function is exported, then use the exported function if it exists. That way, code that is written now will use the exported function if and when it becomes available in a future version of AutoCAD. When the function is not exported, an alternate implementation must be provided. This is not an unusual scenario, and the solution I present for the specific case of AcGiFaceData can be adapted to the more general problem.

In the AcGiFaceData case, the missing functions are virtual functions. Knowing this, it is possible to use a trick to get the address of the real function. In the code below, the function getAcGiFaceData_vtable() constructs a temporary AcGiFaceData object, from which it extracts a pointer to the object’s virtual function table. The virtual function table is just an array of function pointers, so the address of the desired function can be obtained by indexing into the virtual function table. The question is, how far? By counting virtual functions and data members starting from the top of the class hierarchy: in this case, 6 virtual functions in AcRxObject plus 16 virtual functions in AcGiFaceData = 22.

Note that obtaining a function pointer this way relies on Visual C++ implementation details, but this is safe to do since all ObjectARX modules must be compiled in Visual C++.

Following is my solution to the missing AcGiFaceData functions:

#pragma warning(push)
#pragma warning(disable: 4608)
template < typename Src, typename Dest >
Dest force_cast( Src src )
{
union _convertor { Dest d; Src s; _convertor() : d(0), s(0) {} } convertor;
convertor.s = src;
return convertor.d;
}
#pragma warning(pop)

static FARPROC getAcGiFaceData_vtable()
{
static FARPROC rfVTable = (FARPROC**)&AcGiFaceData();
return (rfVTable? rfVTable : NULL);
}

void AcGiFaceData::setTransparency( const AcCmTransparency transparency )
{
typedef void (AcGiFaceData::F_setTransparency)( const AcCmTransparency );
static F_setTransparency pfSetTransparency = force_cast< FARPROC, F_setTransparency >(GetProcAddress( GetModuleHandleA( “acdb18.dll”), “?setTransparency@AcGiFaceData@@UEAAXPEBVAcCmTransparency@@@Z” ));
if( !pfSetTransparency )
{
static FARPROC rfVTable = getAcGiFaceData_vtable();
if( rfVTable )
pfSetTransparency = force_cast< FARPROC, F_setTransparency >( rfVTable[22] );
}
if( pfSetTransparency )
(this->pfSetTransparency)( transparency );
}

AcCmTransparency AcGiFaceData::transparency() const
{
typedef AcCmTransparency (AcGiFaceData::F_transparency)() const;
static F_transparency pfTransparency = force_cast< FARPROC, F_transparency >(GetProcAddress( GetModuleHandleA( “acdb18.dll”), “?transparency@AcGiFaceData@@UEBAPEAVAcCmTransparency@@XZ” ));
if( !pfTransparency )
{
static FARPROC rfVTable = getAcGiFaceData_vtable();
if( rfVTable )
pfTransparency = force_cast< FARPROC, F_transparency >( rfVTable[23] );
}
if( pfTransparency )
return (this->*pfTransparency)();
return NULL;
}