C And C++ Coding Standards

Question

What are best practices with regards to C and C++ coding standards? Should developers be allowed to willy-nilly mix them together. Are there any complications when linking C and C++ object files.

Should things like socket libraries that traditionally is written in C remain in C and kept in seperate source files? That is keeping c code in .c files and c++ code in .cpp files. When mixing c and C++ after being parsed with g++ will there be any performance penalties, since typesafe checks are not done in C? but are in C++. Would would be the best way to link C and C++ source code files.

Answer 1

The biggest issue is calling a C function from C++ code or vice versa. In that case, you want to make sure you mark the function as having "C" linkage using extern "C". You can do this in the header file directly using:

#if defined( __cplusplus )
extern "C" {
#endif

extern int myfunc( const char *param, int another_one );

#if defined( __cplusplus )
}
#endif

You need the #ifs because C code that includes it won't understand extern "C".

If you don't want to (or can't) change the header file, you can do it in the C++ code:

extern "C" {
#include "myfuncheader.h"
}

You can mark a C++ function as having C linkage the same way, and then you can call it from C code. You can't do this for overloaded functions or C++ classes.

Other than that, there should be no problem mixing C and C++. We have a number of decades-old C functions that are still being used by our C++ code.

Answer 2

C++ doesn't do 'typesafe checks' at run time unless you ask for them (by using dynamic_cast). C++ is highly compatible with C, so you may freely call C libraries as you wish and compile C code with a C++ compiler. C++ does not imply 'object-oriented', and you should get no performance penalty from using it.

If you mix code compiled with gcc and with g++, see Graeme's answer.

Answer 3

One should generally assume that c++ can throw exceptions, hence the c wrapper functions in your block, ought to catch them, and morph them into nice error codes that the c caller can digest.

extern "c"
{
    int nice_c_function_interface
    (
        void
    )
    {
        int returnStatus;

        try
        {
             returnStatus = nice_cpp_function();
        }
        catch (NiceCppException& that)
        {
             returnStatus = that.failure_code();  
        }
        catch (...)
        {
            cerr << "Oh Worse! an unexpected unknown exception" << endl;

            returnStatus = -1;  // Horrible unknown failure
        }

        return returnStatus;
    }
}

Answer 4

If you have a function in C++ which calls a function in C which in turn calls another function in C++, and this later function throws an exception which should be caught by the first function, you can have problems unless you told the C compiler to enable generation of the exception handling tables.

For gcc, this is the -fexceptions parameter, which is enabled by default for C++ but disabled by default for C.