Solved

Loading class by "<name>"

Posted on 1997-04-03
4
225 Views
Last Modified: 2010-04-10
I am about to write an ISAPI extension, and can't work out how to dynamically load an instance of a named class whose name is discovered at run-time. All my loaded classes will be derived from the same base class, and I will only be calling the base class methods, so I only need to have the base header (I think) in my 'loader'. All my loaded instances will be implementing these base business methods differently. But I get stuck in thinking this through, as there seems to be no way of saying Declare_Dyncreate("MyClass"), only Declare_Dyncreate(MyClass), which means my loader has to know all the possibles beforehand and have a ginormous switch, and adding a new derived class will mean regenerating my loader.
Am I being naive here in thinking I can dynamically load instances of a common base class, or is there a well-known design pattern that solves this problem?
0
Comment
Question by:dlacey
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 2
  • 2
4 Comments
 

Author Comment

by:dlacey
ID: 1162558
The title of the question origianlly said
Loading class by ".lt name .gt"
but of course the HTML interpreted the .lt and ,gt symbols!
So the question isn't as stupid as the header makes it look.
0
 
LVL 4

Accepted Solution

by:
jos010697 earned 200 total points
ID: 1162559
I don't know what C++ implementation you're using, but
you might have a look at dlopen(), dlsym() and dlclose().

I think (I may be wrong here) that the MS equivalents
are LoadLibrary(), GetProcAddress() and FreeLibrary().

Their usage is quite simple actually:

first open a library: void* lib= dlopen("my_lib");

get a pointer to the symbol:

<symbol_type>* sym= dlsym(lib, "symbol_name");

and use 'sym' as a pointer to the object ...

when you're done using the lib, simply call dlclose(lib)
to get rid of it ...

In your particular case, 'sym' has to be symbol refering to
the class stuff you want to load ...

kind regards,

Jos aka jos@and.nl
0
 

Author Comment

by:dlacey
ID: 1162560
Yes, I'm in the MS camp, so all I have is GetProcAddress(), which is Get Procedure Address, so all I can point to is a procedure (I believe!)
Are you saying that
hModule = LoadLibrary("MyClass")
hMeth1 = GetProcAddress(hModule, "Method1")
then I can
hModule->hMeth1(arg1, arg2, ...)
That seems so weird ...
0
 
LVL 4

Expert Comment

by:jos010697
ID: 1162561
Almost; you have to write a little wrapper function for every
class you want to load dynamically. GetProcAddress is
used to get the address of this little wrapper, while the
wrapper itself takes care of loading the actual class stuff.
The name of the wrapper function serves as a name for
the associated class then ...

BTW, a class is not an object in C++, so you can never
actually 'load' it, using GetProcAddress (or even dlsym()
which is capable of retrieving the adress of _any_ object).

kind regards,

Jos

0

Featured Post

On Demand Webinar - Networking for the Cloud Era

This webinar discusses:
-Common barriers companies experience when moving to the cloud
-How SD-WAN changes the way we look at networks
-Best practices customers should employ moving forward with cloud migration
-What happens behind the scenes of SteelConnect’s one-click button

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

This article will show you some of the more useful Standard Template Library (STL) algorithms through the use of working examples.  You will learn about how these algorithms fit into the STL architecture, how they work with STL containers, and why t…
Introduction This article is a continuation of the C/C++ Visual Studio Express debugger series. Part 1 provided a quick start guide in using the debugger. Part 2 focused on additional topics in breakpoints. As your assignments become a little more …
The goal of the video will be to teach the user the concept of local variables and scope. An example of a locally defined variable will be given as well as an explanation of what scope is in C++. The local variable and concept of scope will be relat…
The viewer will be introduced to the technique of using vectors in C++. The video will cover how to define a vector, store values in the vector and retrieve data from the values stored in the vector.

728 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question