[29] How to mix C and C++
(Part of C++ FAQ Lite, Copyright © 1991-2000, Marshall Cline, cline@parashift.com)

FAQs in section [29]:

[29.1] What do I need to know when mixing C and C++ code?

There are several caveats:

In addition, you'll need to read the rest of this section to find out how to make your C functions callable by C++ and/or your C++ functions callable by C.

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[29.2] How can I include a standard C header file in my C++ code?

To #include a standard header file (such as <cstdio>), you don't have to do anything unusual. E.g.,

 // This is C++ code
 
 #include <cstdio>           // Note: nothing unusual in #include line
 
 int main()
 {
   printf("Hello world\n");  // Note: nothing unusual in the call
 }

Note: Somewhat different guidelines apply for non-system C headers. There are two cases: either you can't change the header, or you can change the header.

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[29.3] How can I include a non-system C header file in my C++ code?

If you are including a C header file that isn't provided by the system, you may need to wrap the #include line in an extern C { /*...*/ } construct. This tells the C++ compiler that the functions declared in the header file are are C functions.

 // This is C++ code
 
 extern "C" {
   // Get declaration for f(int i, char c, float x)
   #include "my-C-code.h"
 }
 
 int main()
 {
   f(7, 'x', 3.14);   // Note: nothing unusual in the call
 }

Note: Somewhat different guidelines apply for C headers provided by the system (such as <cstdio>) and for C headers that you can change.

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[29.4] How can I modify my own C header files so it's easier to #include them in C++ code? UPDATED!

[Recently added the "macros are sometimes useful" paragraph at the end (on 3/00). Click here to go to the next FAQ in the "chain" of recent changes.]

If you are including a C header file that isn't provided by the system, and if you are able to change the C header, you should strongly consider adding the extern C {...} logic inside the header to make it easier for C++ users to #include it into their C++ code. Since a C compiler won't understand the extern C construct, you must wrap the extern C { and } lines in an #ifdef so they won't be seen by normal C compilers.

Step #1: Put the following lines at the very top of your C header file (note: the symbol __cplusplus is #defined if/only-if the compiler is a C++ compiler):

 #ifdef __cplusplus
 extern "C" {
 #endif

Step #2: Put the following lines at the very bottom of your C header file:

 #ifdef __cplusplus
 }
 #endif

Now you can #include your C header without any extern C nonsense in your C++ code:

 // This is C++ code
 
 // Get declaration for f(int i, char c, float x)
 #include "my-C-code.h"   // Note: nothing unusual in #include line
 
 int main()
 {
   f(7, 'x', 3.14);       // Note: nothing unusual in the call
 }

Note: Somewhat different guidelines apply for C headers provided by the system (such as <cstdio>) and for C headers that you can't change.

Note: #define macros are evil in 4 different ways: evil#1, evil#2, evil#3, and evil#4. But they're still useful sometimes. Just wash your hands after using them.

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[29.5] How can I call a non-system C function f(int,char,float) from my C++ code?

If you have an individual C function that you want to call, and for some reason you don't have or don't want to #include a C header file in which that function is declared, you can declare the individual C function in your C code using the extern C syntax. Naturally you need to use the full function prototype:

 extern "C" void f(int i, char c, float x);

A block of several C functions can be grouped via braces:

 extern "C" {
   void   f(int i, char c, float x);
   int    g(char* s, const char* s2);
   double sqrtOfSumOfSquares(double a, double b);
 }

After this you simply call the function just as if it was a C++ function:

 int main()
 {
   f(7, 'x', 3.14);   // Note: nothing unusual in the call
 }

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[29.6] How can I create a C++ function f(int,char,float) that is callable by my C code?

The C++ compiler must know that f(int,char,float) is to be called by a C compiler using the extern C construct:

 // This is C++ code
 
 // Declare f(int,char,float) using extern C:
 extern "C" void f(int i, char c, float x);
 
 // ...
 
 // Define f(int,char,float) in some C++ module:
 void f(int i, char c, float x)
 {
   // ...
 }

The extern C line tells the compiler that the external information sent to the linker should use C calling conventions and name mangling (e.g., preceded by a single underscore). Since name overloading isn't supported by C, you can't make several overloaded functions simultaneously callable by a C program.

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[29.7] Why is the linker giving errors for C/C++ functions being called from C++/C functions?

If you didn't get your extern C right, you'll sometimes get linker errors rather than compiler errors. This is due to the fact that C++ compilers usually "mangle" function names (e.g., to support function overloading) differently than C compilers.

See the previous two FAQs on how to use extern C.

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[29.8] How can I pass an object of a C++ class to/from a C function?

Here's an example (for info on extern C, see the previous two FAQs).

Fred.h:

 /* This header can be read by both C and C++ compilers */
 #ifndef FRED_H
 #define FRED_H
 
 #ifdef __cplusplus
   class Fred {
   public:
     Fred();
     void wilma(int);
   private:
     int a_;
   };
 #else
   typedef
     struct Fred
       Fred;
 #endif
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
 #if defined(__STDC__) || defined(__cplusplus)
   extern void c_function(Fred*);   /* ANSI C prototypes */
   extern Fred* cplusplus_callback_function(Fred*);
 #else
   extern void c_function();        /* K&R style */
   extern Fred* cplusplus_callback_function();
 #endif
 
 #ifdef __cplusplus
 }
 #endif
 
 #endif /*FRED_H*/

Fred.cpp:

 // This is C++ code
 
 #include "Fred.h"
 
 Fred::Fred() : a_(0) { }
 
 void Fred::wilma(int a) { }
 
 Fred* cplusplus_callback_function(Fred* fred)
 {
   fred->wilma(123);
   return fred;
 }

main.cpp:

 // This is C++ code
 
 #include "Fred.h"
 
 int main()
 {
   Fred fred;
   c_function(&fred);
   return 0;
 }

c-function.c:

 /* This is C code */
 
 #include "Fred.h"
 
 void c_function(Fred* fred)
 {
   cplusplus_callback_function(fred);
 }

Passing pointers to C++ objects to/from C functions will fail if you pass and get back something that isn't exactly the same pointer. For example, don't pass a base class pointer and receive back a derived class pointer, since your C compiler won't understand the pointer conversions necessary to handle multiple and/or virtual inheritance.

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[29.9] Can my C function directly access data in an object of a C++ class?

Sometimes.

(For basic info on passing C++ objects to/from C functions, read the previous FAQ).

You can safely access a C++ object's data from a C function if the C++ class:

If the C++ class has any base classes at all (or if any fully contained subobjects have base classes), accessing the data will technically be non-portable, since class layout under inheritance isn't imposed by the language. However in practice, all C++ compilers do it the same way: the base class object appears first (in left-to-right order in the event of multiple inheritance), and member objects follow.

Furthermore, if the class (or any base class) contains any virtual functions, almost all C++ compliers put a void* into the object either at the location of the first virtual function or at the very beginning of the object. Again, this is not required by the language, but it is the way "everyone" does it.

If the class has any virtual base classes, it is even more complicated and less portable. One common implementation technique is for objects to contain an object of the virtual base class (V) last (regardless of where V shows up as a virtual base class in the inheritance hierarchy). The rest of the object's parts appear in the normal order. Every derived class that has V as a virtual base class actually has a pointer to the V part of the final object.

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[29.10] Why do I feel like I'm "further from the machine" in C++ as opposed to C?

Because you are.

As an OO programming language, C++ allows you to model the problem domain itself, which allows you to program in the language of the problem domain rather than in the language of the solution domain.

One of C's great strengths is the fact that it has "no hidden mechanism": what you see is what you get. You can read a C program and "see" every clock cycle. This is not the case in C++; old line C programmers (such as many of us once were) are often ambivalent (can you say, "hostile"?) about this feature. However after they've made the transition to OO thinking, they often realize that although C++ hides some mechanism from the programmer, it also provides a level of abstraction and economy of expression which lowers maintenance costs without destroying run-time performance.

Naturally you can write bad code in any language; C++ doesn't guarantee any particular level of quality, reusability, abstraction, or any other measure of "goodness."

C++ doesn't try to make it impossible for bad programmers to write bad programs; it enables reasonable developers to create superior software.

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Revised Jul 10, 2000