RFC: Allow moved-from strings to be non-empty

[Jonathan Wakely <jwakely at redhat dot com>]

When an SSO string is contained in the small string buffer or has an
unequal allocator a move operation performs a copy, leaving the original
data in the moved-from string. Setting the length of the moved-from
string to zero is not required, so we can avoid two writes (to the
length member and to set the first character to nul) by leaving the
moved-from string unchanged.

This might be surprising to some users, who (wrongly) expect a string
to always be empty after a move. Is that acceptable?

It's worth noting that the current behaviour, where we do more work
than required, also surprises some people, e.g.
https://stackoverflow.com/q/52696413/981959

Some tests need to be adjusted due to the new behaviour, but they have
comments saying they're testing specific implementation details, so I
don't think needing to adjust them is an argument against making the
change:

// NOTE: This makes use of the fact that we know how moveable
// is implemented on string (via swap). If the implementation changed
// this test may begin to fail.


Should we make this change?


	* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI]
	(basic_string::basic_string(basic_string&&)): Only set length of
	source string to zero when allocated storage is transferred.
	(basic_string::operator=(basic_string&&)): Likewise. Split separate
	cases into separate conditionals.
	* testsuite/21_strings/basic_string/cons/char/moveable.cc: Adjust
	expected state of moved-from strings.
	* testsuite/21_strings/basic_string/cons/char/moveable2.cc: Likewise.
	* testsuite/21_strings/basic_string/cons/char/moveable2_c++17.cc:
	Likewise.
	* testsuite/21_strings/basic_string/cons/wchar_t/moveable.cc:
	Likewise.
	* testsuite/21_strings/basic_string/cons/wchar_t/moveable2.cc:
	Likewise.
	* testsuite/21_strings/basic_string/cons/wchar_t/moveable2_c++17.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/assign/char/
	move_assign.cc: Likewise.
	* testsuite/21_strings/basic_string/modifiers/assign/wchar_t/
	move_assign.cc: Likewise.

Attachment: patch.txt
Description: Text document