According to MSVC manual (*1), cl.exe can skip including a header file
when that:
- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.
GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).
Sun C lacked #pragma once for a looong time. Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.
This changeset modifies header files so that each of them include
strictly one #ifndef...#endif. I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]
*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html
Creative use of `@copydoc` Doxygen command and abusing its half-broken C
parser let us delete some lines of documentations, while preserving
document coverages.
C++ keyword `nullptr` represents a null pointer (note also that NULL is
an integer in C++ due to its design flaw). Its type is `std::nullptr_t`,
defined in <cstddef> standard header. Why not support it when the
backend implementation can take a null pointer as an argument.
C++ (and myself) hates macros. If we could do the same thing in both
preprocessor and template, we shall choose template. This particular
part of the ruby header is one of such situations.
Compilation of extension libraries written in C++ are reportedly
broken due to https://github.com/ruby/ruby/pull/2404
The root cause of this issue was that the definition of ANYARGS
differ between C and C++, and that of C++ is incompatible with the
updated ones.
We are using the incompatibility against itself. In C++ two distinct
function prototypes can be overloaded. We provide the old, ANYARGSed
prototypes in addition to the current granular ones; and let the
older ones warn about types.
