[Bug libstdc++/108886] Add basic_string throw logic_error when assigned a nullptr

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108886

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonny Grant from comment #4)
> It sounds like you know the implementation really well.

I wrote it.

> I doubt all uses of std::string are performance intensive.

Just because some aren't, doesn't mean we can ignore the ones that are.

> For performance
> critical code identified by a profiler I just re-write that function in C.

No. std::string is probably the most widely used vocabulary type in the entire
C++ standard library, and "just use C instead" is utterly unacceptable as a
response to performance concerns.

> Usually graphics code.
> 
> There is what might be a performance issue, I don't know if anyone has
> measured, _M_construct() appears to copy copies byte by byte in
> basic_string.tcc:177 rather than using an optimized memcpy(). I've not
> measured this myself.

You can't use memcpy from an input iterator, the data to be copied isn't
contiguous in memory.