C++ portability rules

6. Don't use namespace facility.


Support of namespaces (through the namespace and using keywords) is a relatively new C++ feature, and not supported in many compilers. Don't use it.


7. main() must be in a C++ file.


The first C++ compiler, Cfront, was in fact a very fancy preprocessor for a C compiler. Cfront reads the C++ code, and generates C code that would do the same thing. C++ and C startup sequences are different (for example static constructor functions must be called for C++), and Cfront implements this special startup by noticing the function called "main()", converting it to something else (like "__cpp__main()"), adding another main() that does the special C++ startup things and then calls the original function. Of course for all this to work, Cfront needs to see the main() function, hence main() must be in a C++ file. Most compilers lifted this restriction years ago, and deal with the C++ special initialization duties as a linker issue. But there are a few commercial compilers shipping that are still based on Cfront: HP, and SCO, are examples.


So the workaround is quite simple. Make sure that main() is in a C++ file. On the Unix version of Mozilla, we did this by adding a new C++ file which has only a few lines of code, and calls the main main() which is actually in a C file.


8. Use the common denominator between members of a C/C++ compiler family.


For many of the compiler families we use, the implementation of the C and C++ compilers are completely different, sometimes this means that there are things you can do in the C language, that you cannot do in the C++ language on the same machine. One example is the 'long long' type. On some systems (IBM's compiler used to be one, but I think it's better now), the C compiler supports long long, while the C++ compiler does not. This can make porting a pain, as often times these types are in header files shared between C and C++ files. The only thing you can do is to go with the common denominator that both compilers support. In the special case of long long, we developed a set of macros for supporting 64 bit integers when the long long type is not available. We have to use these macros if either the C or the C++ compiler does not support the special 64 bit type.


9. Don't put C++ comments in C code.


The quickest way to raise the blood pressure of a Netscape Unix engineer is to put C++ comments (// comments) into C files. Yes, this might work on your Microsoft Visual C compiler, but it's wrong, and is not supported by the vast majority of C compilers in the world. Just do not go there.


Many header files will be included by C files and included by C++ files. We think it's a good idea to apply this same rule to those headers. Don't put C++ comments in header files included in C files. You might argue that you could use C++ style comments inside #ifdef __cplusplus blocks, but we are not convinced that is always going to work (some compilers have weird interactions between comment stripping and pre-processing), and it hardly seems worth the effort. Just stick to C style /**/ comments for any header file that is ever likely to be included by a C file.


10. Don't put carriage returns in XP code.


While this is not specific to C++, we have seen this as more of an issue with C++ compilers, see Use the common denominator between members of a C/C++ compiler family. In particular, it causes bustage on at least the IRIX native compiler.


On unix systems, the standard end of line character is line feed ('\n'). The standard on many PC editors is carriage return and line feed ("\r\n"), while the standard on Mac is carriage return ("\r"). The PC compilers seem to be happy either way, but some Unix compilers just choke when they see a carriage return (they do not recognize the character as white space). So, we have a rule that you cannot check in carriage returns into any cross platform code. This rule is not enforced on the Windows front end code, as that code is only ever compiled on a PC. The Mac compilers seem to be happy either way, but the same rule applies as for the PC - no carriage returns in cross platform code.


MacCVS, WinCVS, and cygwin CVS when configured to use DOS line endings automatically convert to and from platform line endings, so you don't need to worry. However, if you use cygwin CVS configured for Unix line endings, or command line cvs on Mac OS X, you need to be somewhat careful.


11. Put a new line at end-of-file.


Not having a new-line char at the end of file breaks .h files with the Sun WorkShop compiler and it breaks .cpp files on HP.