Combining C++ and C - how does #ifdef __cplusplus

Question

I'm working on a project that has a lot of legacy C code. We've started writing in C++, with the intent to eventually convert the legacy code, as well. I'm a little confused about how the C and C++ interact. I understand that by wrapping the C code with extern "C" the C++ compiler will not mangle the C code's names, but I'm not entirely sure how to implement this.

So, at the top of each C header file (after the include guards), we have

#ifdef __cplusplus
extern "C" {
#endif

and at the bottom, we write

#ifdef __cplusplus
}
#endif

In between the two, we have all of our includes, typedefs, and function prototypes. I have a few questions, to see if I'm understanding this correctly:

1. If I have a C++ file A.hh which includes a C header file B.h, includes another C header file C.h, how does this work? I think that when the compiler steps into B.h, __cplusplus will be defined, so it will wrap the code with extern "C" (and __cplusplus will not be defined inside this block). So, when it steps into C.h, __cplusplus will not be defined and the code will not be wrapped in extern "C". Is this correct?

2. Is there anything wrong with wrapping a piece of code with extern "C" { extern "C" { .. } }? What will the second extern "C" do?

3. We don't put this wrapper around the .c files, just the .h files. So, what happens if a function doesn't have a prototype? Does the compiler think that it's a C++ function?

4. We are also using some third-party code which is written in C, and does not have this sort of wrapper around it. Any time I include a header from that library, I've been putting an extern "C" around the #include. Is this the right way to deal with that?

5. Finally, is this set up a good idea? Is there anything else we should do? We're going to be mixing C and C++ for the foreseeable future, and I want to make sure we're covering all our bases.

Answer 1

1. extern "C" doesn't change the presence or absence of the __cplusplus macro. It just changes the linkage and name-mangling of the wrapped declarations.

2. You can nest extern "C" blocks quite happily.

3. If you compile your .c files as C++ then anything not in an extern "C" block, and without an extern "C" prototype will be treated as a C++ function. If you compile them as C then of course everything will be a C function.

4. Yes

5. You can safely mix C and C++ in this way.

Answer 2

A couple of gotchas that are colloraries to Andrew Shelansky's excellent answer and to disagree a little with doesn't really change the way that the compiler reads the code

Because your function prototypes are compiled as C, you can't have overloading of the same function names with different parameters - that's one of the key features of the name mangling of the compiler. It is described as a linkage issue but that is not quite true - you will get errors from both the compiler and the linker.

The compiler errors will be if you try to use C++ features of prototype declaration such as overloading.

The linker errors will occur later because your function will appear to not be found, if you do not have the extern "C" wrapper around declarations and the header is included in a mixture of C and C++ source.

One reason to discourage people from using the compile C as C++ setting is because this means their source code is no longer portable. That setting is a project setting and so if a .c file is dropped into another project, it will not be compiled as c++. I would rather people take the time to rename file suffixes to .cpp.

Answer 3

It's about the ABI, in order to let both C and C++ application use C interfaces without any issue.

Since C language is very easy, code generation was stable for many years for different compilers, such as GCC, Borland C\C++, MSVC etc.

While C++ becomes more and more popular, a lot things must be added into the new C++ domain (for example finally the Cfront was abandoned at AT&T because C could not cover all the features it needs). Such as template feature, and compilation-time code generation, from the past, the different compiler vendors actually did the actual implementation of C++ compiler and linker separately, the actual ABIs are not compatible at all to the C++ program at different platforms.

People might still like to implement the actual program in C++ but still keep the old C interface and ABI as usual, the header file has to declare extern "C" {}, it tells the compiler generate compatible/old/simple/easy C ABI for the interface functions if the compiler is C compiler not C++ compiler.

Answer 4

extern "C" doesn't really change the way that the compiler reads the code. If your code is in a .c file, it will be compiled as C, if it is in a .cpp file, it will be compiled as C++ (unless you do something strange to your configuration).

What extern "C" does is affect linkage. C++ functions, when compiled, have their names mangled -- this is what makes overloading possible. The function name gets modified based on the types and number of parameters, so that two functions with the same name will have different symbol names.

Code inside an extern "C" is still C++ code. There are limitations on what you can do in an extern "C" block, but they're all about linkage. You can't define any new symbols that can't be built with C linkage. That means no classes or templates, for example.

extern "C" blocks nest nicely. There's also extern "C++" if you find yourself hopelessly trapped inside of extern "C" regions, but it isn't such a good idea from a cleanliness perspective.

Now, specifically regarding your numbered questions:

Regarding #1: __cplusplus should be defined inside of extern "C" blocks. This doesn't matter, though, since the blocks should nest neatly.

Regarding #2: __cplusplus will be defined for any compilation unit that is being run through the C++ compiler. Generally, that means .cpp files and any files being included by that .cpp file. The same .h (or .hh or .hpp or what-have-you) could be interpreted as C or C++ at different times, if different compilation units include them. If you want the prototypes in the .h file to refer to C symbol names, then they must have extern "C" when being interpreted as C++, and they should not have extern "C" when being interpreted as C -- hence the #ifdef __cplusplus checking.

To answer your question #3: functions without prototypes will have C++ linkage if they are in .cpp files and not inside of an extern "C" block. This is fine, though, because if it has no prototype, it can only be called by other functions in the same file, and then you don't generally care what the linkage looks like, because you aren't planning on having that function be called by anything outside the same compilation unit anyway.

For #4, you've got it exactly. If you are including a header for code that has C linkage (such as code that was compiled by a C compiler), then you must extern "C" the header -- that way you will be able to link with the library. (Otherwise, your linker would be looking for functions with names like _Z1hic when you were looking for void h(int, char)

5: This sort of mixing is a common reason to use extern "C", and I don't see anything wrong with doing it this way -- just make sure you understand what you are doing.