Hi there, thanks again for the quick (and helpful) response. I'm getting ever closer, but still have a few more questions. Here is where I'm at: (I have attached a visual studio project that demonstrates most of this)
1) I can copy the vole::object to another local variable, but not to a member variable in my class
2) I can access the vole::object through a pointer, but only until the original object goes out of scope and gets destroyed. I then throw an exception.
3) Do I have to use wide character arrays for all of the arguments to vole::object::invoke()? How should I handle arguments that I want to pass around internally as normal strings?
4) Do I have to use SAFEARRAY to interface with QuickOPC? It seems somewhat cumbersome for simply reading and writing a bunch of tag values. Is there an option of using stl::vectors or even just plain C arrays?
5) If I am using OPC locally on a single computer, do I likely need to use the whole VTQ interface? Can I get away with "ReadMultipleItemValues" and "WriteMultipleItemValues"?
6) What is the best reference document for getting the proper syntax for QuickOPC function calls? I have been looking at "Concepts-QuickOPC-Classic" (mostly pages 38-45) which has verbal descriptions, but not the kinds of full function prototypes I am used to seeing for library documentation.
7) I have temporarily given up on using a non-microsoft compiler because I don't have time to fight that battle right now. From what I have read online, vole/stlsoft should work just fine with gcc, but I couldn't get it working in the time I had. It is still very important for me to use as much standard C++ as possible so that I can migrate later.
Thanks again,
Neal

OK, I think I am finally back to working on my opc interface again. Based upon your vole/stlsoft sample code, I am able to read data values from my PLC into a standard c++ program. (Which is a great step forward, thank you!) However, I have run into a few additional questions. Hopefully, these are easy ones....
1) I am trying to encapsulate the opc interface (mainly to hide plc tag names) in a simple c++ class. My starting assumption is that I should only create the connection once (say, in the constructor) and then subsequent calls to the class should be able to jump right to readItemValue calls. (Is this correct?) However, the object::create static functional call returns an object (rather than a pointer) and I am having trouble keeping it in scope unless I call object::create in the same function as readItemValue. I tried creating a member variable within my class of type vole::object and assigning it with object:create in my constructor, but the compiler complains about accessing a private = operator. I can create a pointer (vole::object*) and point it to easyDAClient, but that only seems to work until the original easyDAClient goes out of scope and gets destroyed. Any suggestions? How attrocious is it to call object::create before every read or write?
2) While vole/stlsoft seem to be working, I cannot claim to actually understand them yet and their documentation is not very helpful. How do I translate things like multi-item read and write operations from the atl-based example at the beginning of this thread to the vole/stlsoft syntax?
Thank you so much for all of your help. In the end, my use of opc should be relatively simple (just read and write data from a local PLC) so I hope that I am getting quite close.
Neal

Thank you! I was able to load, compile, and run your MSVS project in VS 2010 express! I have not yet tried to do anything useful with it (like communicate with our real opc server) but now that the compiling problems are out of the way I can actually try once I have some time (which unfortunately might be a while...)
I may try to compare your (working) VS project file with my (not working) VS project file to try and understand what I was doing wrong in the setup. My hope (possibly overly optimistic) is that understanding this might also shed some light on the troubles I was having using gcc. Any ideas or suggestions anyone has on this topic would of course be greatly appreciated. I have attached my super-simple (and not working) VS project in case that is helpful.
Thanks again for all of your help.

After moving to a new building and getting sucked into a few other projects, I am finally back to trying to get QuickOPC to work via StlSoft/Vole. Unforunately, I don't seem to be getting very far and I suspect that it is because I am missing something very basic.
My goal had been to use the mingw/gcc compiler since I am stuck in the windows world for now. From what little documentation I can find on StlSoft, it claims to support gcc natively, but since I was having so many problems I decided to try and get it working in Visual Studio first since that is what you tested with.
In both Visual C++ 2010 Express (free) and an old version of Visual Studio (2005) I created a bare minimum CLR Console app from the startup wizard and added the comstl and vole includes as below:
// VoleTest.cpp : main project file.
#include "stdafx.h"
#include
#include
using namespace System;
int main(array ^args)
{
Console::WriteLine(L"Hello World");
return 0;
}
The comstl include doesn't create a problem right away, but as soon as I uncomment the vole.hpp include I get a bunch of linker errors

---

Build started: Project: VoleTest, Configuration: Debug Win32

---

Linking...
VoleTest.obj : error LNK2028: unresolved token (0A0005FF) "extern "C" long __stdcall VariantCopy(struct tagVARIANT *,struct tagVARIANT *)" (?VariantCopy@@$$J18YGJPAUtagVARIANT@@0@Z) referenced in function "public: __thiscall stlsoft::comstl_project::variant::variant(class stlsoft::comstl_project::variant const &)" (??0variant@comstl_project@stlsoft@@$$FQAE@ABV012@@Z)
VoleTest.obj : error LNK2028: unresolved token (0A000602) "extern "C" long __stdcall VariantClear(struct tagVARIANT *)" (?VariantClear@@$$J14YGJPAUtagVARIANT@@@Z) referenced in function "public: static void __cdecl stlsoft::comstl_project::VARIANT_policy::clear(struct tagVARIANT *)" (?clear@VARIANT_policy@comstl_project@stlsoft@@$$FSAXPAUtagVARIANT@@@Z)
VoleTest.obj : error LNK2028: unresolved token (0A000604) "extern "C" void __stdcall VariantInit(struct tagVARIANT *)" (?VariantInit@@$$J14YGXPAUtagVARIANT@@@Z) referenced in function "public: static void __cdecl stlsoft::comstl_project::VARIANT_policy::init(struct tagVARIANT *)" (?init@VARIANT_policy@comstl_project@stlsoft@@$$FSAXPAUtagVARIANT@@@Z)
VoleTest.obj : error LNK2028: unresolved token (0A000692) "extern "C" unsigned int __stdcall SysStringLen(wchar_t *)" (?SysStringLen@@$$J14YGIPA_W@Z) referenced in function "public: unsigned int __thiscall stlsoft::comstl_project::bstr::length(void)const " (?length@bstr@comstl_project@stlsoft@@$$FQBEIXZ)
VoleTest.obj : error LNK2028: unresolved token (0A000696) "extern "C" void __stdcall SysFreeString(wchar_t *)" (?SysFreeString@@$$J14YGXPA_W@Z) referenced in function "public: __thiscall stlsoft::comstl_project::c_str_VARIANT_proxy_w::~c_str_VARIANT_proxy_w(void)" (??1c_str_VARIANT_proxy_w@comstl_project@stlsoft@@$$FQAE@XZ)
VoleTest.obj : error LNK2028: unresolved token (0A000697) "extern "C" wchar_t * __stdcall SysAllocString(wchar_t const *)" (?SysAllocString@@$$J14YGPA_WPB_W@Z) referenced in function "public: __thiscall stlsoft::comstl_project::errorinfo_desc::errorinfo_desc(class stlsoft::comstl_project::errorinfo_desc const &)" (??0errorinfo_desc@comstl_project@stlsoft@@$$FQAE@ABV012@@Z)
VoleTest.obj : error LNK2019: unresolved external symbol "extern "C" wchar_t * __stdcall SysAllocString(wchar_t const *)" (?SysAllocString@@$$J14YGPA_WPB_W@Z) referenced in function "public: __thiscall stlsoft::comstl_project::errorinfo_desc::errorinfo_desc(class stlsoft::comstl_project::errorinfo_desc const &)" (??0errorinfo_desc@comstl_project@stlsoft@@$$FQAE@ABV012@@Z)
VoleTest.obj : error LNK2019: unresolved external symbol "extern "C" void __stdcall SysFreeString(wchar_t *)" (?SysFreeString@@$$J14YGXPA_W@Z) referenced in function "public: __thiscall stlsoft::comstl_project::c_str_VARIANT_proxy_w::~c_str_VARIANT_proxy_w(void)" (??1c_str_VARIANT_proxy_w@comstl_project@stlsoft@@$$FQAE@XZ)
VoleTest.obj : error LNK2019: unresolved external symbol "extern "C" unsigned int __stdcall SysStringLen(wchar_t *)" (?SysStringLen@@$$J14YGIPA_W@Z) referenced in function "public: unsigned int __thiscall stlsoft::comstl_project::bstr::length(void)const " (?length@bstr@comstl_project@stlsoft@@$$FQBEIXZ)
VoleTest.obj : error LNK2019: unresolved external symbol "extern "C" void __stdcall VariantInit(struct tagVARIANT *)" (?VariantInit@@$$J14YGXPAUtagVARIANT@@@Z) referenced in function "public: static void __cdecl stlsoft::comstl_project::VARIANT_policy::init(struct tagVARIANT *)" (?init@VARIANT_policy@comstl_project@stlsoft@@$$FSAXPAUtagVARIANT@@@Z)
VoleTest.obj : error LNK2019: unresolved external symbol "extern "C" long __stdcall VariantClear(struct tagVARIANT *)" (?VariantClear@@$$J14YGJPAUtagVARIANT@@@Z) referenced in function "public: static void __cdecl stlsoft::comstl_project::VARIANT_policy::clear(struct tagVARIANT *)" (?clear@VARIANT_policy@comstl_project@stlsoft@@$$FSAXPAUtagVARIANT@@@Z)
VoleTest.obj : error LNK2019: unresolved external symbol "extern "C" long __stdcall VariantCopy(struct tagVARIANT *,struct tagVARIANT *)" (?VariantCopy@@$$J18YGJPAUtagVARIANT@@0@Z) referenced in function "public: __thiscall stlsoft::comstl_project::variant::variant(class stlsoft::comstl_project::variant const &)" (??0variant@comstl_project@stlsoft@@$$FQAE@ABV012@@Z)
C:\Users\tanner\Documents\Visual Studio 2005\Projects\VoleTest\Debug\VoleTest.exe : fatal error LNK1120: 12 unresolved externals
Build log was saved at "file://c:\Users\tanner\Documents\Visual Studio 2005\Projects\VoleTest\VoleTest\Debug\BuildLog.htm"
VoleTest - 13 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
I have downloaded the stlsoft and vole headers and added their paths as include directories (as evidenced by the fact that the #include did not fail) but I am clearly doing something wrong in setting up the project. As is probably obvious, I am a complete neofite with Visual Studio (part of why I wanted to use gcc) so please forgive my ignornace. Does anyone have any suggestions of what I am missing? Can you send me the complete visual studio project/solution?
Thanks again for all of your help. I still desperately want to get an opc client running because the alternative looks even worse....
Neal

Thanks for the quick response and the sample code. I tried it very briefly yesterday and it compiled right away but I got a bunch of link errors from the stlsoft lib (which is odd since it is a header-only library). There are tons of cases online of people using this library with the compilers I am trying to use, so it is likely that I simply do not have my environment set up right. We're in the middle of moving buildings at the moment, so it may be a few days before I have time to look into the linker issues. I'll make sure to reply to this thread once I figure it out.