How to cause a linker error when an externally-defined macro is inconsistent across translation units?

2 weeks ago 11

ARTICLE AD BOX

I'm writing a header-only library which defines class X in header x.hpp. The implementation of X is different for different standard versions:

#if __cplusplus < 201703 class X { int a; }; #else class X { char b; short c; }; #endif

When x.hpp is included in different client translation units a.cpp and b.cpp, which are compiled with ${CXX} -std=c++11 -c a.cpp and ${CXX} -std=c++20 -c b.cpp and linked together, this silently produces a broken program since ODR violations are IF-NDR.

In practice I can use inline namespaces like so:

#if __cplusplus < 201703 inline namespace before_cpp17 { class X { int a; }; } #else inline namespace cpp17_or_later { class X { char b; short c; }; } #endif

But this only helps when X is used in the client code in a way that affects name mangling of some non-inline function (e.g. as a parameter) so that the linker fails to find the function definition. It does not help when X is used as a base or a data member of some class.

What are my options? Can I put something in x.hpp which will cause linker errors then the standard version is inconsistent regardless of how the client code uses X?