P0935R0 Eradicating Unnecessarily Explicit Default Constructors

[StephanTLavavej]

P0935R0 Eradicating Unnecessarily Explicit Default Constructors

[NathanSWard]

This sounds like a good next issue I can tackle.
This seems similar to #69 where I'll provide the _HAS_CXX20 block with the new non-explicit constructor and the constructor with the non-default parameter.

[StephanTLavavej]

I wonder whether we should implement this unconditionally. Implementers have long-standing permission from the Standard to implement member functions (including constructors) with different signatures than depicted, as long as the behavior is the same. Splitting up default arguments into separate overloads is an allowed transformation.

Unconditional implementation would be unlikely to disrupt 14/17 code, since making a default constructor implicit has basically no effect for someone who didn’t need it. The downside would be portability - users might take advantage of it, and then have problems porting to other STLs.

So perhaps the best thing to do would be to unconditionally split signatures, but then mark the default constructor with an _EXPLICIT_BEFORE_20 macro, avoiding as much code duplication as possible.

@BillyONeal, @CaseyCarter, @barcharcraz - thoughts?

[CaseyCarter]

Mild preference for _EXPLICIT_BEFORE_20 for portability. We should probably back-channel a consensus with libc++ and libstdc++, however: if they implement unconditionally and we do not, we may end up hampering rather than helping portability.

[BillyONeal]

Mild preference to treat it as a DR. (And admonish lib(std)?c++ to do the same)

[StephanTLavavej]

I've emailed some libc++ and libstdc++ maintainers about implementing this unconditionally.

[StephanTLavavej]

libstdc++ has implemented this unconditionally, so we should do the same thing - no macros or preprocessor logic. (We have an "implemented unconditionally" section in yvals_core.h.)

[NathanSWard]

@StephanTLavavej thanks for following up with the other library vendors! An unconditional implementation it is!