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I have a C++ dll, lib and the corresponding header file. I would like to create a dummy wrapper dll that simply returns True from some of the functions in the dll, so that when testing, I can replace the dll with the test one and have the application still work without recompilation.
Some function however (error handling) I would like to keep. My approach is as follows.
1. Copy the .h file in my local project folder and remove the extern keyword for the functions I would like to override, than add implementation for those functions.
2. All functions are declared __declspec( dllexport ) so that they are included in the dll.
3. I include the original lib file as an Additional dependency in the project, so I can link to the function definitions for the functions I'm not overriding. The project is set to DLL (I'm using VS)
Everything compiles fine, but when I use depends to look at the dll, I only see the functions I've implemented, not the ones from the lib.
Any ideas?
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So you expect the dll to automagically implement the functions you didn't write? If you find a compiler to do that let me know (private email please, I don't want everyone else to know).
Phil
The opinions expressed in this post are not necessarily those of the author, especially if you find them impolite, inaccurate or inflammatory.
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You really have a fantastic signature! Great!
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I expect to link to the static lib file and export those implementations. Or are those just stub functions that load the dll code?
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Ah ha, just seen one point of confusion you've got...
When you produce a DLL you can produce something called an import library. This is nothing like an ordinary static library (although you can add object files to it if you want) but essentially just a lump of data describing the functions exported from the DLL - essentially just the function's names and position in the exported function table. They don't contain any code.
So if you link with a DLLs import library then all that means is that the corresponding DLL will be automatically loaded when the program starts and the loader can fixup the addresses of the imported functions.
Cheers,
Ash
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Yeah I realized that after a while 
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I wouldn't try using library interpositioning for this sort of thing. While you can implement a DLL that calls selectively forwards functions to another DLL it ends up being a bit of a headache [1]. The approach I tend to use is...
- wrap the DLL in a class
- extract an interface class out of the class you've implemented (all the functions are pure virtual)
- implement the interface again, passing the bits you want through to the DLL and reimplementing the bits you don't want to
- select at runtime or compile time (depending on the type of testing you want to do) which implementation of the interface you want to use
This method is something to consider with any third party library so when it doesn't work properly you've got a central point to either replace it or patch its functionality.
Cheers,
Ash
[1] If you really want to do use library interpositioning then...
- reimplement the entire interface of the wrapped DLL, starting off by just returning succeeded, true, whatever is appropriate
- reimplement the functions you want to pass through by (a) manually loading the wrapped DLL and (b) calling the wrapped function
- reimplement the bits you don't want to pass through however you want
This saves the balls ache of link order of the wrapping and wrapped libraries.
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So I can't simply use the external function definitions to expose those functions in the dll and statically link the lib file to the dll for the actual implementation??
Wrapping the dll in a class sounds like an overkill, I wanted something quick and dirty. If I try to use the library interpositioning you described at the bottom of your post, won't the functions I import from the wrapped dll clash with the functions I defined in the wrapper, being the same name and signature??
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Not sure what you're talking about in the first paragraph (sorry!). When you define a function you're actually writing the code for it. At least in C++ terms, not sure what the C standard says about definitions.
Wrapping the DLL in a class may or may not be overkill. It's guarenteed to work and gives you a nice boundary to unit test around by adding something that's easily substitutable. If the DLL has a couple of hundred functions you're trying to use then it might be a headache (but then the DLL itself is likely to already be that) but for 20 or so functions it's less than an hours work.
As to the interpositioning I described... Why would there be any clash of names? You're not linking with the wrapped DLL so there are no names from it to clash with.
Cheers,
Ash
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So doing dll import would not name clash with my local functions ?
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Nope, I didn't say that. I said that if you don't link with the DLL then there'll be no names to clash with.
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depends only shows exported function from current dll.
so if you want to show nested dll function then you should need to export the function.
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how can i develop a file exe?
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That's a really vague question, so I'll give you a vague answer: by writing code and compiling it. Now if you want a more precise answer, you'll need to give more details about your problem.
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 By following these steps:
- choose a programming language
- buy a good book about that language and study it
- write down your first project (usually each book starts with a very simple Hello World application)
- compile your project to produce the executable file
That's it! 
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i'm using visual studio but i have problem with the code project i'm learning le function
thanks
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Do you mean how to use Visual Studio? Which language are you using (VC++, VC#, ...)?
We can speak italian if you prefer...
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se parli in taliano va meglio.con visual studio creo i project ma non riesco a trasformarli con il comando compile
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Il comando Compile non è quello di cui hai bisogno: esso esegue soltanto la compilazione del file che attualmente hai aperto nell'editor (se guardi nella cartella dei files intermedi troverai qualcosa tipo nomefile.obj).
Per ottenere il file eseguibile finale devi usare il comando Build oppure Rebuild.
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il problema è che debbo imparare i codici da inserire nel file cercando su internet c'è di tutto
ma non so usare i comandi come include o le funzioni numeriche
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In questo modo però rischi di diventare vecchio prima di aver imparato! Prova a partire da qui:
http://www.cerca-manuali.it/manuale-guida/c.htm[^]
Non li ho guardati tutti, ma sono in italiano e dovrebbero darti una panoramica di quello che serve per iniziare.
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veramente vorrei imparare senza manuali tu dici che è possibile?
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Per "imparare" servono sia teoria che pratica.
La sola teoria o la sola pratica sono ... masturbazioni. Possono essere divertenti, ma non producono nulla che duri a lungo.
L'"esperienza" si fa passando dall'una all'altra alternativamente su cose via via più complesse.
Se non hai tempo (ne passione) per questo, cambia mestiere.
Da quanto vedo, il modo assolutamente approssimativo con cui esprimi i concetti anche basilari ti sta già creando problemi di comunicazione ... chi ti deve rispondere fatica a cogliere di quale argomento parli. Non perché non lo conosci, ma perché usi una "lingua" tutta tua. Fatta sulla sola "pratica" che solo tu conosci.
Se insisti moooolto a lungo potrai anche diventare un grande esperto, che però non riuscirà mai a "vendere" nulla di ciò che produce e che difficilmente troverà compagni di avventura con cui lavorare su cose più complesse.
Non puoi imparare senza "studiare" ciò che altri hanno già "imparato" prima di te.
2 bugs found.
> recompile ...
65534 bugs found.
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Sauro Viti wrote: We can speak italian if you prefer
Yeah great idea; now all the non-Italian speakers have no way of figuring out whether your suggestions are useful or not. 
It's time for a new signature.
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