C++ using C code using double underscores in defines and identifiers

Question

I understand that in C++ double underscores in identifiers are reserved for the compiler. I have some C code which has characteristics similar to this in the corresponding header files:

extern "C" {
    #define HELLO__THERE 1
    int hello__out__there( int );
}

I will be using this header in a C++ project, and plan to be doing things in C++ like:

if (HELLO__THERE == abc) 
    hello__out__there(foo);

Is this acceptable behavior in C++, covered by the standard?

Answer 1

In the C++03 standard 17.4.3.1.2 Global names, that use of underscores is defined as reserved:

Each name that contains a double underscore (_ _) or begins with an underscore followed by an upper- case letter (2.11) is reserved to the implementation for any use.

Being reserved means that it might be used in any conforming implementation and therefore it is not advisable to use it.

Answer 2

You should be fine, unless by some fluke chance that one of the defines has clashes with your compiler's one. If that is the case, it'll likely be a warning or error (depending on your compiler's configuration) that there'll be a duplicate symbol.

Hope it helps. Cheers!