Can I have configurable static linkage in C++? - c++

I know this sounds like "can I have dynamic static linkage" which makes little sense, but let me explain.
I am looking for options to explore and I am aware that there might be something out there that I'm not aware of.
My goal is to have a modular code base where plugins would be provided as static libs, to avoid having exposed dll and result in a single library which clients could swallow in their code.
I imagine having a config file listing all the desired plugins, feed that to a script and boom : a magic sln file with everything ready to be built.
My initial idea is to have a 'main deck' that would know every possible plugin interface and link with them all. Plugins not yet implemented or required by the client would be dead-end/no-op implementations, while required plugins would be implementations realizing the interfaces called by the 'main deck'.
I think that would work, but I find conceptually horrible the idea of linking dead implementations for the sake of modularity.
The main issue I see is at that 'main deck' level : how could I remove useless headers to prevent useless linking or add newly developed ones without editing the code each time? I cannot figure this out without a ton of macros or generating some source files.
Could other patterns solve that issue?

I think there is no possible way that doesn't involve macro magic and a complex build system.
If I understand correctly, what you want to do is similar to a library I have used, rocksdb. At build time you can specify what modules/packages you want and it will build them into the static library for you. Check out what they do and see if it is along the lines of what you want.

Related

Is it bad practice if the executable depends on a library that is already used by another dependency?

My current project has two libraries libAAA and libBBB.
The former, libAAA, contains basic math and mechanics for my project (e.g. the rules for chess). The latter, libBBB, depends on libAAA but adds GUI elements (e.g. how single figures are drawn).
The executable depends on both, libAAA and libBBB. E.g. libBBB is used to setup the chess game while libBBB is used to define how it looks (console, qt, gtk,...).
Is it OK to have such redundant dependencies or is it probably bad practice?
edit:
I ask that because I like how easy it is to decouple a GUI representation from the mechanics. E.g. if I would like to change the look or the toolkit, I could just add libHHH, make another executable and that's it.
This is fine.
Let's look at some alternatives:
Pull in all the code from libAAA into libBBB: This is alright if libAAA isn't intended to be used in other contexts, and their respective sizes allow this to be done in a practical manner. In many contexts though, e.g. if libBBB is much larger and libAAA is meant as a small reusable library for many projects independently of the UI framework used in libBBB, this won't work.
Copy the code used by libBBB into it directly: Please don't. This would mean two separate libraries that do a lot of the same things, two places to update in the future, more confusion for you in the future or for anyone else trying to understand it, etc.
Bring the libBBB code into the application itself: This is not always possible or practical, depending on your development stack and environment. Many applications may also need libraries that were programmed and compiled in a different language for performance or other reasons.
I'm probably missing a couple other alternative options, but keeping both dependencies despite the redundancy seems the best option to me.
As you stated in the edit, this also allows you to swap out libBBB for another UI library that also uses libAAA(or another lib if need be) without tearing down any of your core application code or invalidating any other code in your dependencies.

Should I use a Static Library or DLL if I'm creating an API?

I'm creating an API to make it easier to find specifications about your system (CPU, GPU, etc.) using C++. This API will be distributed using GitHub.
In this scenario, which type of library should I use: Static or DLL (Dynamic)? Also, what are the pros and cons of each?
It depends on a few things, but like in the previous comments, you can leave the choice to the user.
A DLL is easier to integrate since the user just has to do a LoadLibrary() to start using it.
The problem with the DLL is that if you compiled it without using the default libraries as static and if you don't provide the redistributables, the user will get frustrated because of SxS problems. It may just so happen that there may not be any issues with SxS but you never know.
If you give out the lib file, it is possible that some compilation options may conflict with yours, unless you only use vanilla options.
All in all, at the end of the day, both options are viable and similar. Just depends on how the user wants to use your API.

Discovering Symbol Usage

Issue
I have recently found myself working with a large, unfamiliar, multi-department, C++ codebase in need of better organization. I would like to discover a way to map which symbols are used by which source files for any given header. This is in the hope that if only one department uses a given function, then it can be moved out of the shared area and into that department's area.
Attempts
My first thoughts were to use the symbol table: ie. compile the project and dump the symbols for each object file. From there I figured I could simply write a script to check if the symbols from my header file were used. While this approach seems viable, it would require me to create a list of symbols I am looking for from the headers. With my limited knowledge, I am unsure of how to automate such a process, and with hundreds of headers files to test, doing it manually is out of the question.
Questions
Is my approach valid? If so..
What can I use to generate the symbol names from my header file?
If not..
What else can I do?
Additionally, while I am using Linux, most of the development teams work in Windows only environments. What utilities could I use on both platforms?
Any and all help is greatly appreciated.
When I need to clean up APIs I sometimes use information from callcatcher. It basically builds a database of all symbols while compiling and allows you to determine what symbols are used in some build product.
I sometimes also use DXR (code on github, an example installation) to browse what code defined where is used how. In contrast to callcatcher with DXR you can drill down to much finer detail. Setting up DXR is pretty heavy duty, but might be worth it if you have enough code to work with.
On the other side of the spectrum there are tools like cscope. Even though it doesn't work super nicely with C++ code it is still very useful. If you deal with more than a couple 100kloc you will quickly feel limited though.
If I had to pick only one of these tools and would be working on a large code base (>1Mloc) I would definitely pick DXR.
You can get a reasonable start on the information that you've described by using doxygen.
Even for source that doesn't contain the doxygen formatted comments the documentation created can contain a list of places (ie. source files) where a particular symbol is used.
And, as doxygen can be used to generate html documentation, navigating through your source tree becomes trivial. It's can be even better if you enable the dot functionality to generate relationship diagrams for the classes in your source tree.
very old-school, simple, and possibly unix only, but are you aware of etags? there's also gnu global which i think is similar.
the gnu global link refers to the "comparison with similar tools" discussion here which might also be useful.

How should I integrate with and package this third-party library in a Win32 C++ app?

We have a (very large) existing codebase for a custom ActiveX control, and I'd like to integrate libkml into it for the sake of interacting with KML mapping data, rather than reinventing the wheel. The problem is, I'm a relatively new Windows developer, and coming from the Linux world, I'm really not sure what the right way of integrating a third party library is. Thankfully, libkml does provide MSVCC projects for compiling it, so porting isn't a problem. I guess I have a couple choices that I can think of:
Build and link the library directly. We already have a solution with project files in it for the "main" project; I could add the libkml projects to that solution, but I'd rather not. It's very unlikely that the libkml code will change in relation to our app's code.
Statically link to the .lib files produced by the libkml build. This is unattractive, since there are six .lib files that come out of the libkml solution and it seems inelegant to manually specify them in the linker options, etc.
Package the code as-is in a DLL. Maybe with COM? It seems like if I did this without any translation, I'd end up with a lot of overhead, and since I'm fairly unfamiliar with COM, I don't know how much work would be involved in exposing all the functionality I'd like to use via COM. The library is fairly big, has a lot of classes it uses, and if I had to manually write code to expose it all, I'd be hesitant to go this route.
Write wrapper code to to abstract the functionality I need, package that in a COM DLL, and interact with that. This seems sensible, I suppose, but it's difficult to determine how much abstraction I need since I haven't written the code that would use libkml yet.
Let me reiterate: I haven't yet written the code that will interact with libkml yet, so this is mostly experimental. Options 1 and 2 are also complicated by the fact that libkml relies additionally on three more external libraries that are also in .lib files (that I had to recompile anyways to get the code generation flags to line up). The goal obviously is to get the code to work, but maintainability and source tree organization are also goals, so I'm leaning towards options 3 and 4, but I don't know the best way to approach those on Windows.
Typing six file names, or using the declarative style with #pragma comment(lib, "foo.lib") is small potatoes compared to the work you'll have to do to turn this into a DLL or COM server.
The distribution is heavily biased towards using this as a static link library. There are only spotty declarations available to turn this into a DLL with __declspec(dllexport). They exist only in the 3rd party dependencies. All using different #defines of course, you'll by typing a bunch of names in the preprocessor definitions for the projects.
Furthermore, you'll have a hard time actually getting this DLL loaded at runtime since you are using it in a COM server. The search path for DLLs will be the client app's when COM creates your control instance, not likely to be anywhere near close to the place you deployed the DLL.
Making it a COM server is a lot of work, you'll have to write all the interface glue yourself. Again, nothing already in the source code that helps with this at all.
You can also wrap all the functionality you need in a non-COM-dll. Visual studio supports creating a static wrapper library which, when linked, will make your program use the dll. This way you only have one dependency to specify instead of six.
Other than that, what is wrong with specifying six dependencies. I would assume that there is a good reason that these are six separate libraries instead of one, so it is prudent to specify exactly which parts you actually use.
Maybe I'm missing something here, but I really don't see what is wrong with (1). I think that even if you had multiple projects that were using libkml, just insert the project file for libkml into your solution file, specify the dependencies, and you should be done. It's dead simple. Even solution (2) is dead simple. If the libraries ever change, you rebuild - you're going to need to do that anyway.
I'm failing to see how (3) or (4) are necessary or even desired. To me, it sounds like a lot of work for goals (source tree organization and maintainability) that I'm not even sure that those options really meet. In fact, you said yourself that "It's very unlikely that the libkml code will change in relation to our app's code."
What I've found over the years is to just keep things simple. If rebuilding KML is potentially time consuming, grab the libs and just statically link to the libraries. Yes, there are other dependencies, but you'll set this up once and be done, hopefully never to worry about it again. Otherwise, stick it in the project and move on. I think that it's worthwhile to ask whether spending a lot of time on this issue is worth the trouble.

C++ internal code reuse: compile everything or share the library / dynamic library?

General question:
For unmanaged C++, what's better for internal code sharing?
Reuse code by sharing the actual source code? OR
Reuse code by sharing the library / dynamic library (+ all the header files)
Whichever it is: what's your strategy for reducing duplicate code (copy-paste syndrome), code bloat?
Specific example:
Here's how we share the code in my organization:
We reuse code by sharing the actual source code.
We develop on Windows using VS2008, though our project actually needs to be cross-platform. We have many projects (.vcproj) committed to the repository; some might have its own repository, some might be part of a repository. For each deliverable solution (.sln) (e.g. something that we deliver to the customer), it will svn:externals all the necessary projects (.vcproj) from the repository to assemble the "final" product.
This works fine, but I'm quite worried about eventually the code size for each solution could get quite huge (right now our total code size is about 75K SLOC).
Also one thing to note is that we prevent all transitive dependency. That is, each project (.vcproj) that is not an actual solution (.sln) is not allowed to svn:externals any other project even if it depends on it. This is because you could have 2 projects (.vcproj) that might depend on the same library (i.e. Boost) or project (.vcproj), thus when you svn:externals both projects into a single solution, svn:externals will do it twice. So we carefully document all dependencies for each project, and it's up to guy that creates the solution (.sln) to ensure all dependencies (including transitive) are svn:externals as part of the solution.
If we reuse code by using .lib , .dll instead, this would obviously reduce the code size for each solution, as well as eliminiate the transitive dependency mentioned above where applicable (exceptions are, for example, third-party library/framework that use dll like Intel TBB and the default Qt)
Addendum: (read if you wish)
Another motivation to share source code might be summed up best by Dr. GUI:
On top of that, what C++ makes easy is
not creation of reusable binary
components; rather, C++ makes it
relatively easy to reuse source code.
Note that most major C++ libraries are
shipped in source form, not compiled
form. It's all too often necessary to
look at that source in order to
inherit correctly from an object—and
it's all too easy (and often
necessary) to rely on implementation
details of the original library when
you reuse it. As if that isn't bad
enough, it's often tempting (or
necessary) to modify the original
source and do a private build of the
library. (How many private builds of
MFC are there? The world will never
know . . .)
Maybe this is why when you look at libraries like Intel Math Kernel library, in their "lib" folder, they have "vc7", "vc8", "vc9" for each of the Visual Studio version. Scary stuff.
Or how about this assertion:
C++ is notoriously non-accommodating
when it comes to plugins. C++ is
extremely platform-specific and
compiler-specific. The C++ standard
doesn't specify an Application Binary
Interface (ABI), which means that C++
libraries from different compilers or
even different versions of the same
compiler are incompatible. Add to that
the fact that C++ has no concept of
dynamic loading and each platform
provide its own solution (incompatible
with others) and you get the picture.
What's your thoughts on the above assertion? Does something like Java or .NET face these kinds of problems? e.g. if I produce a JAR file from Netbeans, will it work if I import it into IntelliJ as long as I ensure that both have compatible JRE/JDK?
People seem to think that C specifies an ABI. It doesn't, and I'm not aware of any standardised compiled language that does. To answer your main question, use of libraries is of course the way to go - I can't imagine doing anything else.
One good reason to share the source code: Templates are one of C++'s best features because they are an elegant way around the rigidity of static typing, but by their nature are a source-level construct. If you focus on binary-level interfaces instead of source-level interfaces, your use of templates will be limited.
We do the same. Trying to use binaries can be a real problem if you need to use shared code on different platforms, build environments, or even if you need different build options such as static vs. dynamic linking to the C runtime, different structure packing settings, etc..
I typically set projects up to build as much from source on-demand as possible, even with third-party code such as zlib and libpng. For those things that must be built separately, e.g. Boost, I typically have to build 4 or 8 different sets of binaries for the various combinations of settings needed (debug/release, VS7.1/VS9, static/dynamic), and manage the binaries along with the debugging information files in source control.
Of course, if everyone sharing your code is using the same tools on the same platform with the same options, then it's a different story.
I never saw shared libraries as a way to reuse code from an old project into a new one. I always thought it was more about sharing a library between different applications that you're developing at about the same time, to minimize bloat.
As far as copy-paste syndrome goes, if I copy and paste it in more than a couple places, it needs to be its own function. That's independent of whether the library is shared or not.
When we reuse code from an old project, we always bring it in as source. There's always something that needs tweaking, and its usually safer to tweak a project-specific version than to tweak a shared version that can wind up breaking the previous project. Going back and fixing the previous project is out of the question because 1) it worked (and shipped) already, 2) it's no longer funded, and 3) the test hardware needed may no longer be available.
For example, we had a communication library that had an API for sending a "message", a block of data with a message ID, over a socket, pipe, whatever:
void Foo:Send(unsigned messageID, const void* buffer, size_t bufSize);
But in a later project, we needed an optimization: the message needed to consist of several blocks of data in different parts of memory concatenated together, and we couldn't (and didn't want to, anyway) do the pointer math to create the data in its "assembled" form in the first place, and the process of copying the parts together into a unified buffer was taking too long. So we added a new API:
void Foo:SendMultiple(unsigned messageID, const void** buffer, size_t* bufSize);
Which would assemble the buffers into a message and send it. (The base class's method allocated a temporary buffer, copied the parts together, and called Foo::Send(); subclasses could use this as a default or override it with their own, e.g. the class that sent the message on a socket would just call send() for each buffer, eliminating a lot of copies.)
Now, by doing this, we have the option of backporting (copying, really) the changes to the older version, but we're not required to backport. This gives the managers flexibility, based on the time and funding constraints they have.
EDIT: After reading Neil's comment, I thought of something that we do that I need to clarify.
In our code, we do lots of "libraries". LOTS of them. One big program I wrote had something like 50 of them. Because, for us and with our build setup, they're easy.
We use a tool that auto-generates makefiles on the fly, taking care of dependencies and almost everything. If there's anything strange that needs to be done, we write a file with the exceptions, usually just a few lines.
It works like this: The tool finds everything in the directory that looks like a source file, generates dependencies if the file changed, and spits out the needed rules. Then it makes a rule to take eveything and ar/ranlib it into a libxxx.a file, named after the directory. All the objects and library are put in a subdirectory that is named after the target platform (this makes cross-compilation easy to support). This process is then repeated for every subdirectory (except the object file subdirs). Then the top-level directory gets linked with all the subdirs' libraries into the executable, and a symlink is created, again, naked after the top-level directory.
So directories are libraries. To use a library in a program, make a symbolic link to it. Painless. Ergo, everything's partitioned into libraries from the outset. If you want a shared lib, you put a ".so" suffix on the directory name.
To pull in a library from another project, I just use a Subversion external to fetch the needed directories. The symlinks are relative, so as long as I don't leave something behind it still works. When we ship, we lock the external reference to a specific revision of the parent.
If we need to add functionality to a library, we can do one of several things. We can revise the parent (if it's still an active project and thus testable), tell Subversion to use the newer revision and fix any bugs that pop up. Or we can just clone the code, replacing the external link, if messing with the parent is too risky. Either way, it still looks like a "library" to us, but I'm not sure that it matches the spirit of a library.
We're in the process of moving to Mercurial, which has no "externals" mechanism so we have to either clone the libraries in the first place, use rsync to keep the code synced between the different repositories, or force a common directory structure so you can have hg pull from multiple parents. The last option seems to be working pretty well.