https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67195
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to Jeffrey Walton from comment #5) > When I parsed the page on macros, I found it to be an interesting > implementation detail. And I thought it was useful to know cpp or g++ > fetched the macro from c++config.h. (A related factoid is LLVM uses > <ciso646>). Technically it uses <__config>, which is its equivalent of our <bits/c++config.h>. The reason they suggest including <ciso646> is that it's a public header that does nothing except include the internal implementation header <__config>. > But because I was not explicitly told to include c++config.h or another > standard library header for the define, I did not make the leap it was a > requirement. > > For me (naively), 'cpp' or 'g++ -dM' provide the preporcessor macros used to > compile a source file, and the way they are delivered to me is > indistinguishable from magic. But for us it is done using sufficiently advanced technology not magic, and that technology is a header file. Since you could in theory be using g++ with a different standard library implementation other than libstdc++ it would make no sense for g++ to define the libstdc++ macros, and would be completely wrong! They have to come from the library itself because only once you include a library header file is it known which library you are using.
