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

2014-12-19 14:06 645 查看

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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

[code]#ifdef __cplusplus
extern "C" {
#endif

and at the bottom, we write

[code]#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:

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?

Is there anything wrong with wrapping a piece of code with

extern
 "C" { extern "C" { .. } }

? What will the second

extern
 "C"

do?

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?

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?

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.

c++ c preprocessor

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this question

asked Sep 24 '10 at 17:03

dublev

7672712

Have a look at C++
FQA: mixing C and C++ and C++
FAQ: mixing C and C++. – Lazer Sep
24 '10 at 19:25

Thanks guys, I appreciate the help! – dublev Sep
24 '10 at 20:54

add
a comment

3 Answers

active oldest votes

up
vote91down
voteaccepted

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.

PS：也就是说cpp里的函数，如果没有被其他.c文件的调用不需要在前面加extern "C"，如果该函数需要被其他.c文件调用，则需要在.cpp文件里的函数实现上加上extern "C"

//xxx.cpp的实现

extern "C" void C_fun()

{

}

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.

103down
votefavorite

62

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

[code]#ifdef __cplusplus
extern "C" {
#endif

and at the bottom, we write

[code]#ifdef __cplusplus
}
#endif

__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?

Is there anything wrong with wrapping a piece of code with

extern
 "C" { extern "C" { .. } }

? What will the second

extern
 "C"

extern
 "C"

share|improve
this question

asked Sep 24 '10 at 17:03

dublev

7672712

Have a look at C++
FQA: mixing C and C++ and C++
FAQ: mixing C and C++. – Lazer Sep
24 '10 at 19:25

Thanks guys, I appreciate the help! – dublev Sep
24 '10 at 20:54

add
a comment

3 Answers

active oldest votes

up
vote91down
voteaccepted

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"

extern
 "C"

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"

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.

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