On 14/06/2021 5:34 pm, Rainer Deyke wrote:
You're assuming that by not specifying an explicit link from B to the standard library that it will inherently have the same settings as P. You're even mostly correct, because the standard library has a bit of special handling in the build tools.
No, I'm saying that by not having a separate compilation step, there is literally no way for B to have different settings than P, because there is nothing that could be compiled with different settings. This is true regardless of the build system, because it is true at the compile level. There is no set of arguments that the build system can pass to the compiler that would cause B to be built with different settings than P.
B is a header-only library. To use B, the C++ preprocessor literally pastes the code from B into P. Therefore, B /has/ no build settings. There are only the build settings from P, which are applied indiscriminately to P's own code and the code from B that P #includes.
That's actually completely false. The preprocessor itself *is* build settings -- defines can be defined differently (along with other compiler options) between the two builds. And yes, there may still be two builds. There is the build of P itself (which includes A) and the build of another library C that is linked into P but also includes A. Both of these can be via B or separately, and both can cause different code for B to be generated if the settings differ, which is an ODR violation. (Or cause two distinct copies of B to be generated, where C is a shared library with private visibility, for example.) For some examples, there's usually a compiler option to choose whether the standard library is linked as static or as shared, which in turn causes different #defines to be defined that will affect compilation. Another example is things that can vary due to the C++ language selected, or whether or not you've defined various "posix extension" flags, or library-specific configuration defines. Or something as simple as compiling one with exception support and one without. Having a header-only library is not a silver bullet to avoid these kinds of compilation incompatibilities. In fact it makes some of them worse.