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So I've seen some C libraries written so that they compile without any changes with a C++ compiler, for example Lua.

What are some advantages and disadvantages of doing this?

A few basic ones I see are:

Advantages:

  • library can be used with any C or C++ project with no extra effort (without using extern "C" for example).

Disadvantages:

  • Harder to maintain because you need it compile as both (involves things like extra casts which can hide bugs)
  • Either limit yourself to C/C++ (sacrificing things like flexible array members), or end up with non-standard C++.

But I'm sure there's more to consider than just the above advantages and disadvantages. What sort of cases would doing this be appropriate, instead of using either pure C or C++ for the library?

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  • // Harder to maintain because you need it compile as both // is why open-source organizations usually have a well-maintained and well-resourced build farm to test each code change on a diverse collection of build environments, compilers and versions. The other disadvantages and concerns are all valid.
    – rwong
    May 10, 2019 at 3:17
  • @rwong I'm not saying you shouldn't test it in various environments etc. I more meant you have to do extra work in your code to compile it as both (adding extra casts that you wouldn't need in C for example).
    – CoffeeTableEspresso
    May 10, 2019 at 17:28

5 Answers 5

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Well, there are many disadvantages:

  1. You cannot write good, idiomatic C, as it must also be valid C++.
  2. Dito for C++, but worse.
  3. Maintainers need to know C, and all the differences to C++. That is in effect a different third composite-language featuring the disadvantages of either plus all incompatibilities.
    I wonder if there is any good documentation for that? Especially as every combination of C version and C++ version is a different challenge.
  4. If you use multiple languages, you have to compile it as C and include it within a manually added extern "C"-block anyway.

Why not simply write it in whatever language you want, featuring a C-interface, and optionally adding a C++ facade on top of it for ease of use?

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  • 1
    david.tribble.com/text/cdiffs.htm and en.wikipedia.org/wiki/Compatibility_of_C_and_C%2B%2B do a pretty good job of covering the differences. Note that most compilers will support basically all C features as an extension when compiling with C++ (which makes C++ almost a superset of C when compiling with these compilers).
    – CoffeeTableEspresso
    May 9, 2019 at 20:27
  • @CoffeeTableEspresso Yes, david tribble's text seems quite good for listing differences between C99 and C++98. But while the first is still often used, the two decades really make a difference for C++. The wikipedia text even on cursory reading seems a bit biased in favor of C++ and plain incorrect in many important points.
    – Deduplicator
    May 9, 2019 at 21:07
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    Is there really much difference between "idiomatic C" and "idiomatic C modified just enough to make it also valid C++"? Casting the result of malloc is one.
    – user253751
    May 9, 2019 at 22:09
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    How does writing in idiomatic C and then writing a C++ wrapper to contain the weirdness (or vice versa) stack up?
    – Dan Is Fiddling By Firelight
    May 9, 2019 at 23:02
  • @immibis you lose things like flexible array members and designated initializers, which are very very important for writing idiomatic C. It's not just the casts.
    – CoffeeTableEspresso
    May 9, 2019 at 23:38
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A major advantage:

The authors only have to support one version of the code

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    This is possible when compiling exclusively as C or C++; some work may need to be done to support wrappers, but it is definitely still “one version of the code.”
    – gntskn
    May 10, 2019 at 0:07
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The point is - why would anyone do it?

This is a lot of effort for what would ultimately be hard to maintain code which is not good C or C++.

Point in case: error conditions. C++ uses exceptions while C does not support them.

Write a C library, add an ifdefed extern "C" in the header and that's it. Add a separate C++ wrapper.

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What sort of cases would doing this be appropriate, instead of using either pure C or C++ for the library?

When you expect that there will be

  • users of your library
  • on a platform that doesn't have both a C compiler and a C++ compiler, which each produce object files compatible with the other
  • wishing to write programs using the language you didn't.

Otherwise you should give C to C compilers, and C++ to C++ compilers.

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Advantages compared to what?

You can compare it to "write a C library". Assume you have a library that is written in C, without care for C++.

You can just run it through a C++ compiler. You'll have to make a few changes, if you happened to name a variable "class" or "template". You'll have to make a few more changes because type checking may be stricter in C++. You may have to make a few casts for the results of malloc or the arguments to free. The last item, some people will tell you it's not idiomatic C - because it might hide the fact that you missed including some header file. That's not a valid fear today.

I think all in all it will make your code better, at quite little cost.

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    I think you overestimate the compatibility of C and C++. You have to add a cast for every malloc, and you lose very nice C features like flexible array members and designated initializers. The stricter type checking is of course much nicer in a lot of cases (such as with enums), but most people turn on stricter type checking when writing pure C anyways.
    – CoffeeTableEspresso
    May 9, 2019 at 20:30

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